iPlan PB commissioning report (2013)

Commissioning of the Pencil Beam Photon Dose Calculation Algorithm for Radiosurgical Treatment Planning with the TrueBeam STx linac

Department of Medical Physics Nova Scotia Cancer Centre QE II Health Sciences Centre Capital District Health Authority (CDHA) Author: Edwin Sham, PhD, MCCPM, DABR Supervisors: James L. Robar, PhD, FCCPM Mammo Yewondwossen, PhD, MCCPM, DABR Chris Thomas, PhD, MCCPM

Chapter 1 Introduction

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1. Introduction

This report describes the commissioning of the Pencil Beam (PB) photon dose calculation algorithm implemented in a commercial advanced radiotherapy treatment planning system (iPlan RT Dose 4.5.1; BrainLab AG; Germany). The BrainLab PB dose algorithm is developed based on publications by Mohan et al. 1-3 to calculate dose distribution in a heterogeneous-density phantom or patient for arbitrarily shaped megavoltage (MV) photon beams. The BrainLab iPlan RT Dose system uses advanced treatment planning techniques in conjunction with the PB algorithm to perform highly conformal 3D dose distributions calculation for both cranial and extra-cranial targets. Treatment planning techniques available in the iPlan Dose system include: • Conformal Beams technique. It uses multiple static beams collimated with

multileaf collimators (MLC) to conform to beam’s eye views (BEV) of the planning target volume (PTV) varying in configuration for different beam angles or orientations.

• Conformal Arcs technique. It adjusts the field aperture with the MLC according to the PTV shape varying over an arc radiation plane, delivered through a gantry rotation at a given treatment couch angle. Multiple non-coplanar arcs (between 4 and 10) are typically used in linac-based stereo-tactic radiosurgery.4-5 Conformal Arcs have both static and dynamic field-shaping options. With the Static Conformal Arcs option, a single, static conformal field is used throughout an arc. This MLC-defined, conformal field shape is taken by averaging the PTV shapes (plus user-specific margins) varying in shape over gantry angles involved in the arc. With the Dynamic Conformal Arcs option, radiation field configurations are continually shaped with the MLC to adjust the varying PTV projections in every 10°-interval during the gantry rotation.

• Intensity Modulated Radiation Therapy (IMRT) technique. It modulates radiation intensities in several static, planar or non-coplanar MLC fields to maximize dose coverage to PTV and minimize doses to surrounding normal tissues and organs at risk. Intensity modulation is calculated with inverse planning optimization and delivered in practice using MLC leaf sequencing.

• HybridArcs technique. It is a combination of the dynamic conformal arcs and IMRT techniques. Inverse-planned IMRT fields are added to dynamic conformal arcs in order to improve 3D isodose distributions for either one of the two techniques alone.

The iPlan PB algorithm is commissioned to perform accurate conformal 3D dose distributions calculation for the new, state-of-the-art stereotactic radio-therapy-dedicated linear accelerator system (TrueBeamTM STx; Varian; Palo


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Alto; USA) installed in the Nova Scotia Cancer Centre (NSCC). The Varian’s TrueBeamTM system integrates real-time tracking and precise treatment delivery to address complex clinical cases, such as lung, prostate, head and neck, etc. With this integration, the latest advanced radiotherapy techniques, e.g., stereotactic radiosurgery or radiotherapy (SRS/SRT), stereotactic body radiotherapy (SBRT), volumetric modulated arc therapy (VMAT) or RapidArc®, and image-guidance radio-therapy (IGRT) can be delivered in high (spatial and dosimetric) precision for these complicated treatment sites.

The TrueBeam system incorporates an on-board imager (OBI) kilo-voltage (kV) imaging system to enhance pre-treatment tumor targeting based on 2D or 3D matching of the high-quality, setup kV-images (planar or cone-beam CT) with reference (planning) images. The TrueBeam STx system installed in our clinic is dedicated to stereotactic irradiation and uses high definition 120 MLC leaves (with the finest resolution of 2.5 mm at the isocenter) to perform small-fields SRS or SBRT procedures in the brain, lung, and prostate cases.

The TrueBeam system is characterized by its versatile, flexible architecture that allows interfaces with multiple technologies for imaging and tumor-specific solutions. Our TrueBeam STx unit (from now on termed “TrueBeam 1”) is interfaced with a combined room-based kV-imaging and robotic patient positioning couch system (ExacTrac®; BrainLab; Germany) to facilitate precise patient set-up and accurate tumor targeting for inter-fractional and also intra-fractional radiotherapy. The BrainLab ExacTrac® IGRT system consists of a pair of room-mounted kV-imaging units to perform 3-D tumor targeting during radiation delivery. Pairs of kV X-ray images are acquired to instantly track the field-to-field tumor displacement or patient movement, thus enabling real-time, intra-fractional motion management. In addition, an ExacTrac® Robotic Patient Alignment system is mounted on top of the linac’s couch support and fully integrated with the electro-mechanical interface of the TrueBeam system to correct for residual positional errors after an initial tattoo- or infrared-based patient setup. This advanced patient positioning system provides two rotational motions (i.e., pitch and roll) in addition to the existing 3D translations and couch rotation, allowing precise patient setup with six degrees of freedom and thus higher level of setup accuracy. Figure 1.1 shows a picture of the TrueBeam 1 linac with the integrating ExacTrac IGRT (imaging and patient-positioning) system.

For cranial SRS, the integration of the ExactTrac IGRT system with the TrueBeam 1 linac offers highly accurate dose delivery on a frameless basis. The use of stereoscopic kV-imaging with the ExacTrac system in conjunction with a BrainLab cranial stereotactic immobilization system achieves sub-millimetric precision required in a SRS procedure. The non-invasive attachment of the immobilization system with the head improves patient’s comfort during treatment.


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Figure 1.1 Picture of the TrueBeam STx linac installed in the Nova Scotia Cancer Centre (NSCC). The linac is equipped with the portal MV imager, kV on-board imager (OBI) as well as ExacTrac imaging and automatic 6D-positioning system to perform multi-options IMRT/IGRT treatments.

Chapter 2 Pencil beam algorithm

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2. Pencil beam algorithm

The pencil beam (PB) algorithm is a model-based dose calculation algorithm developed in the BrainLab’s iPlan RT Dose treatment planning system. Based on the knowledge of linac-specific energy spectra and measured beam characteristic data of the unit, 3D dose distributions for irregularly shaped photon beams in an arbitrary phantom or patient contour with tissue heterogeneities are calculated. Dose calculations are, in principle, performed using 2D convolution of relative primary photon fluence distributions with pencil beam kernels. The convolution PB kernel essentially represents cross-sectional dose distributions of a pencil beam (PB) as a function of depths in a uniform-density medium.

In the PB algorithm, an incident irregular field is divided into many tiny beam-lets called pencil beams. Dose distributions of each pencil beam, known as kernels, are calculated with Monte Carlo (MC) simulation incorporating photon energy spectra specific to a given treatment unit or linac. The MC-calculated pencil beam kernels are modified with a source function correction to account for the influence of the finite focal spot size, curvature of MLC leaves, and linac head scatter on doses. Total 3D dose distributions for an irregular field are calculated by convolution of corrected pencil beam kernels with the primary fluence distribution, using the Fast Fourier Transformation (FFT). The algorithm uses fast ray tracing and adaptive grid methods to enhance dose calculation efficiency and accuracy. With these optimizations, 3D dose distributions for a single, irregularly shaped field can be effectively calculated within milli-seconds. This section dedicates to explain several key parameters constituting the PB dose formalism.

2.1 Pencil beam kernels

A pencil beam is defined as a narrow beam with an infinitesimal cross-sectional field. Assume the pencil beam is mono-energetic with a photon energy E . As the pencil beam impinges on a flat surface of a homogeneous phantom, 3D dose distributions are generated in the medium of the phantom (see Figure 2.1). Point P refers to a location where first collisions (or photon interactions) of the pencil beam take place, at depth

Pt below the phantom

surface. Collision density Pc,

ρ is defined as the number of the first (photon) collisions per unit volume in a small volume surrounding point P and is expressed by:

( ) ( ) ( ) ( )[ ]PPPPc,tEEEtE ×−××Φ= μμρ exp, , (2.1)

where


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Figure 2.1 Definition of differential pencil beam (DPB).

( )EP

Φ is the fluence of the un-attenuated incident photons of energy E at the point P of first (photon) collisions.

μ is the linear attenuation coefficient depending on photon energy.

A differential pencil beam (DPB) refers to a fraction of pencil beam photons that have their first collisions in a small volume surrounding a given point P. DPB kernel is a 3D dose distribution in a medium as a result of the DPB. It depends on three parameters: (1) photon beam energy E ; (2) radial distance PQr between point P of first collisions and the observation point Q; (3) polar angle θ between the incident pencil beam and the ray line joining point P and point Q.

DPB occurs at every depth Pt along the incident pencil beam direction in the

medium. As a result, the dose deposited at the observation point Q due to the pencil beam is a line integral of kernels corresponding to the DPBs generated along the pencil beam axis in the medium and is thus expressed by:

( ) ( ) ( )[ ] ( )PPQDPBPP

dtErktEEEDQ

×××−××∫Φ= ,,exp θμμ , (2.2)


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Equation (2.2) assumes mono-energetic pencil beam spectrum Φ . However, the energy spectrum of a linac is a poly-energetic bremsstrahlung spectrum. Thus, the total dose at the point Q for a realistic poly-energetic pencil beam from the linac is an integration of the DPB doses over the multiple-energy spectrum ( ) EE ddφ . It is known as the pencil beam kernel ( )dyxK ,, and is given by:

( ) ( ) ( ) ( )[ ] ( ) EtErktEEEE

dyxK ddd

dDPB

×∫ ×××−××∫= ,,exp,, θμμφ . (2.3)

Note that the DPB kernels ( )Erk ,,θDPB

were typically pre-calculated with MC simulation in a range of photon energies E between 100 keV and 50 MeV to account for the energy fluence distribution of a typical medical linac.

2.2 Primary fluence distribution

The pencil beam kernel K requires photon fluence distribution, as shown in Equation (2.3). Primary photon fluence of a linac is typically calculated by MC simulation which takes into account the geometry of a linac treatment head as well as incident electron beam characteristics, such as electron beam width and energy spread, etc. The primary photon fluence spectra are scored in a plane perpendicular to the beam’s central axis at the isocenter level.

Photon fluence distribution for an arbitrarily shaped field is calculated by convolving the scored primary fluence distribution with a “geometric kernel” (see Figure 2.2) for the field. This kernel is a 2D matrix with pixel values depending on the field coverage: Pixels located in fully exposed areas have a kernel value of 1; pixels located in fully closed areas have a kernel value of 0; pixels partially blocked by the field (i.e., on the field edge) have fractional values varying between 0 and 1 depending on the extent of the field coverage.

The above photon fluence calculation for an irregular field neglects variation in the fluence spectra relative to the radius from the beam central axis, that is confirmed in previous studies including a MC study by Mohan et al.1 Due to the conical shape of a flattening filter, the spectra soften as the off-axis distance increases. To account for this off-axis beam softening effect, a radial factor is used. The planar photon fluence distribution ( )dyx ,,Φ in air at a level corresponding to an arbitrary depth d in a medium is thus given by:

( ) ( ) ( )drRFSyxdyx ,,,, 0 ×Φ=Φ , (2.4)

where


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Figure 2.2 Geometry kernel for calculation of a photon fluence distribution for an irregular MLC-shaped field. Blue outline refers to the boundary of the MLC field aperture. Kernel value is assigned to 1 for pixels fully exposed in the open field and 0 for voxels fully blocked by the MLC leaves. For voxels partially covered by the leaves, fractional values, proportional to the degree of the leave coverage, are used: the more the leave coverage the smaller the fraction.

( )yx,0

Φ is the fluence distribution at the isocenter which accounts for the field shaping effect of the MLC leaves (see Figure 2.2).

( )drRFS , is the radial factor correcting for the beam softening effect on the photon fluence at a radial distance 22 yxr += from the beam central axis at a level corresponding to the given depth d in the medium.

The radial factors are dose functions that run across the beam central axis at various depths in water. They correct the off-axis variation in the primary photon fluence spectra. Radial factors are directly obtained from the off-axis ratios along the radial distance from the beam central axis and are given by:

( ) ( )( )

( )( )fdD

fdyxDfdDfdrD

drRFS,,0,0,,,

,,0,,, == , (2.5)

where


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Figure 2.3 Geometry for (a) calibration of dose at a reference point Pcal for a reference field Aref and (b) pencil beam modelled calculation of total dose at an arbitrary point Q for an irregular MLC-shaped field in water.

( )fdrD ,, is the radially symmetric dose along the radius 22 yxr += from the beam central axis at a depth d in the medium for a given source-to-surface (SSD) distance f .

2.3 Total dose calculation formalism

Pencil beam kernels K and photon fluence spectra for an irregular field are calculated by MC simulations. A 2D convolution of the MC pre-calculated pencil beam kernels K and total photon fluence distribution Φ results in an idealized dose distribution given by:

( ) ( ) yxdyyxxKdyxdyxIDD ′′−′−′⋅∫∫ ′′Φ= dd,,,,),,( . (2.6)

IDD is essentially a relative 3D dose distribution normalized to 1 at a point of calibration Pcal in water for a given calibration geometry (see Figure 2.3a). For an irregular field, the total dose ( )dyxD ,, at an arbitrary point Q in a medium (see Figure 2.3b) is related to the kernel-based IDD and also basic dosimetric parameters by the following equation:


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( ) ( ) ( )QQtlyxIDD

dfdf

AlTPRAASKdyxD ,,~,~,),,(2

SADSAD

calcal

QmlcjawMU ⋅⎟⎟

⎠

⎞⎜⎜⎝

⎛++

⋅⋅⋅⋅= & , (2.7)

where

MU is monitor unit applied to the linac;

K& is nominal linac output giving an absorbed dose per MU for a reference field

refA of 10×10 cm2 at the isocenter level with a

calibration SSD calf at a depth of calibration

cald in water;

( )mlcjaw

AASt

~, is the total scatter factor for the equivalent square area jaw

A of the collimator jaw opening and the effective square area

mlcA~ of

the MLC field aperture projected at the isocenter level with the source-axis distance SAD of 100 cm for a standard linac;

( )Q

AlTPRQ

~, is the tissue-phantom-ratio at the point of calculation Q in water with the radiological pathlength

Ql of the ray line joining the

source with the point Q for an effective MLC-shaped field area Q

A~ projected at depth d ;

calf is the SSD for the geometry of output calibration;

f is the SSD for the geometry of dose calculation;

IDD is the idealized dose distribution at the radiological depth Ql for

the off-axis distances ( )yx, of the point Q projected at the level of the iso-center with ( )dfxx +⋅= SAD

SAD and ( )dfyy +⋅= SAD

SAD.

2.4 Other correction factors

Several corrections are used to account for deviations from the simplistic geometries assumed in the above dose calculation formalism. These include: (1) tissue inhomogeneity and surface curvature corrections; and (2) source function and radiologic field corrections.

Calculations of DPB dose distributions assume an semi-infinite homogeneous medium of unit density. In a non-uniform density phantom, inhomogeneities are taken into account by primary fluence attenuation correction as well as radiological pathlength correction.


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Figure 2.4 Difference between the radiologic and geometric MLC field. Radiologic field (in red) is measured 50% isodose profile and the geometric field (in black) is MLC-defined field.

Primary fluence attenuation correction accounts for inhomogeneities in the pencil beam path between the source and the point P of first collisions (see Figure 2.1). The total linear attenuation coefficient μ used for a unit-density medium is replaced with the effective linear attenuation coefficient

effμ given

by the coefficients averaged over tissues of various densities in a non-uniform density medium. Heterogeneities also occur between the scattering voxel and computation point Q (see Figure 2.1). Radiological pathlength correction is used to scale the physical distance

PQr by the effective density accounting for

inhomogeneities traversed by the transport of scattered electrons.

Radiological pathlength correction also accounts for curved irregular surfaces irradiated by photon fields. The PB algorithm uses ray-tracing along the ray between the source and the observed calculation point Q (see Figure 2.3) to calculate radiological pathlength l . The radiological pathlength is calculated based on CT Hounsfield units representing relative electron densities. Thus, a correct calibration of the CT simulator used for imaging of patients is crucial to accurate radiologic pathlength calculation in the PB algorithm.


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Source function correction accounts for the influence of finite focal spot size, head scatter, and other effects broadening the penumbra of the beam profile. The function is modeled with a Gaussian distribution with a width σ and amplitude A for a given depth d in water. It is incorporated in the PB dose calculation by convolution with the pencil beam kernels ( )dyxK ,, . The width and amplitude are acquired empirically by fitting calculated dose profiles with measured ones. The pencil beam algorithm requires the widths and amplitudes for two depths in water. Sigma and amplitude values for any intermediate depths are linearly interpolated for dose profile calculations.

Radiological field correction accounts for small deviations of the radiological field from the nominal MLC-defined field as a result of the MLC design (i.e., the round leaf-end and tongue-and-groove). This offset is defined as the static (radiological) leaf shift sΔ and is given by:

( )mlcsss −×=Δ %505.0 , (2.8)

where

%50s is the width of a measured beam profile at the 50% isodose level;

mlcs is the nominal MLC-defined field width.

2.5 Beam modeling data acquisition

Table 2.1 briefly summarizes general beam data required to model the pencil beam algorithm for 3D in-phantom dose distribution calculations for static conformal fields and dynamic conformal arcs techniques. General beam data measurement geometries and dosimeters recommended for characterizing the beam parameters are also described in the table.

The dosimetry data shown in Table 2.1 are essential to a comprehensive pencil beam modeling. Accuracy of the dose calculation is directly dependent on the accuracy and the range of the measured beam data. For detailed information about the beam data measurement as well as review of the pencil beam algorithm, readers are referred to the Brainlab's Physics Technical Reference Guide.6


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Table 2.1 Beam data required for pencil beam modeling for 3D isodose distributions calculations with irregularly shaped linac-photon beams.

Beam data Measurement conditions Modelled paramters

Dosimeters

Linac output SSD: 100 cm; Depth d : 10 cm; Jaws field: (10×10) cm2; MLC field: (10×10) cm2.

K& Calibrated ion chamber.

MLC leakage SSD: 100 cm; Depth d : 10 cm; (Open jaws, open MLCs): (10×10, 10×10) cm2; (Open jaws, closed MLCs): (10×10, 0×0) cm2; (Closed jaws, closed MLCs): (0×0, 0×0) cm2.

Background leakages

Calibrated ion chamber.

PDD SSD: 100 cm; d range: (0-30) cm; MLC field range: (5×5-300×220) mm2; Jaws field range: (8×8-300×220) mm2.

TPR Medium-sized ion chamber for large field PDDs; High-resolution detectors for small fields PDDs.

Scatter factors SSD: 100 cm; Depth d : 10 cm; MLC field range: (5×5-300×220) mm2; Jaws field range: (8×8-300×220) mm2.

tS Ion chamber in combination with a

high-resolution detector for small fields.

Diagonal radial dose profiles

SSD: 100 cm; Jaws field: (40×22) cm2; MLC field: (40×22) cm2.

( )drRFS , Small-sized ion chamber or high-resolution detector.

Transverse dose profiles

SSD: 100 cm; Jaws field: (15×15) cm2; MLC field: Brainlab-specified (see Physics Guide pp.90)6.

Source function and radiological field corrections

High-resolution detector or films.

Chapter 3 Square MLC fields dosimetry verification

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3. Dosimetry verification for MLC-defined square fields

At the NSCC, the nominal dose output of 100 cGy per 100 MU is specified in an isocentric setup with a reference field size of (10×10) cm2 at a reference depth of 5 cm in water for a reference SSD of 95 cm. The output calibration geometry applies to linacs with megavoltage photon energies of 6 MV or lower, including the TrueBeam 1 unit. As a fundamental check of the PB model accuracy, a dose of 100 cGy was prescribed to a reference point in a synthetic water phantom (ID: T41009) under the same output calibration geometry. The number of monitor units (MU) for 6 MV and 6 MV flattening filter free (FFF) modes were both calculated to be 99 MU, as shown in parts a and b, respectively, of Figure 3.1. Recalculation using identical beam setup in a CT-scanned unit-density IMRT phantom (ID: T41010) also results in the same number of calculated MU’s. The 1% difference between the expected (100 MU) and calculated (99 MU) values was observed in both phantom calculations. These discrepancies, though non-negligible but acceptable, may be attributed to uncertainties in the dose calculation.

3.1 Percent depth dose (PDD) distributions

Figures 3.2 and 3.3 compare PB-calculated and measured PDD distributions for 6 MV and 6 MV FFF beams, respectively, for an SSD of 100 cm for various MLC and jaw field settings, ranging from (5×5) mm2 to (220×220) mm2. Two different detectors were used for PDD measurements, depending on the field sizes. For small MLC-defined field areas from (5×5) mm2 to (20×20) mm2, a small-field stereotactic diode (SFD; IBA; Germany) was used. For field areas larger than (20×20) mm2, a 0.13-cc mini-ionization chamber (CC-13; IBA; Germany) was used. PDD data were measured with these two detectors in a 3D water phantom (Blue Phantom2; IBA; Germany). Variable depth resolution was used in the PDD measurements: 2-mm resolution for shallow depths from 0 to 2.5 cm and 5-mm resolution for depths larger than 2.5 cm.

PDDs for MLC-defined square fields were calculated with the PB algorithm in the iPlan RT Dose platform. 3D dose distributions including PDDs were calculated in a synthetic 50-cc water phantom (ID: T41009) using adaptive grid algorithm. Calculated PDD data were sampled with 1-mm resolution in depth. To evaluate degrees of agreement between measured and calculated PDDs, gamma values7 versus depths are plotted for each MLC field in Figures 3.2 and 3.3 for 6 MV and 6 MV FFF beams, respectively. Gammas are evaluated based on criteria of 2-% dose difference and 2-mm distance-to-agreement (DTA). Results show that PDDs calculated by the PB algorithm are in excellent agreement with the measured data with (2%,2-mm)-based gamma values less than 1 for all fields and all depths, except those in the build-up region.


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Figure 3.1 Pencil beam calculation of monitor units in the calibration geometry for (a) 6 MV beam and (b) 6 MV FFF beam of the TrueBeam 1 unit.


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Figure 3.2(a) PB-calculated (in blue line) and measured (in red dots) PDD distributions for 6 MV photon beams for an SSD of 100 cm for various (MLC, jaws) field sizes: (0.5×0.5, 0.8×0.8), (1.0×1.0, 1.2×1.2), (2.0×2.0, 2.2×2.2), (4.0×4.0, 4.2×4.2) cm2. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. PDDs were calculated with the PB algorithm of the iPlan RT dose platform in a unit-density phantom (ID: T41009, Plan02). Measured PDDs were carried out in a water tank (Blue Phantom2; IBA; Germany) with a SFD diode detector for small MLC-defined fields smaller than (2×2) cm2 and a mini-ion chamber for large MLC-defined fields larger than (2×2) cm2.


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Figure 3.2(b) PB-calculated (in blue line) and measured (in red dots) PDD distributions for 6 MV photon beams for an SSD of 100 cm for various (MLC, jaws) field sizes: (6×6, 6×6), (10×10, 10×10), (22×22, 22×22) cm2. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. PDDs were calculated with the PB algorithm of the iPlan RT dose platform in a unit-density phantom (ID: T41009, Plan02). Measured PDDs were carried out in a water tank (Blue Phantom2; IBA; Germany) with a SFD diode detector for small MLC-defined fields smaller than (2×2) cm2 and a mini-ion chamber for large MLC-defined fields larger than (2×2) cm2.


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Figure 3.3(a) PB-calculated (in blue line) and measured (in red dots) PDD distributions for 6 MV FFF photon beams for an SSD of 100 cm for various (MLC, jaws) field sizes: (0.5×0.5, 0.8×0.8), (1.0×1.0, 1.2×1.2), (2.0×2.0, 2.2×2.2), (4.0×4.0, 4.2×4.2) cm2. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. PDDs were calculated with the PB algorithm of the iPlan RT dose platform in a unit-density phantom (ID: T41009, Plan03). Measured PDDs were carried out in a water tank (Blue Phantom2; IBA; Germany) with a SFD diode detector for small MLC-defined fields smaller than (2×2) cm2 and a mini-ion chamber for large MLC-defined fields larger than (2×2) cm2.


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Figure 3.3(b) PB-calculated (in blue line) and measured (in red dots) PDD distributions for 6 MV FFF photon beams for an SSD of 100 cm for various (MLC, jaws) field sizes: (6×6, 6×6), (10×10, 10×10), (22×22, 22×22) cm2. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. PDDs were calculated with the PB algorithm of the iPlan RT dose platform in a unit-density phantom (ID: T41009, Plan03). Measured PDDs were carried out in a water tank (Blue Phantom2; IBA; Germany) with a SFD diode detector for small MLC-defined fields smaller than (2×2) cm2 and a mini-ion chamber for large MLC-defined fields larger than (2×2) cm2.


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3.2 Lateral dose profile distributions

Figures 3.4 and 3.5 compare calculated and measured lateral dose profiles for 6 MV and 6 MV FFF beams, respectively, for an SSD of 100 cm for various MLC-defined square field sizes, ranging from (5×5) mm2 to (220×220) mm2 and for various depths in water: 1.5 ( maxd ); 5; 10; and 20 cm. Measured and also calculated profiles were acquired in two orthogonal axes: (1) the cross-plane axis parallel to the MLC leaf direction (or the X-jaws direction) and (2) the in-plane axis perpendicular to the MLC direction ( or the Y-jaws direction). For accuracy, all beam profiles were measured in the scanning 3D water tank (IBA Blue Phantom2) with the high-resolution SFD-diode. Scanning step sizes of the tank are variable depending on the scanned profile regions: step sizes of (2-5) mm were used for uniform-dose areas, while step sizes of (0.5-1) mm were used for high-dose gradient regions, i.e., penumbras.

3D dose distributions including PDDs and beam profiles at various depths in water (ID: T41009; Plan02 and Plan 03) for the MLC-defined square fields of the 6 MV and 6 MV FFF photon beams were calculated with the PB model in the iPlan RT Dose treatment planning system. Calculated dose profiles at depths of 1.5 cm, 5 cm, 10 cm, and 20 cm for comparison with diode-measured data were sampled with very high spatial resolution of 0.2 mm. This very fine resolution facilitates precise gamma evaluation across profiles, in particular to sharp-gradient penumbra region where a high spatial resolution and small step size of the scanning diode detector were used.

For both 6 MV and 6 MV FFF photon beams of the TrueBeam 1 linac, diode-measured beam profiles agree well with the PB-calculated profiles for all MLC field areas ranging from small (5×5) mm2 to large (220×220) mm2, as shown in Figures 3.4 and 3.5, respectively. Gamma values based on (2%, 2-mm) criteria are generally less than 1 in the central uniform-dose region for all dose profiles. Higher gamma values greater than 1 occur most frequently in the sharp dose-gradient penumbra regions but also occasionally in the transmission tails, especially for larger depths (e.g., 20 cm) in water and for larger field sizes (e.g., 220×220 mm2). The higher γ discrepancies between measured and calculated doses in the transmission region for the large field may be attributed to the energy-dependent response of the diode as a result of increased phantom scatter (of lower photon energies) for increased field size. In general, discrepancies between measured and calculated dose profiles increase as depth increases. Overall, agreements are good based on the mean gamma values shown in the figures for each beam profile in comparison.

.


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Figure 3.4(a) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (0.5×0.5, 0.8×0.8) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


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Figure 3.4(b) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (0.5×0.5, 0.8×0.8) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


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Figure 3.4(c) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (1.0×1.0, 1.2×1.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐11

Figure 3.4(d) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (1.0×1.0, 1.2×1.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐12

Figure 3.4(e) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (2.0×2.0, 2.2×2.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐13

Figure 3.4(f) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (2.0×2.0, 2.2×2.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐14

Figure 3.4(g) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (4.0×4.0, 4.2×4.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐15

Figure 3.4(h) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (4.0×4.0, 4.2×4.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐16

Figure 3.4(i) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (6×6, 6×6) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐17

Figure 3.4(j) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (6×6, 6×6) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐18

Figure 3.4(k) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (10×10, 10×10) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐19

Figure 3.4(l) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (10×10, 10×10) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐20

Figure 3.4(m) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (22×22, 22×22) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐21

Figure 3.4(n) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (22×22, 22×22) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan02) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐22

Figure 3.5(a) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (0.5×0.5, 0.8×0.8) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐23

Figure 3.5(b) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (0.5×0.5, 0.8×0.8) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐24

Figure 3.5(c) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (1.0×1.0, 1.2×1.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐25

Figure 3.5(d) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (1.0×1.0, 1.2×1.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐26

Figure 3.5(e) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (2.0×2.0, 2.2×2.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐27

Figure 3.5(f) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (2.0×2.0, 2.2×2.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐28

Figure 3.5(g) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (4.0×4.0, 4.2×4.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐29

Figure 3.5(h) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (4.0×4.0, 4.2×4.2) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐30

Figure 3.5(i) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (6×6, 6×6) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐31

Figure 3.5(j) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (6×6, 6×6) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐32

Figure 3.5(k) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (10×10, 10×10) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐33

Figure 3.5(l) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (10×10, 10×10) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐34

Figure 3.5(m) PB-calculated (in blue line) and SFD-diode-measured (in red dots) cross-plane (leaf-parallel or X-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (22×22, 22×22) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.


3‐35

Figure 3.5(n) PB-calculated (in blue line) and SFD-diode-measured (in red dots) in-plane (leaf-perpendicular or Y-jaws) dose profile distributions for 6 MV FFF photon beams of TrueBeam 1 unit for an SSD of 100 cm for (MLC, jaws) field sizes of (22×22, 22×22) cm2 at various depths in water: 1.5; 5; 10; 20 cm. Gamma values (in magenta dashed line) based on (2%, 2 mm) criteria are also plotted in the graph. All dose profiles were normalized to 1 at the central axis point. Profiles were calculated with the PB algorithm in a unit-density phantom (ID: T41009, Plan03) with high spatial resolution of 0.2 mm. Profiles were measured with the SFD diode in the 3D water phantom with a large step size of (2-5) mm in uniform dose regions and a small step size of 1 mm in high dose gradient (or penumbra) regions.

Chapter 4 Irregular MLC fields dosimetry verification

4‐1

4. Dosimetric verification for irregular MLC-shaped fields

Dosimetry calculated with the iPlan pencil beam (PB) dose algorithm was verified using a 2-D ion chambers array detector system (I’mRT MatriXX; IBA; Germany). The I’mRT MatriXX detector is used in conjunction with the plastic water phantom (MULTICube; IBA; Germany) for 2D dosimetric verification with rotational therapy techniques, including IMRT, VMAT, as well as conformal fields and arcs techniques used in the iPlan RT Dose treatment planning system. Figure 4.1 shows the setup of the MultiCube-MatriXX dosimetry system for dosimetry measurement in the TrueBeam 1 linac’s room.

The I’mRT MatriXX dosimeter measures 2D dose distributions for both jaw-defined fields and MLC-defined (static or sliding) fields. Using a movie mode with a time resolution down to 20 ms, 2D dose maps can be measured for dynamic fields used in rotational therapy such as Dynamic conformal arcs or VMAT.

Figure 4.1 Setup of the MatriXX ion-chambers array detector embedded in a plastic water phantom (MULTICube; CIRS; USA) for dosimetric verification of MLC-defined complex fields. The solid water layer overlying the MatriXX system is 11 cm thick, and the backscatter layer is 7.5 cm thick.


4‐2

4.1 Calibration of the MatriXX system

Accurate dosimetry with the MatriXX system requires output calibration in the reference irradiation geometry. The ion chambers array underlying the MatriXX measurement surface is set up at the isocenter level. The MatriXX system is sandwiched inside the MULTICube plastic water phantom which has 8-cm thick solid water underneath the MatriXX system and 11-cm thick solid water layer on top of the MatriXX system. A (10×10) cm2 jaw-defined field with retracted MLC leaves is used for the output calibration.

With this measurement setup, a reference dose for the given linac beam (i.e., the 6-MV or the 6-MV FFF beam) is entered in the dose analysis software (OmniPro I’mRT; IBA; Germany) to calibrate the MatriXX system. As recommended8, the reference dose value is best obtained from direct linac calibration measurement. 9 Alternatively, it can also be referenced by dose calculation in a treatment planning system which, however, incorporates heterogeneity corrections for the materials in the MatriXX system.

In the commissioning process, reference doses were obtained from a simple TPR ratio measured directly with the MatriXX system independent of planned dose calculation. Assuming the linac output is accurately calibrated and the effect of heterogeneity in the body of the MatriXX-MULTICube system on the dose distribution is small, reference doses of 80.6 cGy and 78 cGy were measured for 6 MV and 6 MV FFF beam, respectively, and were used subsequently for calibration of the detector. Figure 4.2 compares the resulting measured dose distributions with the ones calculated with the PB algorithm for the calibration (10×10 cm2) fields. Differences between the MatriXX-measured and PB-calculated doses are less than 1% for both 6 MV and 6 MV FFF fields. Table 4.1 summarizes the ratio of measured dose to PB-calculated dose for consecutive calibrations of the MatriXX system used during the commissiong measurements. Similar dose differences (of < 1%) were found. These differences may be attributed to two possible reasons: (1) small offset of the linac output; (2) the inhomogeneity effect corrected by the PB dose calculation but not taken into account in measurements with the MatriXX system.


4‐3

Figure 4.2(a) Comparison of MatriXX-measured dose plane (coronal) and PB-calculated dose plane for a single jaw-defined (10×10) cm2, 6-MVfield in a calibration setup of the MatriXX detector. Both measured and calculated doses were normalized to 100% at the central axis point. Gamma results were evaluated based on dose difference of 3% and distance-to-agreement (DTA) of 3 mm. Dose difference between measurement and calculation at the normalization point is 0.5% for this calibration setup.


4‐4

Figure 4.2(b) Comparison of MatriXX-measured dose plane (coronal) and PB-calculated dose plane for a single jaw-defined (10×10) cm2, 6-MV FFF field in a calibration setup of the MatriXX detector. Both measured and calculated doses were normalized to 100% at the central axis point. Gamma results were evaluated based on dose difference of 3% and distance-to-agreement (DTA) of 3 mm. Dose difference between measurement and calculation at the normalization point is 0.8% for this calibration setup.


4‐5

Table 4.1 Reference doses used for calibration of the MatriXX system in dosimetric measurements for commissioning the pencil beam (PB) algorithm of the iPlan treatment planning system. Measured 2D dose map was compared with the PB-calculated dose map for a field size of (10×10) cm2 in a calibration setup of the MatriXX detector (dref: 11 cm; SSDref: 89 cm; 100 MU). Differences in the cental axis dose between the MatriXX measurement and PB dose calculation (Dmeas/Dplan -1) for each calibration setup for 6 MV and 6 MV FFF photon beams are shown in the table. Measured dose is larger than the planned dose by less than or equal to 1% in all cases.

6 MV 6 MV FFF

Calibration date

Reference dose (cGy)

Dose difference (%)

Reference dose (cGy)

Dose difference (%)

May 10, 2013 80.6 0.5% 78.4 0.8% May 11, 2013 80.5 0.5% 78.1 0.9%

May 12, 2013 80.5 0.6% 78.0 0.6%

May 13, 2013 80.6 0.1% 78.1 1.0%

May 15, 2013 80.7 0.7% 78.0 0.9% May 18, 2013 80.5 0.6% 77.7 0.6%

May 19, 2013 80.5 0.4% 77.7 0.6%

4.2 Verification for circular fields

Dose distributions for MLC-defined circular fields were calculated with the PB dose algorithm. Circular fields of 1, 2, 4, and 8 cm diameter were used. In-phantom doses were calculated in the phantom body of the I’mRT matrix (ID: T41010; Plan1). PB-calculated dose distributions were sampled on the array detector plane with a spatial resolution of 1 mm. They were compared with the dose distributions measured directly with the MatriXX system. The comparison results are shown for the four diameter fields in Figures 4.3 to 4.6, for both 6 MV and 6 MV-FFF beams.

Gamma analysis was evaluated based on 3%-dose difference and 3-mm DTA. For large circular fields with diameters of 2 cm and greater, good agreements between the MatriXX-measured and PB-calculated dose distributions are achieved, as shown in Figures 4.3 to 4.5. For the smallest 1-cm diameter field, the central-axis peak dose measured with the MatriXX detector was smaller than the PB-calculated peak dose for both 6 MV and 6 MV-FFF fields, as shown in Figures 4.6(a) and (b), respectively. It is due to the volume averaging effect of the finite-size ion chambers with cross-sectional areas of the air cavity of 7-mm diameter. It is also this effect that results in the measured beam profiles appearing less sharp than the calculated profiles in the penumbra regions for all diameter fields.


4‐6

Figure 4.3(a) MatriXX-measured vs PB-calculated dose distributions for a 8-cm diameter MLC-defined field of 6 MV beam.


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Figure 4.3(b) MatriXX-measured vs PB-calculated dose distributions for a 8-cm diameter MLC-defined field of 6 MV FFF beam.


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4.3 Verification for complex “letter” fields

MLC-defined fields shaped as “letters” were created. Dose distributions for these irregular “letter” fields were calculated with the PB dose algorithm in the iPlan RT Dose planning system. Four “letter” field shapes were defined with small field dimesions of the order of (4×4) cm2: (1) “I” shape; (2) “H” shape; (3) “Z” shape; (4) “N” shape. In-phantom dose distributions were calculated in the CT body of the I’mRT MatriXX (ID: T41010; Plan2). PB-calculated dose distributions were sampled on the chambers array plane with a fine spatial resolution of 1 mm. They were exported to the OmniPro I’mRT dose analysis software and compared with dose distributions measured directly with the MatriXX system. The comparison results are shown for the four “letter” fields in Figures 4.7 to 4.10, for both 6 MV and 6 MV-FFF beams.

Overall, the MatriXX-measured dose distributions agree well with the PB-calculated dose distributions for all “letter” fields. Differences of the point dose at the normalization point in uniform dose region between measurement and calculation are within 2% for all fields for both 6 MV and 6 MV FFF beams. The measured point dose is typically smaller than the PB-calculated dose due to the volume averaging effect of the ion chambers exposed in these small fields.

Gamma analysis based on 3% dose difference and 3-mm DTA was evaluated for comparison between MatriXX measurement and PB calculation for these small “letter” fields. The gamma statistics for each field are shown in Figures 4.7 to 4.10. Gammas with values greater than 1 refer to pixels where typical sharp dose gradient locates. In comparsion with circular diameter fields where dose gradients are isotropic in radial directions, the gamma statistics for these “letter” fields with anisotropic and sharper dose gradients are less superior. For example, for the “Z” shaped MLC-defined field as shown in Figure 4.9, measured dose profile agree well with the calculated profile in X-direction parallel to the MLC leaf motion. However, along the Y-direction perpendicular to the MLC leaf motion, the measured and calculated dose profiles differ, particularly in the dip region where the diagonal arm of the “Z” shape meets the horizontal arm. Similar discrepancy occur in the profiles for the “N” shaped field, albeit in opposite (X-jaw) direction. Obviously, the MatriXX detector lacks sufficient resolving power to measure the sharp dose gradient over a small region. Gamma results can be improved when higher-resolution, 2D dose detectors are used, such as radiographic or radiochromic films. Film measurements may follow up to confirm this speculation.


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Figure 4.7(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “I-shaped” field of 6 MV beam.


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Figure 4.7(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “I-shaped” field of 6 MV FFF beam.


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Figure 4.8(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “H-shaped” field of 6 MV beam.


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Figure 4.8(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “H-shaped” field of 6 MV FFF beam.


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Figure 4.9(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “Z-shaped” field of 6 MV beam.


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Figure 4.9(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “Z-shaped” field of 6 MV FFF beam.


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Figure 4.10(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “N-shaped” field of 6 MV beam.


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Figure 4.10(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined “N-shaped” field of 6 MV FFF beam.


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4.4 Verification for complex “checker-board” fields

“Checker-board” field patterns were created with MLC for further dosimetric verification for static, complex-shaped fields. Four patterns were designed by arranging four small MLC-shaped squared fields in orderly fashion. Each squared field has a dimension of (1.5×1.5) cm2. The “check-board” patterns were defined within a jaw dimension of (6.4×6.4) cm2. In-phantom dose distributions were calculated inside the CT body of the MULTICube-MatriXX dosimetric system with the PB dose algorithm (ID: T41010; Plan 3). The PB-calculated 2D dose distributions with a spatial resolution of (1×1) mm2 were exported to the OmniPro-I’mRT dose analysis software for comparison with direct measurement data. Measured versus calculated dose distributions along with the gamma analyses are shown in Figures 4.11 to 4.14 for these four “check-board” field patterns for both 6 MV and 6 MV FFF photon beams.

In comparison, point doses at the normalization point show good agreement between the MatriXX measurement and PB dose calculation for all four “checker-board” field patterns, within 2% in general cases and within 3.3% in the laregest discrepancy. The checker-board patterns were formed with simpler squared fields with more isotropic dose gradients; therefore, gamma statistics improve for these fields in comparison with the more anisotropic dose-varying “letter” fields. More than 98% of the total number of pixels has gamma value less than 1 for all fields (see Figures 4.11 to 4.14) for both 6 MV and 6 MV-FFF beams. Gamma values were evaluated based on 3% dose difference and 3-mm DTA.

Chapter 4_____________________________________________________________________________________ Irregular MLC fields dosimetry verification

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Figure 4.11(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (I) of 6 MV beam.


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Figure 4.11(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (I) of 6 MV FFF beam.


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Figure 4.12(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (II) of 6 MV beam.


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Figure 4.12(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (II) of 6 MV FFF beam.


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Figure 4.13(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (III) of 6 MV beam.


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Figure 4.13(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (III) of 6 MV FFF beam.


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Figure 4.14(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (IV) of 6 MV beam.


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Figure 4.14(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined checker-board field pattern (IV) of 6 MV FFF beam.


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4.5 Verification for complex “number” fields

Lastly, “number” fields shaped by the high-definition MLC’s of the TrueBeam 1 unit were used in dosimetric verification for single, irregularly MLC-shaped fields. Nine “number” fields with shapes ranging from numbers “1” to “9” were created by the MLC (ID: T41010; Plan04). Two sets of these 9 “number” fields were formed based on the font sizes of the numbers: (1) Large font size group with jaws-defined field dimensions of the order of (4×7) cm2 and (2) Small font size group with jaws-defined field dimensions of the order of (2×4) cm2. In-phantom dose distributions were calculated in the CT body of the solid-water embedded MatriXX detector (ID: T41011). PB-calculated dose distributions were sampled on the chambers array plane with a fine spatial resolution of 1 mm. They were exported to the OmniPro I’mRT dose analysis software and compared with dose distributions measured directly with the MatriXX system. The comparison results are shown for the 9 “number” fields with two font size ranges in Figures 4.15 to 4.23, parts (a) and (b) for 6 MV beams with large and small font sizes, respectively, and parts (c) and (d) for 6 MV-FFF beams with large and small font sizes, respectively.

Comparison shows that MatriXX-measured dose distributions agree well with the PB-dose distributions for all larger-sized “number” fields. Difference in the normalization point dose between measurement and calculation is small, within 2.2%. Distribution of gamma values based on 3% dose difference and 3-mm DTA varies depending on the complexity of the field shape. For round figure configurations (such as numbers “6” and “8”), gamma statistics are excellent with more than 97% of the total number of pixels with gamma values less than 1. For figure configurations with high irregularity (e.g., numbers “2” and “5”), gamma distributions slightly degrade primarily due to very steep dose gradients which cannot be adequately measured with the current finite-resolution MatriXX system.

As shown in Figures 4.15 to 4.23, when the field configurations were reduced by ½ in size of the order of (2-4) cm, discrepancies between the measured and PB-calculated dose distributions, including the normalization point doses, are more significant. These results demonstrate the limitation of the currently used MatriXX system for use in dosimetry verification for small fields. The smaller spatial resolution of 7 mm for this dosimeter prevents its application to dosimetry for fields as small as 2 cm, unless the field shapes are uniform or rounded like configurations of “6” and “8” as shown in parts (b) and (d) of Figures 4.20 and 4.22, respectively. For such small and highly irregular field shapes, it is recommended to use high-resolution films to evaulate 2D dose distributions and a small-field ion chamber to evaulate an in-phantom point dose located in a uniform dose region.


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Figure 4.15(a) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined, number “1” field in large font size of 6 MV beam.


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Figure 4.15(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined, number “1” field in small font size of 6 MV beam.


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Figure 4.15(c) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined, number “1” field in large font size of 6 MV FFF beam.


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Figure 4.15(d) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined, number “1” field in small font size of 6 MV FFF beam.


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Figure 4.23(b) MatriXX-measured vs PB-calculated dose distributions for a MLC-defined, number “9” field in small field size of 6 MV beam.


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Chapter 4 Irregular MLC fields dosimetry verification _

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4.6 Summary

In summary, a total of 42 plans for single, static, MLC-defined complex fields were used in commissioning the PB dose algorithm of the iPlan RT Dose treatment planning system (21 plans for 6 MV beam and 21 identical plans for 6 MV FFF beams). 2D isodose distributions with fine spatial resolution of (1×1) mm2 were calculated with the PB algorithm for each plan and compared with the dose distributions measured directly with the ion-chamber arrays (MatriXX) dosimetery system.

The accuracy of the commissioned PB dose calculation algorithm is evaluated from a comparison of the measured and calculated dose distributions in two aspects: (1) Point dose difference; (2) Gamma (γ) map analysis. Point dose difference refers to the percentage difference in the absolute dose at a point of normalization in a phantom between the measurement and PB calculation. Gamma map analysis evaluates difference in relative isodose distributions between the measurement and PB calculation in terms of dose and distance. These QA results for each plan were evaluated and are presented in the corresponding figure in this chapter.

QA results (i.e., point dose difference and gamma analysis) of all plans are summarized in the form of frequency histograms in Figure 4.24 to evaluate the peformance of the PB dose calculation algorithm. Out of a total 42 plans, 76% and 93% of all plans have point dose difference between measurement and calculation within ±2% and ±3%, respectively. In addition, 74% of all plans have more than 97% of all pixels passing the gamma values (less than 1) based on the criteria of 3% dose difference and 3-mm DTA. The median percentage of pixel population meeting the gamma criteria is 97.2%. In conclusion, agreements between the measured and PB-calculated dose distributions are good for single, MLC-shaped complex fields. The PB dose algorithm is validated for these fields.

Chapter 4 Irregular MLC fields dosimetry verification _

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0

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4

6

8

10

12

14

(-4%,-3%) (-3%,-2%) (-2%,-1%) (-1%,0%) (0%,1%) (1%,2%) (2%,3%) (3%,4%)

Point dose difference (%)

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Percentage of pixels in passing (γ < 1) range

Num

ber o

f plan

s

Figure 4.24 Frequency histograms summarizing quality assurance (QA) results of a total 42 plans with single, static, MLC-defined irregular fields for both 6 MV and 6 MV FFF beams. The top histogram shows number of plans with difference of the normalization point dose between the MatriXX measurement and the PB calculation in 1%-difference interval. The bottom histogram shows the number of plans with total number of pixels (in percentage) having γ less than 1 in various percentage ranges from 93% to 100%.

Chapter 5 Patientspecific plans dosimetry verification

5‐1

5. Dosimetric verification for patient-specific treatment plans

Validation of the iPlan pencil beam (PB) dose calculation algorithm extends from application to single, static MLC-defined fields to application to patient-specific treatment plans. The accuracy of the PB algorithm was investigated in the iPlan RT Dose platform for two radiosurgery-dedicated techniques: (1) Static Conformal Fields (SCF) technique and (2) Dynamic Conformal Arcs (DCA) technique. Patient-specific treatment plans with multiple planning target volumes (PTV) were used for the SCF and DCA techniques with both 6 MV and 6 MV FFF photon beams from the Varian’s TrueBeam 1 system. In-patient dose distributions were calculated with the PB algorithm using the non-uniform density CT image data set for the particular patient.

To verify the accuracy of the PB dose calculation model, each treatment plan was mapped to the CT body phantom (ID: T41011) of the MultiCube-MatriXX dosimetry system. The calculated in-phantom dose distributions on the plane of the ion-chambers array were compared with the corresponding 2D planar dose distributions measured directly with the MatriXX system. Both absolute doses and gamma distributions were evaluated for validation purpose. Figure 5.1 shows the beam setup for the SCF and DCA radiosurgical techniques for in-phantom dose calculation and verification.


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Figure 5.1 In-phantom dose calculation and verification geometry for two radiosurgical techniques: (a) Static Conformal Fields (SCF) technique and (b) Dynamic Conformal Arcs (DCA) technique with 6 MV and 6 MV FFF beams from the TrueBeam 1 linac. Dose distributions were calculated with the pencil beam (PB) algorithm in a CT body phantom of the MatriXX dosimeter (ID: T41011). The PB-calculated dose distributions on the plane of the ion-chambers array were compared directly with the same 2D planar dose distributions measured directly with the MatriXX system to validate the dose calculation accuracy of the PB model.

The MatriXX system was mounted on the special Brainlab 6D robotic couch for dosimetry verification of the SCF and DCA techniques. Given that some treatment fields were incident into the MatriXX body through the treatment couch, a model of the couch with the associated electron density information was incorporated in the in-phantom dose distributions calculation with the PB algorithm (see Figure 5.1). Both SCF and DCA radiosurgical techniques are characterized by multiple non-coplanar fields. This means both the gantry and the couch are rotated at specific planned angular positions for treatment irradiation. The MatriXX system is set up with a particular couch position during verification. Some combined gantry-couch angular positions may result in a collision of the MatriXX detector with the gantry and thus should not be incorporated in associated treatment plans. Table 5.1 shows the range of gantry angles of collision clearance for a set of couch angles available for use in the SCF and DCA radiosurgical techniques.


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Table 5.1 Range of gantry angles at given couch angles used in the SCF and DCA radiosurgical techniques devoid of the gantry collision with the MatriXX system mounted on the Brainlab 6D robotic treatment couch. Note that this data applies to the MatriXX system set up at a particular couch position (Vertical: 11.07 cm; Longitudinal: 111.42 cm; Lateral: 1000.00 cm; Rotation: 0.00°). If the MatriXX system is changed to a different setup position, this data may vary.

Couch angle (°) Range of gantry angle (°) avoiding collision

90° (179°, 310°) counter‐clockwise (ccw)

75° (179°, 310°) ccw

60° (179°, 305°) ccw

45° (179°, 300°) ccw

30° (179°, 295°) ccw

15° (179°, 181°) ccw

0° (179°, 181°) ccw

345° (179°, 181°) ccw

330° (70°, 181°) ccw

315° (60°, 181°) ccw

300° (55°, 181°) ccw

285° (50°, 181°) ccw

270° (45°, 181°) ccw

5.1 Two-brain-metastases plan verification

Treatment plan (ID: 169073) of a patient with two brain metastases (anterior and posterior) was used to verify the accuracy of dose calculation of the PB algorithm for SCF and DCA radiosurgical treatments. The anterior brain metastasis PTV (PTVa) has a volume of 12.3 cm3 with the equivalent radius of 1.4 cm and was prescribed with a total dose of 22.5 Gy in a single fraction. The posterior brain metastasis PTV (PTVp) has a volume of 3.8 cm3 with the equivalent radius of 1 cm and was prescribed with a single fractionated dose of 30 Gy. Figures 5.2 and 5.3 show the field setups of both the SCF and DCA radiosurgical techniques used in treating the PTVa and PTVp, respectively. In-patient 3D dose distributions calculated with the PB dose algorithm in the three orthogonal (axial; sagittal; coronal) planes are also displayed (in color wash form) in the figures. The corresponding treatment parameters used for both radiosurgical techniques were summarized in the setup reports printed from a commissioned Record-and-Verify Oncology information system (ARIA®; Varian; USA). Figure 5.4 and 5.5 shows the ARIA-printed setup forms for the SCF and DCA techniques used to treat the anterior PTV and posterior PTV, respectively, in this patient plan (ARIA ID: T41010; Course ID: C5-169073).


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Figure 5.2 Beam setup of two radiosurgical techniques: (a) SCF and (b) DCA for dose calculation with the PB algorithm in a patient-specific treatment plan (ID: 169073). All fields are isocentrically setup at an anterior planning target volume (PTVa) prescribed with a single fractionated dose of 22.5 Gy. The PB-calculated 3D dose distributions are shown in the form of colorwash and isodose lines (10%, 30%, 50%, 80%, 90%, and 95%).


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Figure 5.3 Beam setup of two radiosurgical techniques: (a) SCF and (b) DCA for dose calculation with the PB algorithm in a patient-specific treatment plan (ID: 169073). All fields are isocentrically setup at a posterior planning target volume (PTVp) prescribed with a single fractionated dose of 30 Gy. The PB-calculated 3D dose distributions are shown in the form of colorwash and isodose lines (10%, 30%, 50%, 80%, 90%, and 95%).


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Figure 5.4 Aria-printed setup report summarizing the treatment parameter used for in-phantom verification with the MatriXX system for the SCF and DCA techniques used in conjunction with the PB dose calculation algorithm to treat the anterior planning target volume (PTVa) of the test patient case (ID: 169073).


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Figure 5.5 Aria-printed setup report summarizing the treatment parameters used for in-phantom verification with the MatriXX system for the SCF and DCA techniques used in conjunction with the PB dose calculation algorithm to treat a posterior planning target volume (PTVp) of a test patient case (ID: 169073).


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Table 2 Summary of dosimetry verification results of two-brain-metastases treatment plans (iPlan ID: 169073; Eclipse Plan/Course IDs: T41010/C5-169073) with two radiosurgical techniques (SCF and DCA) for two X-ray beam mode (6 MV and 6 MV FFF).

Plan #

PTV type Technique Beammode

Dmeas‐MatriXX Dplan‐PBC Dmeas/Dplan Area % (γ≤1)

1 PTVa SCF 6 MV 1310.5 cGy 1304.0 cGy 1.005 94.7%

2 PTVa SCF 6 MV FFF

1274.9 cGy 1303.1 cGy 0.978 98.7%

3 PTVa DCA 6 MV 1436.9 cGy 1460.4 cGy 0.984 98.2%

4 PTVa DCA 6 MV FFF

1371.0 cGy 1344.6 cGy 1.020 97.9%

5 PTVp SCF 6 MV 2026.0 cGy 2040.2 cGy 0.993 95.9%

6 PTVp SCF 6 MV FFF

2020.4 cGy 2073.9 cGy 0.974 97.8%

7 PTVp DCA 6 MV 1790.8 cGy 1811.7 cGy 0.988 97.8%

8 PTVp DCA 6 MV FFF

1740.4 cGy 1708.9 cGy 1.018 98.2%

Approved SCF- and DCA-treatment plans for both the PTVa and PTVp with 6-MV and 6-MV FFF X-ray beams of the TrueBeam 1 linac were mapped to the CT phantom of the MatriXX dosimeter (ID: T41011) for plan verification. In-phantom 3D dose distributions were calculated with the PB model in the iPlan RT4.5 Phantom QA platform. A 2D, PB-calculated dose plane on the level of the ion-chambers array was used to compare with the same planar dose distributions measured with the ion chambers in the MatriXX system. Figures 5.6 to 5.13 shows results of comparison between the measured and calculated dose distributions for the two radiosurgical techniques (SCF and DCA) for the two photon beam modes (6 MV and 6 MV FFF) for the two PTVs (PTVa and PTVp). Table 5.2 summarizes the dosimetric verification results in terms of measured-to-planned dose ratio and pixels area percentage of passed gamma level (γ ≤ 1, for 3%-dose difference and 3-mm DTA).

Overall, the MatriXX-measured and PB-calculated dose distributions agree well for all treatment plans of this patient case. Absolute dose differences are acceptable, within ±4% tolerance level for the MatriXX system used. Pixel areas percentage with γ ≤ 1 are also acceptable, greater than 95% tolerance level for all plans except for one (PTVa; SCF; 6 MV). A closer examination of the QA result for this plan (see Figure 5.6) shows most of the pixel areas with failed gamma values (γ > 1) occur in the peripeheral region of the MatriXX array, where calculated or measured dose levels are relatively low. This pattern that lowers the overall γ-agreeing area percentage is also observed in the other treatment plans (see Figures 5.7 to 5.13).


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Figure 5.6 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV beam to treat the anterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVa SCF 6X).


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Figure 5.7 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV FFF beam to treat the anterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVa SCF 6XFFF).


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Figure 5.8 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV beam to treat the anterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVa DCA 6X).


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Figure 5.9 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV FFF beam to treat the anterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVa DCA 6XFFF).


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Figure 5.10 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV beam to treat the posterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVp SCF 6X).


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Figure 5.11 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV FFF beam to treat the posterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVp SCF 6XFFF).


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Figure 5.12 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV beam to treat the posterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVp DCA 6X).


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Figure 5.13 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV FFF beam to treat the posterior PTV in a patient plan (ID: T41011; Plan: Mapped from CL 169073, PTVp DCA 6XFFF).


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5.2 Three-brain-metastases plan verification

Treatment plans (iPlan ID: T41010-159410) of a patient with three brain PTV’s (frontal, occipital, and cerebellar sinus) were used to further validate the accuracy of the PB dose calculation algorithm applied to the SCF and DCA radiosurgical treatments. The frontal brain PTV (PTVf) has the largest volume size of 39 cm3 with an equivalent radius of 2.1 cm and was prescribed with a total dose of 14 Gy in a single fraction. The occipital brain PTV (PTVo) has an intermediate volume size of 10.5 cm3 with the equivalent radius of 1.4 cm and was prescribed with a total, single fractionated dose of 20 Gy. The (cerebellar) sinus brain PTV (PTVs) has the smallest volume size of 1.1 cm3 with the equivalent radius of 0.6 cm and was prescribed with a total, single fractionated dose of 24 Gy. Figures 5.14 to 5.16 show various field setups of the SCF and DCA techniques used in the treatment plans for the three different size PTVs: PTVf, PTVo, and PTVs, respectively. In-patient 3D dose distributions calculated with the investigated PB dose algorithm in the three orthogonal (axial; sagittal; coronal) planes are also displayed (in color wash form) in these figures. The corresponding treatment parameters used for both radiosurgical techniques were summarized in the setup reports printed from the ARIA Record-and-Verify information system. Figure 5.17 to 5.19 show the ARIA-printed setup forms for the SCF and DCA techniques used to treat PTVf, PTVo, and PTVs, respectively, in the associated patient plan (ARIA ID: T41010; Course ID: C6-159410).

Approved SCF-based and DCA-based treatment plans for the three brain PTV’s (PTVf, PTVo,and PTVs) with 6-MV and 6-MV FFF X-ray beams of the TrueBeam 1 linac were mapped to the CT phantom of the MatriXX dosimeter (ID: T41011) for dosimetry verification. In-phantom 3D dose distributions were calculated with the inestigated PB model in the iPlan RT4.5 Phantom QA platform. The 2D, PB-calculated dose plane lying on the level of the ion-chambers array was used to compare with the same planar dose distribution measured with the ion chambers in the MatriXX system. Figures 5.20 to 5.31 shows QA results of comparison between the measured and calculated dose distributions for the two radiosurgical techniques (SCF and DCA) with the two photon beam modes (6 MV and 6 MV FFF) for the three various sized PTV’s (PTVf, PTVo, and PTVs). Table 5.3 summarizes dosimetric verification results expressed in terms of the ratio of the absolute point dose measured with the MatriXX ion chambers array (Dmeas) to the same (in-phantom) point dose calculated with the PB algorithm (Dplan) as well as the percentage of total pixels with passed gamma level (γ ≤ 1) based on the (3%-dose difference, 3-mm DTA) criterion.


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Figure 5.14 Beam setup of two radiosurgical techniques: (a) SCF and (b) DCA for dose calculation with the PB algorithm in a patient-specific treatment plan (ID: T41010-159410). All fields are isocentrically localized at a planning target volume (PTVf) located on the frontal lobe. The PTVf was prescribed with a single fractionated dose of 14 Gy. The PB-calculated 3D dose distributions are shown in the form of colorwash and isodose lines (10%, 30%, 50%, 80%, 90%, and 95%).


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Figure 5.15 Beam setup of two radiosurgical techniques: (a) SCF and (b) DCA for dose calculation with the PB algorithm in a patient-specific treatment plan (ID: T41010-159410). All fields are isocentrically localized at a planning target volume (PTVo) located on the occipital lobe. The PTVo was prescribed with a single fractionated dose of 20 Gy. The PB-calculated 3D dose distributions are shown in the form of colorwash and isodose lines (10%, 30%, 50%, 80%, 90%, and 95%).


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Figure 5.16 Beam setup of two radiosurgical techniques: (a) SCF and (b) DCA for dose calculation with the PB algorithm in a patient-specific treatment plan (ID: T41010-159410). All fields are isocentrically localized at a planning target volume (PTVs) located on the cerebellar sinus. The PTVs was prescribed with a single fractionated dose of 24 Gy. The PB-calculated 3D dose distributions are shown in the form of colorwash and isodose lines (10%, 30%, 50%, 80%, 90%, and 95%).


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Figure 5.17 Aria-printed setup report summarizing the treatment parameter used for in-phantom verification with the MatriXX system for the SCF and DCA techniques used in conjunction with the PB dose calculation algorithm to treat the frontal planning target volume (PTVf) of the test patient case (iPlan/ARIA ID: T41010-159410).


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Figure 5.18 Aria-printed setup report summarizing the treatment parameter used for in-phantom verification with the MatriXX system for the SCF and DCA techniques used in conjunction with the PB dose calculation algorithm to treat the occipital planning target volume (PTVo) of the test patient case (iPlan/ARIA ID: T41010-159410).


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Figure 5.19 Aria-printed setup report summarizing the treatment parameter used for in-phantom verification with the MatriXX system for the SCF and DCA techniques used in conjunction with the PB dose calculation algorithm to treat the (cerebellar) sinus planning target volume (PTVs) of the test patient case (iPlan/ARIA ID: T41010-159410).


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Figure 5.20 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV beam to treat the frontal PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVf SCF 6X).


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Figure 5.21 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV FFF beam to treat the frontal PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVf SCF 6XFFF).


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Figure 5.22 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV beam to treat the frontal PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVf DCA 6X).


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Figure 5.23 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV FFF beam to treat the frontal PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVf DCA 6XFFF).


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Figure 5.24 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV beam to treat the occipital PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVo SCF 6X).


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Figure 5.25 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV FFF beam to treat the occipital PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVo SCF 6XFFF).


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Figure 5.26 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV beam to treat the occipital PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVo DCA 6X).


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Figure 5.27 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV FFF beam to treat the occipital PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVo DCA 6XFFF).


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Figure 5.28 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV beam to treat the sinus PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVs SCF 6X).


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Figure 5.29 MatriXX-measured vs PB-calculated in-phantom dose distributions for the SCF technique with the 6 MV FFF beam to treat the sinus PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVs SCF 6XFFF).


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Figure 5.30 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV beam to treat the sinus PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVs DCA 6X).


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Figure 5.31 MatriXX-measured vs PB-calculated in-phantom dose distributions for the DCA technique with the 6 MV FFF beam to treat the sinus PTV in a mapped treatment plan (ID: T41011; Plan: Mapped from T41010-159410, PTVs DCA 6XFFF).


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Table 5.3 Summary of dosimetry verification results of three-brain-tumors treatment plans (iPlan ID: T41010-159410; Eclipse Plan/Course IDs: T41010/C6-159410) with the two radiosurgical techniques (SCF and DCA) for two X-ray beam mode (6 MV and 6 MV FFF) of the TrueBeam 1 linac. QA results are expressed in terms of (1) the ratio of measured dose to planned dose at a normalization point defined in the uniform dose region of the isodose distributions and (2) gamma distribution reflecting dosimetric and spatial difference between measured and calculated dose distributions. Plan # PTV type Volume

(eqv. radius) Radiosurgical technique

Photon beam mode

Dmeas(MatriXX) Dplan(PB) Dmeas/Dplan % of pixels with γ≤1

1 PTVf (frontal)

39 cm3 (2.1 cm)

SCF 6 MV 798.8 cGy 809.7 cGy 0.987 99.4%

2 PTVf (frontal)

39 cm3 (2.1 cm)

SCF 6 MV FFF 754.4 cGy 756.6 cGy 0.997 98.6%

3 PTVf (frontal)

39 cm3 (2.1 cm)

DCA 6 MV 881.3 cGy 889.0 cGy 0.991 99.8%

4 PTVf (frontal)

39 cm3 (2.1 cm)

DCA 6 MV FFF 824.9 cGy 828.7 cGy 0.995 99.9%

5 PTVo (occipital)

10.5 cm3 (1.4 cm)

SCF 6 MV 1344.2 cGy 1346.3 cGy 0.998 97.6%

6 PTVo (occipital)

10.5 cm3 (1.4 cm)

SCF 6 MV FFF 1290.6 cGy 1271.9 cGy 1.015 97.6%

7 PTVo (occipital)

10.5 cm3 (1.4 cm)

DCA 6 MV 1276.9 cGy 1270.1 cGy 1.005 99.5%

8 PTVo (occipital)

10.5 cm3 (1.4 cm)

DCA 6 MV FFF 1234.0 cGy 1200.2 cGy 1.028 99.0%

9 PTVs (sinus)

1.1 cm3 (0.6 cm)

SCF 6 MV 1461.0 cGy 1556.4 cGy 0.939 99.1%

10 PTVs (sinus)

1.1 cm3 (0.6 cm)

SCF 6 MV FFF 1448.4 cGy 1488.5 cGy 0.973 99.0%

11 PTVs (sinus)

1.1 cm3 (0.6 cm)

DCA 6 MV 1515.8 cGy 1564.4 cGy 0.969 100%

12 PTVs (sinus)

1.1 cm3 (0.6 cm)

DCA 6 MV FFF 1486.5 cGy 1479.7 cGy 1.005 100%


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For these three-PTV’s treatment plans (ID: T41010-159410), dose agreements between the MatriXX-measured and PB-calculated dose distributions vary depending on the size of the PTV: the larger the PTV size the better the dose agreement. For the large- and medium-size PTV’s (frontal PTVf and occipital PTVo), the measured and calculated in-phantom dose distributions are in excellent agreement: (1) the measured dose to planned dose ratio within ±3% and (2) more than 97% of total pixels with passed (3%,3-mm) based gamma level (γ ≤ 1), as shown in the QA results summarized in Table 5.3.

For the smallest PTV size of 1 cm3, discrepancies of the absolute dose at the normalization point between the MatriXX-measurement and PB-calculation become larger, up to a maximum value of 6% for the plan #9 (PTVs; SCF; 6 MV) as shown in Table 5.3 as well as Figure 5.28. It is expected the measured point dose is smaller than the PB-calculated dose for this small PTV or field because of the volume averaging effect of the finite-size (0.7-cm) resolution of the MatriXX detector. However, the influence of the volume averaging effect may not be significant for the conformal (SCF and DCA) techniques employed in commissioning the PB algorithm. It is because all fields involved in both SCF and DCA techniques are OPEN conformal fields without dynamic MLC leaf movement. Therefore, the dose distributions surrounding the isocenter should appear isotropic in shape, enhancing better agreement of dose defined inside the uniform dose region. This explains why the measured dose may also agree well with the calculated dose even for the small PTV size, as shown from the QA results for Plan #12 (PTVs; DCA; 6 MV FFF) in Table 5.3 and Figure 5.31. Of course, this circumstance can only stand as long as the open, conformal field size is comparable to or larger than the resolution of the detector (i.e., 7 mm for the MatriXX system used). For the PTV or field size smaller than the MatriXX detector resolution, higher-resolution dosimeters should be used, such as the mini- or micro-ionization chambers (with active volume of 0.125 cc and 0.03 cc) for point dose characterization and films for dose distributions measurements.

Gamma distributions based on the (3-%, 3-mm) criterion are excellent for all treatment plans even for the plans (#9 to #12) with the smallest PTV size, as shown in Table 5.3. Percentage of total pixels with passed gamma level (γ ≤ 1) is more than 97% for all cases. The good agreements between the measured dose distributions and calculated dose distributions in terms of point dose ratio and gamma map therefore confirm the accuracy of the PB algorithm for use in the conformal radiosurgical techniques (SCF and DCA) for this patient planning case.


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Figure 5.32 Frequency histograms summarizing QA results: (a) point dose difference and (b) pixels percentage of passing γ range for a total of 20 patient-specific plans using the PB dose algorithm to calculate in-patient dose distributions for conformal radiosurgical techniques (SCF and DCA) with 6 MV and 6 MV FFF beams from the TrueBeam 1 linac.

5.3 Summary

In summary, a total of 20 patient-specific radiosurgical treatment plans were used for verification of dose distributions calculated with the iPlan PB dose algorithm for two conformal radiosurgery techniques (SCF and DCA). Figure 5.32 summarizes the dosimetric verification results based on comparisons of measured and PB-calculated dose distributions for these 20 patient-specific plans. Tolerance levels for dose verification with the MatriXX system are ±4% for absolute dose difference and 95% of pixels with (3-%,3-mm) based γ less than 1. Out of a total 20 plans, one plan fails to meet the absolute dose agreement, while another fails to meet the gamma requirement. The rest 18 plans successfully meet both dose and gamma requirements. Passing rate for plan verification reaches 90%.

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Chapter 6 Conclusion

6‐1

6. Conclusion

The Pencil Beam (PB) photon dose calculation algorithm implemented in the BrainLab’s iPlan RT Dose treatment planning system was commissioned to perform in-patient 3D dose distributions for two radiosurgical techniques: (1) Conformal Static Fields (CSF) technique and (2) Dynamic Conformal Arcs (DCA) technique. The algorithm was investigated extensively by comparison of a series of dosimetric measurements with PB calculations for 6 MV and 6 MV Flattening Filter Free (FFF) beams of the Varian’s TrueBeam STx linac under three specific testing field conditions: (1) MLC-defined square fields; (2) MLC-defined irregularly shaped fields; (3) patient-specific CSF and DCA radiosurgical treatment plans.

Measurement of the percentage depth dose (PDD) and lateral beam profiles for MLC-defined square fields, ranging from (5×5) mm2 to (220×220) mm2, were compared with calculations of the same dosimetry parameters with the PB algorithm in a unit-density water phantom. Excellent agreement between measured and PB-calculated PDD distributions is achieved for small to large fields for both 6 MV and 6 MV FFF beams. Beam profiles in central uniform dose regions show good agreements between diode-measurements and PB-calculations. Larger discrepancies appear most frequently in the sharp dose-gradient penumbras, indicating the difficulty of the PB algorithm in modeling these profile regions. High dose discrepancies also occur in transmission tails of the beam profiles but are only limited to the largest (220×220) mm2 field, which seems to indicate a variation in diode’s response for this field. The use of a small-field ion chamber for comparison may confirm this speculation.

Dosimetry for MLC-defined irregular fields as well as plan-specific CSF and DCA radiosurgical treatment plans was verified using the 2D calibrated ion-chambers array I’mRT MatriXX detector. 31 treatment plans with a variety of MLC field configurations were used for both 6 MV and 6 MV FFF beams, totaling 62 treatment plans for verification. All treatment plans were mapped to the CT image of the MatriXX dosimeter for in-phantom dose distributions calculation with the PB algorithm. The PB-calculated dose plane intercepted on the ion-chambers array plane were compared with the dose distribution measured directly with the MatriXX detector. Dosimetric comparisons were evaluated using: (1) absolute dose difference at a point in uniform dose region and (2) gamma (γ) distributions. Tolerance levels for dosimetric verification with the MatriXX system are ±4% for absolute dose difference and >95% population of pixels with pass γ (≤ 1) values based on (3% dose, 3-mm DTA) criteria.

Chapter 6 Conclusion

6‐2

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Figure 6.1 Frequency histograms showing dosimetric verification results in terms of absolute dose difference and percentage of pixels in passing range (γ<1) for 62 treatment plans with MLC-defined conformal fields tested in the commissioning of the PB algorithm for the TrueBeam 1 STx linac.

Figure 6.1 summarizes verification results of absolute point dose difference and %-gamma distribution in histogram forms for all 62 tested treatment plans. Overall, 98% (61/62) of all plans with absolute dose difference within ±4% and 89% (55/62) of all plans with 95% or more population with passing gammas show good dosimetric agreement and thus confirm the accuracy of the PB algorithm for calculation of 3D radiosurgical dose distributions for 6 MV and 6 MV FFF beams of the TrueBeam 1 (STx) linac. In conclusion, the PB algorithm is validated for clinical treatment planning with the Conformal Beams and Dynamic Conformal Arcs techniques.

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4 E.B. Podgorsak and M.B. Podgorsak, “Stereotactic

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5 E.B. Podgorsak and M.B. Podgorsak, “Special procedures

and techniques in radiotherapy” in Radiation Oncology Physics: A Handbook for Teachers and Students, edited by E.B. Podgorsak (International Atomic Energy Agency, Vienna, 2005), pp. 505-548.

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Brainlab AG, Germany (2011). 7 D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, “A

technique for the quantitative evaluation of dose distributions,” Med. Phys. 25, 656-661 (1998).

8 P. R. Almond, P. J. Biggs, B. M. Coursey, W. F. Hanson,

M. S. Huq, R. Nath, and D. W. O. Rogers, “AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams,” Med. Phys. 26, 1847-1870 (1999).

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ID: P-07-002-510-001 08, IBA Dosimetry GmbH, Germany (2011).

Documents

iPlan PB commissioning report (2013)