Evaluation of Motorized Wedge for a new generation

Sudan Academy of Sciences

Evaluation of Motorized Wedge for a new generation telecobalt machine

By

Sumia Elshiekh Mohammed Elhassan

^a&jll (>»&jll &\ ^uu

Sudan Academy of Sciences Atomic Energy Council

Evaluation of Motorized Wedge for a new generation telecobalt machine

By

Sumia Elshiekh Mohammed Elhassan

A thesis submitted as partial fulfillment for the requirements of M.Sc. degree in Medical Physics

Supervisor: Mr. Mustafa M.Elhasan

JAN 2008

Sudan Academy of Sciences Atomic Energy Council

Examination Committee

Name Dr. Farouk Idris Habbani

\trfjMr. Muhanad M. Eltayeb

Mr. Mustafa M. Elhasan

Title External Examiner

Academic Affairs Representative

Supervisor

JAN 2008

Acknowledgements

This dissertation couJd not have been written without Mr. Mustafa M.Elhasan

who not only served as my supervisor but also encouraged and challenged me

throughout my academic program. He and others from Sudan academy of sciences

patiently guided me through the dissertation process, never accepting less than my best

efforts. I thank them all.

I am also greatly indebted to Prof. Nasr Eldin Elwali, Ms.Fawzia Elsadig,

Ms.Eshraga Elfaki, and Ms.Afrah Ahmed for their encouragement and support.

Last, but not least, I thank my family: my parents, for unconditional support and

encouragement to pursue my interests that my research should always be useful and

serve good purposes for all humankind.

I

DEDICATION

ii

ABSTRACT

A new model of the telecobalt unit, Theratron Equinox-100, (MDS Nordion,

Canada) equipped with upper and lower asymmetric jaws and a single 60 degree

motorized wedge (MW), have been evaluated. Motorized wedge was commissioned in

Pinnacle3 (Philips) 3D treatment planning system (TPS). The profiles and central axis

depth dose (CADD) were measured with Wellhofer. Blue water phantom for various

field sizes using 0.13 cc thimble ionization chamber (Scanditronix Wellhofer, Uppsala,

Sweden) and the data were commissioned in Pinnacle3.The profiles for wedge beam

were measured for 5X5, 10X10 and 15X15 cm2 field sizes at 5 and 10 cm depths

measured with 2D Array (two dimensional detector array with 729 vented ionization

chambers with a size of 5X5X5 mm ) PTW, Germany.and compared with calculated

profiles . A homogenous phantom was generated in Pinnacle .The dose was calculated

in this phantom at 10 cm depth for field sizes of 5X5, 10X10 and 15X15 cm2 for

particular motorized wedge angle ( MW) 15, 30, 45 and 60 degree and the respective

open and MW beam weight, the absolute dose of 2 Gy was calculated by TPS, Pinnacle

using collapsed cone convolution (cc convolution) algorithm with a grid size of 4 mm,

and compared with measured dose in a water phantom at 10 cm depth with a 0.6 cc

thimble ion chamber FC-65-G and DOSE1 electrometer for field sizes of 5X5, 10X10

and 15X15 cm2 using IAEA dosimetry protocol TRS-398.The variation of measured

and calculated doses at 10cm depth were within acceptable limit.

The motorized wedge was successfully commissioned in Pinnacle .

Ill

IV

CONTENTS

Acknowledgements I

Dedication II

Abstract Ill

Abstract in Arabic IV

Contents V

List of tables VIII

List of figures IX

Abbreviations XI

CHAPTER 1

Introduction

1.1 Preface 1

1.2 The aim of the study 2

1.3 Gamma Ray Beams and Units 2

1.3.1 Basic Properties of Gamma Rays 2

1.3.2 Teletherapy machines 2

1.3.3 Teletherapy sources and encapsulation 3

1.3.4 Duration of source use and replacement 3

1.3.5 Physical properties of cobalt-60 3

1.3.6 Teletherapy source housing 4

1.3.7 Dose delivery with teletherapy machines 4

1.4 Cobalt-60 Teletherapy Units versus Linacs 4

1.5 Design of modern Co-60 Devices 5

1.5.1Design of Motorized wedge 5

1.6 Beam characteristics of motorized wedge 8

1.6.1 Isodose curves 8

1.6.2 Wedge filters 8

1.7 Clinical application of Motorized wedge 10

1.8 Dosimetric Measurements 11

CHAPTER 2

Absolute and Relative Dosimetry

2.1Clinical consideration for photon beam 12

2.1.1 Central axis wedge transmission factors 12

V

2.1.2 Transverse beam profiles/off-axis energy changes 13

2.1.3 Off-Axis Ratio and beam profiles 13

2.2.5 Beam flatness 15

2.2.6 Beam symmetry 16

2.2.7 Central axis depth doses in water: source to surface distance set -up 16

2.8 Percentage depth dose 16

2.9 Determination of absorbed dose under reference conditions 18

2.10 Absorbed dose at z max 19

2.11 Values for kQ, Q 0 19

2.6 Radiation Dose Calculation 19

2.6.1 Convolution / superposition method 20

2.6.1.1 Terma 20

2.6.1.3 Energy Deposition Kernels 20

2.6.1.4 The convolution process 21

2.6.2 Collapsed Cone Convolution 21

2.6.2.1 Algorithm overview 22

CHAPTER 3

Materials and Methods

3.1 External beam therapy Equipment 24

3.2 Water phantoms 24

3.2.1 The blue phantom 24

3.2.1.1 OmniPro-Accept system 25

3.2.2 WP 34 water phantom 25

3.3 Detector system 26

3.3.1 CC 13 ionization chamber '. 27

3.3.2 FC-65-G Farmer chamber 27

3.3.3 2D-ARRAY 28

3.4 Therapy electrometer 28

3.4.1 DOSE 1 28

3.4.2 PTW UNIDOS 29

3.5 Dosimetric Measurements 29

CHAPTER 4

Results and Discussion 32

VI

4.2 Discussion 49

CHAPTER 5

Conclusion 50

References 51

Appendix 54

VII

LIST OF TABLES

Table: 4.1 Motorized 60° wedge factor for various field sizes at 10 cm depth.

Table: 4.2 Weight factors for motorized wedge 15°, 30°, 45° and 60° for clinical

Implementation.

Table: 4.3 Absolute dose verification in machine for motorized wedge 15 °, 30°, 45°

and 60° at 10cm depth for various field sizes.

VIII

LIST OF FIGURES

Figure: 1.1 Isodose curves for a wedged 6 MV photon beam. The isodoses have

been normalized to zmax with the wedge in place.

Figure: 2.1 An example of typical dose profiles measured at various depths in water

for two field sizes (10 x 10 and 30 x 30 cm2) and a 10 MV X ray beam.

Figure: 2.2 Geometry for PDD measurement and definition. Point Q is an arbitrary

point on the beam central axis at depth z, point P is the point at zmax on

the beam central axis. The field size A is defined on the surface of the

phantom.

Figure: 2.3 PDDs for radiotherapy beams are usually tabulated for square fields

However, the majority of fields used in radiotherapy are rectangular or

irregularly shaped. The concept of equivalent squares is used to

determine the square field that will be equivalent to the given rectangular or

irregular field

Figure: 2.4 The collapsed cone.

Figure: 3.1 The blue water phantom

Figure: 3.2 WP34 Water phantoms

Figure: 3.3 Compact Chamber CC13

Figure: 3.4 FC-65-P, Farmer chamber

Figure: 4.1(a) In-plane beam profiles at 0.75, 5, 10 and 20 cm depths for

symmetric fields for (a) 5x5 (b) 10x10 and (c) 20x20 cm field size measured in

the blue water phantom

Figure: 4.1(b) Cross-plane beam profiles at 0.75, 5, 10 and 20 cm depths for

symmetric and fields for (a) 5x5 (b) 10x10 and (c) 20x20 cm field sizes

measured in the blue water phantom.

Figure: 4.2 The CADD for symmetry field sizes 5x5, 10x10 and 15x15cm2

Figure: 4.3 The CADD for MW taken by WP for 60° at various field sizes.

Figure: 4.4(a) In-plane beam profiles at 0.65, 5, 10, 20 and 25 cm depths for wedged

field 5x5 cm

Figure: 4.4(b) Cross-plane beam profiles at 0.65, 5, 10, 20 and 25 cm depths for

wedged field 5x5cm

IX


field 10x10 cm2


wedged field 10x10 cm2


field 15x15 cm2


wedged field 15x15 cm2

Figure: 4.7 In line beam profiles at 5 and 10 cm depths for wedged field sizes for (a)

5x5 (b) 10x10 and (c) 15x15 cm2 field sizes measured by2D-ARRAYwith

build up material, and calculated by Pinnacle .

Figure: 4.8 Cross beam profiles at 5 and 10 cm depths for wedged fields for

(a) 5x5 (b) 10x10 and (c) 15x15 cm2 field sizes measured with 2D-ARRAY with

build up material, and calculated by Pinnacle3.

X

ABBREVIATIONS

RICK Radiation and Isotopes Center of Khartoum

TCU Telecobalt Unit

60Co COBALT-60

MW Motorized Wedge

UW Universal Wedge

TPS Treatment Planning System

3-D TPS 3 Dimensions Treatment Planning System

CADD Central Axis Depth Dose

IAKR Air Kerma Rate Constant

SAD Source Axis Distance

SSD Source Surface Distance

KeV Kilo electron volt

MeV Mega electron volt

MLC Multi Leave Collimator

MU Monitor Unit

WF Wedge Factor

PDD Percentage Depth Dose

TPR Tissue Phantom Ratio

TMR Tissue Maximum Ratio

WP Water Phantom

2D-ARRAY 2-Dimention ARRAY

TERMA Total Energy Released per unit Mass

IAEA International Atomic Energy Agency

OAR Off Axis Ratio

AAPM American Association of Physicists in Medicine

XI

CHAPTER 1

INTRODUCTION

1.1 Preface

Radiation and Isotopes Center of Khartoum (RICK) is one of the two

radiotherapy centers in Sudan that continue to expand. Recently a new model of a

fully computerized isocentric teletherapy cobalt-60 unit, Equinox-100 model was

installed.

Equinox-100 (MDS Nordion, Canada) incorporates unique features, such as:

o availability of smaller field size (0.5 cm x 0.5 cm),

o fully software controlled operation which also keeps the record of

vital operational history of the unit,

o software controlled collimator, couch and gantry movement and

calibration facilities,

o availability of upper and lower asymmetric jaws,

o availability of motorized wedge (MW), a single physical wedge (60°)

which can generate desired angles (from 0° up to 60°) with the

combination of open and wedged beam time,

o ANAVA ™ treatment positioning table with carbon fiber top,

o collision detection system, and

o simulate feature for arc treatment [36].

The new model was commissioned in Pinnacle , the three-dimensional

computerized Treatment Planning System (TPS) for the purpose of the dose

calculation after the therapy machine is accepted and before it can be placed in

clinical service. An extensive set of radiation measurements was acquired to

characterize the machine performance and to confirm the beam characteristics against

the machine manufacturer's specification.

Earlier models of the teletherapy cobalt-60 unit (TCU) have large been

routinely used physical wedge, but more recently MW has been introduced. MW is a

single physical wedge (60°), which could generate desired angles (0 to 60 °) with the

combination of open and wedged beam [10, 20]. The isodose curves can be tilted to

get the desired wedge angle with the help of 3 dimensional treatment planning

system(3D-TPS), measured central axis depth doses (CADD), wedge profile for 60 °

MW and adjusted beam weight[4]. Before clinical introduction of MW in the

1

department, it was required to verify the performance of the treatment planning

process with MW.

1.2 The Aim of the Study

• To measure central axis depth doses (CADD) for square field sizes (3*3cm2

to 15*15cm ) and a maximum wedged rectangular field size (18.5*20cm ).

• To measure the off axis cross plane profile and in plane profiles for same field

sizes at various depths.

• To compare measured beam profiles with the TPS profiles.

• To measure MW 60° factor for various field sizes.

• Absolute dose verification, the dose measured for a field size of 10* 10cm2

with water phantom at 10cm depth with chamber was compared with TPS

calculated dose.

1.3 GAMMA RAY BEAMS AND GAMMA RAY UNITS

1.3.1 Basic properties of gamma rays

For use in external beam radiotherapy, y-rays are obtained from specially

designed and built sources that contain a suitable, artificially produced radioactive

material.

t The parent source material undergoes f>- decay, resulting in excited daughter nuclei

that attain ground state through emission of y- rays.

• The important characteristics of radioisotopes in external beam radiotherapy are:

— High y ray energy;

— High specific activity;

— Relatively long half-life;

— Large specific air kerma rate constant TAKR. 60Co is the most widely used source, since it offers the most practical approach to

external beam radiotherapy, considering the energy of emitted photons, half-life,

specific activity and means of production [29] .

1.3.2Teletherapy machines

Treatment machines incorporating y-ray sources for use in external beam

radiotherapy are called teletherapy machines. They are most often mounted

isocentrically, allowing the beam to rotate about the patient at a fixed SAD. Modern

teletherapy machines have SADs of 80 or 100 cm.The main components of a

teletherapy machine are: a radioactive source; source housing, including beam

2

collimator and source movement mechanism; a gantry and stand in isocentric

machines or a housing support assembly in stand-alone machines; a patient support

assembly; and a machine console [29].

1.3.3 Teletherapy sources and encapsulation

The most widely used teletherapy source uses 60Co radionuclide contained

inside a cylindrical stainless steel capsule and sealed by welding. A double welded

seal is used to prevent any leakage of the radioactive material.

• To facilitate interchange of sources from one teletherapy machine to another and

from one isotope production facility to another, standard source capsules have been

developed.

• The typical diameter of the cylindrical teletherapy source is between land 2 cm; the

height of the cylinder is about 2.5 cm. The smaller the source diameter, the smaller is

its physical penumbra and the more expensive is the source. Often a diameter of 1.5

cm is chosen as a compromise between the cost and penumbra.

• Typical source activities are of the order of 5000 -10.000 Ci (185-370 TBq) and

provide a typical dose rate at 80 cm from the teletherapy source of the order of 100—

200 cGy/min. Often the output of a teletherapy machine is stated in Rmm (roentgens

per minute at 1 m) as a rough guide for the source strength [29].

1.3.4 Duration of source used and replacement

• Teletherapy sources are usually replaced within one half-life after they are installed;

however, financial considerations often result in longer source usage.

• The Co radionuclide in a teletherapy source decays with a half-life of 5.26 years

into 60Ni with the emission of electrons (P-particles) with a maximum energy of 320

keV and two y-rays with energies of 1.17 MeV and 1.33 MeV. The emitted y-rays

constitute the therapy beam; the electrons are absorbed in the cobalt source or the

source capsule, where they produce relatively low energy and essentially negligible

bremsstrahlung x -rays and characteristic x-rays [29].

1.3.5 Physical properties of cobalt-60

Cobalt is a dense (mass density 8900 kg/m ) silver white metal with a high

melting point (1500 K); it is non - radioactive ore is rarely found in naturally

occurring rock formation. The chemical symbol Co, (atomic number 27, atomic

weight 58.933 amu) is named for a globin, Kobalt, known in Germanic legend for

steeling silver [36].

3

When cobalt-59 is bombarded with neutrons in a nuclear reactor, the resulting product

is radioactive cobalt-60, with a half-life of 5.261 years and exposure rate constant of

UlRmVCi"1 [36].

1.3.6 Teletherapy source housing

The housing for the teletherapy source is called the source head, and consists of

a steel shell with lead for shielding purposes and a mechanism for bringing the source

in front of the collimator opening to produce the clinical y-ray beam.

Currently two methods are in use for moving the teletherapy source from the

beam off into the beam on position and back:

(i) A source on a sliding drawer and.

(ii) A source on a rotating cylinder.

Both methods incorporate a safety feature in which the beam is terminated

automatically in the event of a power failure or emergency.

• When the source is in the beam off position, a light source appears in the beam on

position above the collimator opening, allowing an optical visualization of the

radiation field, as defined by the machine collimators and any special shielding

blocks.

• Some radiation will escape from the unit even when the source is in the beam off

position. The head leakage typically amounts to less than 1 mR/h (0.01 mSv/h) at 1 m

from the source. International regulations require that the average leakage of a

teletherapy machine head be less than 2 mR/h (0.02 mSv/h) at 1 m from the source

[29].

1.3.7 Dose delivery with teletherapy machines

The prescribed target dose is delivered with the help of two treatment timers:

primary and secondary. The primary timer actually controls the treatment time, the

secondary timer serves as a backup timer in case of the primary timer's failure.

The set treatment time must incorporate the shutter error, which accounts for the

travel time of the source from the beam off position towards the beam on position at

the start of irradiation and for the reverse travel at the end of irradiation [29].

1.4 Cobalt-60 Teletherapy Units versus Linacs

After the inception of radiotherapy soon after the discovery of x-rays by

Roentgen in 1895, the technology of radiation production was first aimed towards

4

ever-higher photon energies and intensities and more recently towards

computerization and intensity modulated beam delivery. During the first 50 years of

radiotherapy, technological progress was relatively slow and mainly based on x-ray

tubes, van de Graaff generators and betatrons.

The first truly practical megavoltage therapy machine was the 60Co teletherapy

machine developed in Canada in the 1950s. The invention of 60Co teletherapy

provided a tremendous boost in the quest for higher photon energies and placed the

Co unit in the forefront of radiotherapy for a number of years, mainly because it

incorporated a radioactive source that is characterized by features extremely useful for

radiotherapy.

The important features of Co teletherapy machines can be summarized as follows:

• Relatively high-energy y-ray emission;

• Relatively long half-life;

• Relatively high specific activity;

• Relatively simple means of production.

Linacs were developed concurrently by two groups: W.W. Hansen's group at

Stanford University in the USA and D.D. Fry's group at the Telecommunications

Research Establishment in the UK. Both groups were interested in linacs for research

purposes and profited heavily from the microwave radar technology developed during

World War II, using 3000 MHz as the design frequency.

The potential for the use of linacs in radiotherapy became apparent in the 1950s, and

the first clinical linac was installed in the 1950s at the Hammersmith Hospital in

London. During subsequent years, the linac eclipsed the cobalt unit and became the

most widely used radiation source in modern radiotherapy, with several thousand

units in clinical practice around the world today. In contrast to a 6 Co unit, which

provides essentially only one y energy of 1.25 MeV, a linac can provide either

megavoltage electron or x- ray therapy with a wide range of energies.

In comparison with Co machines, linacs have become very complex in design:

— In part because of the multimodality capabilities that have evolved and are

available on most modern machines.

— In part because of an increased use of computer logic and microprocessors in the

control systems of these machines.

— In part because of added features, such as high dose rate modes, multileaf

collimation, electron arc therapy and the dynamic treatment option, which is

5

characterized by a controlled motion on the collimators (dynamic wedge), MLC

leaves, gantry or table while the beam is turned on.

Despite the clear technological and practical advantages of linacs over 60Co machines,

the latter still occupy an important place in the radiotherapy armamentarium, mainly

because of the considerably lower capital, installation and maintenance costs of 60Co

machines compared with linacs. In the developing world, 60Co machines, because of

their relatively lower costs, simplicity of design and ease of operation, are likely to

play an important role in cancer therapy for the foreseeable future.

Many modern features of linacs, such as MLCs, dynamic wedges and dynamic

operation, could be installed on modern 60Co machines to allow, at a lower cost, a

similar sophistication in treatment as linacs. It is unfortunate that manufacturers of 60Co units are very slow in reacting to new technological developments in

radiotherapy, conceding pre-eminence to linac manufacturers even in areas where it

would be much easier and more practical to run 60Co machines than linacs [29].

Employment of the wedge fraction method results in the delivery of small

numbers of monitor units to the beam's central axis; therefore, wedge profile stability

and delivered dose with low numbers of monitor units was also investigated. The

wedge fraction was proven to be the most efficient method when the time taken for

both planning and treatment delivery were taken into consideration [8].

1.5 Design of Modern Co-60 Devices

1.5.1 Design of M W

The MW provides a 60° nominal wedge angle. The maximum field size covered

by the MW is 18.5 x 20 cm . Alternately, it can be moved in and out of the radiation

fields. It consists of a 60-degree wedge mounted in the asymmetric collimator below

the lead leaves and above the tungsten trimmer bars. For convenience of access, the

MW and UW filters are inserted transversely. When the collimator is at zero angle

position, the MW is oriented with the thin edge directed to the left when facing the

gantry and the UW is oriented with the thin edge directed to the right when facing the

gantry [19].

Basically the concept of MW is not new; however, it is a new concept in cobalt units.

On the graphical user interface control console of the Equinox-80, the treatment time

is set for total open beam time and the motorized wedge beam time. In Equniox-100

machine, the desired MW angle is achieved as follows:

6

• The source is exposed for the open beam time

• The source is retracted

• The MW goes to the in position

• The source moves to the fully exposed position

• The source remains in the fully exposed position for the wedged time

• At the end of the wedged time, the source is retracted

• The MW goes to the out position and the treatment is complete.

In pinnacle , motorized wedge with 60° was commissioned with the measured CADD

and the profile data. The dose distribution of fields containing a MW is calculated on

the basis of the weighted dose percentages for the wedged and open parts of the field.

The pinnacle then displays the monitor unit (MU) values and reference dose for both

the open and wedged part of a field containing a MW.

The weight factor is a parameter used for MW to indicate the dose percentage of the

wedged and open part of a field containing a wedge. The weight factors range from 0

to 1.0 and the factor indicates the following:

0 - fully open field,

1 - fully wedged field

other values - partly wedged field.

For example, a weight factor of 0.5 for a 60° MW is approximately equivalent to a

field containing a 30° UW or a standard wedge. The dose box in pinnacle shows the

coefficient used to indicate the MUs of the wedged and open part of the field. The

weight factor box shows the weight of the wedged part of the field. The coefficient

indicating the MUs of the wedged and open part of the field is calculated as follows:

D = MUwedge / M U i o t a l 1 - 1

Where, M U T o t a l = M U Open + M U Wedge

By using the appropriate beam weights, it is possible to generate isodose distributions,

which exhibit effective wedge angles ranging from 0 to 60° [19]. A wedged isodose

distribution of any wedge angle up to a nominal 60° can be created through the

appropriate combination of a wedged and an open segment [26].The universal wedge

equation may be used to determine the beam weights. However, the beam weight

7

factor may differ from TPS to TPS based on the algorithm and the calculation module

used. This universal wedge equation relates the beam weight of the wedged field to

the effective wedge angle produced by combining the wedged field to the non-wedged

field and is expressed as

Tan (theta) = B .tan (theta w) 1-2

where, B is the normalized weight imposed on wedged field, theta w is the maximum

wedge angle of the wedge filter and theta is the effective wedge angle. Thus the

equation (1-2) can also be written as:

Wedge beam weight = {tan (desired angle) / tan (motorized wedge angle)} 1 - 3

In pinnacle3, the 3D dose distribution in a homogeneous phantom is calculated using

the selected calculation model. The dose distributions are calculated for the field sizes

selected from among the original set of CADD and profile measurements.

1.6 Beam characteristics of motorized wedges

1.6.1 Isodose curves

Isodose curves are lines that join points of equal dose. They offer a planar

representation of the dose distribution and easily show the behavior of one beam or a

combination of beams with different shielding, wedges, bolus, etc.

Isodose curves can be measured in water directly or can be calculated from PDD and

beam profile data. A set of isodose curves is valid for a given treatment machine,

beam energy, SSD and field size.

While isodose curves can be made to display the actual dose in grays, it is more

common to present them normalized to 100% at a fixed point. Two such common

point normalizations are as follows:

• Normalization to 100% at the depth of dose maximum on the central axis;

• Normalization at the isocentre.

1.6.2 Wedge filters

Special filters of absorbing blocks are placed in the path of a beam to modify its

isodose distribution. The most commonly used beam modifying device is a wedge

filter [11]. Three types of wedge filter are currently in use: manual, motorized and

dynamic.

• A physical wedge is an angled piece of lead or steel that is placed in the

8

beam to produce a gradient in radiation intensity. Manual intervention is

required to place physical wedges on the treatment unit's collimator

assembly.

• A motorized wedge is a similar device, a physical wedge integrated into

the head of the unit and controlled remotely.

• A dynamic wedge produces the same wedged intensity gradient by having

one jaw closed gradually while the beam is on.

The following applies to all wedges:

• The wedge angle is commonly defined as the angle between the 50% isodose line

and the perpendicular to the beam central axis. Wedge angles in the range from 10° to

60° are commonly available.

It is feasible to design wedges that tilt the isodose curves for treatment fields sizes

ranging from about 5*5cm2 to about 20cm (along the wedge gradient)*40cm (along

the uniform beam profile) through angles of up to 60°.

Wedges have proven to be a valuable beam modification device for central beam

radiotherapy and are standard feature with all radiotherapy equipment [16].

A typical isodose distribution for a wedged beam is shown in Figure 1.1.

9

Figure. 1.1 Isodose curves for a wedged 6 MV photon beam. The isodoses have been normalized to zmax with the wedge in place.

1.7 Clinical application of motorized wedge

There are two main uses of wedges:

• Wedges can be used to compensate for a sloping surface, as, for example, in

nasopharyngeal treatments, in which wedges are used to compensate for

decreased thickness interiorly [29].

• A wedge pair of beams is also useful in the treatment of relatively low lying

lesions, in which two beams are placed at an angle (of less than 180°) called

the hinge angle). The optimal wedge angle (assuming a flat patient surface)

may be estimated from: 90° - 1/2 (hinge angle). The wedge factor (WF) is

defined as the ratio of dose at a specified depth (usually zmax) on the central

axis with the wedge in the beam to the dose under the same conditions without

the wedge. This factor is used in MU calculations to compensate for the

reduction in beam transmission produced by the wedge. The WF depends on

the depth and field size [29].

' S ' | l '

10

1.8 Dosimetric measurements

Measurements for motorized 60 degree wedge are carried out using blue water

phantom utilizing electrometer and thimble ionization chamber. With the motorized

wedge setting and a source to surface distance (SSD) of 90 cm, central axis depth

doses (CADD) for various square field sizes of 3x3, 5x5, 7x7, 8x8, 10x10,

12x12,15x15,18x18cm and the maximum wedged rectangular field size

(18.5x20)cm are performed.

Beam profiles in the in-plane as well as in cross-plane directions are measured for the

same field sizes at different depths of 0.65, 5, 10, 15, 20, and 25 cm.

Measured CADD and profiles are compared with the data computed in the TPS. The

60 degree MW factor is measured [19].

11

CHAPTER 2

ABSOLUTE AND RELATIVE DOSIMETRY

A radiation dosimeter is defined as any device that is capable of providing a

reading M that is a measure of the dose D deposited in the dosimeter's sensitive

volume V by ionizing radiation.

—A dosimeter that produces a signal from which the dose in its sensitive volume can

be determined without requiring calibration in a known field of radiation is referred to

as an absolute dosimeter;

—Dosimeters requiring calibration in a known radiation field are called relative

dosimeters [29].

2.1Clinical consideration for photon beam

2.1.1 Central axis wedge transmission factors

These are defined as the ratio of doses with and without the wedge, at a point in

phantom along the central axis of the beam. Such a factor should be measured in

phantom at a suitable depth beyond the depth of maximum dose [11]. Wedges are

used to shape the dose distribution of radiation treatment fields. The central axis

wedge transmission factor is the ratio of the dose at a specified depth on the central

axis of a specified field size with the wedge in the beam to the dose for the same

conditions without the wedge in the beam [29].

Central axis wedge transmission factors determined for one field size at one

depth are frequently used to calculate beam-on times or MU settings for all wedged

fields and depths. However, central axis wedge transmission factors may be a function

of both depth and field size. The field size variation may depend not only on the width

of the field along the gradient of the wedge but also on the length of the field. In other

words, the central axis wedge transmission factor for a given wedge for a 10 x 10 cm

field may differ from the central axis wedge transmission factor for a 10 x 20 cm2

field even when the 10 cm is along the wedge gradient in both cases. These

dependencies require measuring central axis PDDs with the wedge in the beam for a

range of field sizes. The dose with the wedge in the beam can then be related to the

calibrated dose rate by measuring the central axis wedge transmission factor at one

depth for each field size. To measure the central axis wedge transmission factor for a

given field size at one depth the ionization chamber should be placed on the central

12

axis of the beam with its axis-aligned parallel to a constant thickness of the wedge.

Measurements should be performed with the wedge in its original position and with

the wedge rotated through 180°. This set of measurements verifies that the wedge and

the ionization chamber are correctly positioned. The wedge position may be rotated

through 180° by rotating the collimator or by rotating the wedge itself. Rotation of the

wedge itself reveals whether the side rails are symmetrically positioned about the

collimator axis of rotation. Rotation of the collimator verifies that the ionization

chamber is positioned on the collimator axis of rotation. The measured values should

be the same for the two wedge orientations. If the values differ by more than 5% for a

60° wedge or 2% for a 30° wedge, then the wedge or ionization chamber is not

positioned correctly and the situation should be corrected. Otherwise it is usually

adequate to take the average value of the two wedge orientations as the correct value

[29].

2.1.2 Transverse beam profiles/off-axis energy changes

The distribution of dose at any point within a radiation beam is required to be

known for treatment planning. Transverse beam profiles are measured to characterize

the dose at points off the central axis. Frequently off-axis data are normalized to the

dose on the central axis at the same depth. These data are referred to as OARs, which

are combined with central axis data to generate isodose curves.

The number of profiles and the depths at which these profiles are measured will

depend on the requirements of the TPS. Some systems require these profiles at a few

equally spaced depths, others require several profiles at specified depths and some

require only one off-axis profile for the largest field size measured in air with a

buildup cap. Transverse beam profiles should be measured in addition to those on

which the beam model was determined to verify the accuracy of the TPS algorithms.

Of course, these profiles should be measured for both open and wedged fields. The

profiles of the wedged field can then be combined with the central axis PDD values

for wedged fields to generate wedged isodose curves. Any change in wedge factor

(WF) with depth is then included in the isodose curves [29].

2.1.3 Off-Axis Ratio and beam profiles

Dose distributions along the beam central axis give only part of the information

required for an accurate dose description inside the patient. Dose distributions in 2-D

13

and 3-D are determined with central axis data in conjunction with off-axis dose

profiles [29]. In the simplest form, the off-axis data are given with beam profiles

measured perpendicularly to the beam central axis at a given depth in a phantom. The

depths of measurement are typically at zmax and 10 cm for verification of compliance

with machine specifications, in addition to other depths required by the particular

treatment planning system (TPS) used in the department. An example of typical dose

profiles measured at various depths in water for two field sizes (10 x 10 and 30 x 30

cm2) and a 10 MV x-ray beam is shown in Fig.2.1. Combining a central axis dose

distribution with off-axis data results in a volume dose matrix that provides 2-D and

3-D information on the dose distribution. The off-axis ratio (OAR) is usually defined

as the ratio of dose at an off-axis point to the dose on the central beam axis at the

same depth in a phantom. Megavoltage x-ray beam profiles consist of three distinct

regions: central, penumbra and umbra.

-30 -20 -10 0 10 20 30

Distance from central axis (cm)

Figure. 2.1 An example of typical dose profiles measured at various depths in water

for two field sizes (10 x 10 and 30 x 30 cm2) and a 10 MV x-ray beam.

14

Standard linac specifications generally require that F be less than 3% when measured

in a water phantom at a depth of 10 cm and an SSD of 100 cm for the largest field size

available (usually 40 x 40 cm2).

Compliance with the flatness specifications at a depth of 10 cm in water results in

'over-flattening' at zmax, which manifests itself in the form of 'horns' in the profile,

and in 'under-flattening', which progressively worsens as the depth z increases from

10 cm to larger depths beyond 10 cm, as evident from the profiles for the 30 x 30 cm2

field in Fig.2.1.The typical limitation on beam horns in the zmax profile is 5% for a 40

x 40 cm2 field at SSD = 100 cm. The over flattening and under-flattening of the beam

profiles is caused by the lower beam effective energies in off-axis directions

compared with those in the central axis direction [29].

2.3 Beam symmetry

The beam symmetry S is usually determined at zmax, which represents the most

sensitive depth for assessment of this beam uniformity parameter. A typical symmetry

specification is that any two dose points on a beam profile, equidistant from the

central axis point, are within 2% of each other. Alternately, areas under the zmax beam

profile on each side (left and right) of the central axis extending to the 50% dose level

(normalized to 100% at the central axis point) are determined and S is then calculated

from:

S = 100 * {area ieft- arearjght / area |eft + area right} 2 -2

The areas under the zmax profiles can often be determined using an automatic

option on the water tank-scanning device (3-D isodose plotter).

Alternatively, using a planimeter or even counting squares on graph paper with a hard

copy of the profile are practical options [29].

2.4 Central axis depth doses in water source to surface distance set -up

2.4.1 Percentage depth dose

Central axis dose distributions inside the patient or phantom are usually

normalized to Dmax = 100% at the depth of dose maximum zmax and then

referred to as the PDD distributions. The PDD is thus defined as follows:

PDD(z,A, f,hv) = 100—^-= 100-^- 2 "3

Where DQ, D'Q are the dose and dose rate, respectively, at point Q at depth z on the

central axis of the phantom and DP, D'P are the dose and dose rate at point P at zmax on

16

the central axis of the phantom. The geometry for PDD definition is shown in Fig.2.2.

Point Q is an arbitrary point at depth z on the beam central axis; point P represents the

specific dose reference point at z = zmax on the beam central axis. The PDD depends on

four parameters: depth in a phantom z, field size A, SSD (often designated with j) and

photon beam energy hx>. The PDD ranges in value from 0 at z = oo to 100 at z = zmax.

The dose at point Q contains two components: primary and scatter.

• The primary component may be expressed as [29]:

DPRI

PDDm = 1 0 0 ^ — = 100 Dw

\

f + z™,

f + 'z

V VeffK2 zmax ) 2-4

eff where /u is the effective linear attenuation coefficient for the primary

eff 60 r beam in the phantom material (ju for a Co beam in water is 0.0657 cm )

A A i i

Source

/ : \

i

i i

Q+

_ .

\ \

= SSD

1

r « V ..

Figure.2.2 Geometry for PDD measurement and definition. Point Q is an arbitrary

point on the beam central axis at depth z; point P is the point at zmax on the beam

central axis. The field size A is defined on the surface of the phantom.

• The scatter component reflects the relative contribution of the scattered

radiation to the dose at point Q. As shown in Fig.2.3. for constant A,fand hv the PDD

first increases from the surface to z = zmax and then decreases with a further increase in

z. The depth of dose maximum and the surface dose depend on the beam energy; the

larger the beam energy, the larger the depth of dose maximum and the lower the

surface dose.

—For constant z,/and hx> the PDD increases with increasing A because of

increased scatter contribution to points on the central axis.

17

—For constant z, A and hx> the PDD increases with increasing/because of

a decreasing effect of z on the inverse square factor, which governs the

primary component of the photon beam.

—For constant z, A and/the PDD beyond zmax increases with beam energy

because of a decrease in beam attenuation (i.e. because of an increase in

beam penetrating power).

An example of PDD distribution for 10 x 10 cm fields and various

depths is given in Fig.2.3 The size of the buildup region increases with beam energy

and the surface dose decreases with beam energy.

-C-A

D,™,* = 1 OO

Z f v » > ; Depth 03

Fig.2.3 PDD distribution for 10x10cm2 field and various depths.

The PDDs for radiotherapy beams are usually tabulated for square fields however, the

majority of fields used in radiotherapy are rectangular or irregularly shaped. The

concept of equivalent squares is used to determine the square field that will be

equivalent to the given rectangular or irregular field.

2.5 Determination of absorbed dose under reference conditions

The absorbed dose to water at the reference depth z ref in water, in a photon

beam of quality Q and in the absence of the chamber, is given by

D w, Q = MQ N DI Wf gk Q,Qo 2-5

where MQ is the reading of the dosimeter with the reference point of the chamber

positioned at zrej and corrected for the influence quantities temperature and pressure,

electrometer calibration, polarity effect and ion recombination. ND: „,, Q0 is the

calibration factor in terms of absorbed dose to water for the dosimeter at the reference

18

quality Q0, and kgtg0 is a chamber-specific factor which corrects for the difference

between the reference beam quality Q0 and the actual quality being used, Q [14] .

2.5.1 Absorbed dose at z max

However, clinical dosimetry calculations are often referenced to the depth of

dose maximum zmax (or at some other depth). To determine the absorbed dose at the

appropriate depth the user should, for a given beam, use the central axis percentage

depth-dose (PDD) data for SSD set-ups and tissue-phantom ratios (TPR) or tissue-

maximum ratios (TMR) for SAD set-ups [14].

2.5.2 Values for k Q, Q 0

When the reference quality Q0 is 60Co, k Q„ QO is denoted by k g and A^ w> o 0 is

denoted by NDi „, . These values have been adapted from the calculations of Andreo

[20] and can be used at the reference depths. A sleeve of PMMA 0.5 mm thick has

been used in the calculations for all the chambers which are not waterproof; for sleeve

thicknesses up to 1 mm the change in kg is not greater than about 0.1%. Values of kg

for non-tabulated qualities may be obtained by interpolation. It is emphasized that

calculated kg values cannot distinguish chamber-to-chamber variations within a given

chamber type and their use necessarily involves larger uncertainties than directly

measured values.

It should be noted that there is no value of Q that corresponds to 60Co where all the kg

values are equal to 1.000. While in principle there is a value of TPR20.10 that would

correspond to a pure 60Co spectrum, the response of a particular chamber in an

accelerator beam of the same TPR20J0 depends on its energy response over the whole

spectrum, and will not necessarily be the same as for Co. In addition there is

considerable disagreement in the literature as to what the TPR20,10 of a 60Co beam is

(0.568 for the beam at the NPL [86]; 0.572 in BJR 17 [87], BJR 25 [81] and at

ARPANSA [88]; 0.578 at NRC [89]; 0.579 in Johns and Cunningham's textbook [90],

etc.), so that a single reference value cannot be used [14].

2.6 Radiation dose calculation

There are three main methods of dose calculation in use today. One class of

algorithms is known as convolution, another as superposition, and the last as Monte

Carlo analysis.

Monte Carlo methods are the most accurate and also the most computationally

intensive methods available. They are not used in clinical practice due to the

19

enormous computational load they generate. Convolution and superposition methods

are slightly less accurate, but can typically be computed in much less time than Monte

Carlo methods.

2.6.1 Convolution / superposition method

Several variations of the convolution algorithm exist [1, 22, 6, 25], but all have two

essential components : one representing the energy imparted to the medium by

interactions of primary photons ( usually called the terma) and one representing the

energy deposited about a primary photon interaction site (the kernel).[1, 3,22,25].

2.6.1.1 Terma

Terma is an acronym for total energy released per unit mass and represents the energy

that is (a) imparted to secondary charged particles and (b) retained by the scattered

photon when primary photons interact in a unit mass. For each primary photon

interaction, the sum of these energies is, of course, equal to that of the primary photon

itself. The terma can thus be thought of as the energy lost out of the primary beam in a

unit mass. A fundamental definition of the terma at a point is the negative of the

divergence of the energy fluence per unit mass, as this is the energy carried into a unit

mass minus the energy carried out .If the energy fluence of primary photons at a point

r is T(r) then the terma is given by

T(r) = -{l/p(r)} • . ¥ ( r ) 2-6

Since the energy fluence has an exponential fall off with depth, evaluating the above

equation gives

T(r)={[j i /p](r)}¥(r) 2-7

where u / p is the mass attenuation coefficient

when performing a terma calculation, the polyenergetic nature of the beam must be

taken into account by incrementing the terma for each energy component. Hence, if

the energy fluence of primary photons in spectral component n at a point r is ¥„ (r),

the terma is given by

T(r') = ZN ^(r 'Hu/p] , , . 2-8 n=l

where [u / p] n is the mass attenuation coefficient for energy component n.

2.6.1.3 Energy deposition kernels

Kernel is the kinetic energy deposited about a primary photon interaction site. Energy

deposition kernels can be easily obtained using Monte Carlo simulation, where

photons are forced to interact at an appropriate point within an array, and the energy

20

subsequently deposited with in each voxel in the array is recorded. This is equal to the

total primary photon energy since energy imparted includes both that transferred to

charged particles and that required by the scattered photon. The recorded energy in

each voxel is that divided by the total energy imparted, giving the fractional energy

for each voxel [13, 12, 21]. If the total energy imparted by primary photons in the

chosen "interaction" voxel is Etot and the primary energy deposited in a voxel at a

vectorial displacement Ar is Ep (Ar) the corresponding primary kernel value is:

Hp(Ar) = Ep(Ar)/E tot 2-9

This expression can also be applied to the scattered kernel, where the subscription p is

replaced with s.

2.6.1.4 The convolution process

In a convolution calculation, the dose at each point in the medium is calculated by

considering primary photon interactions throughout the irradiated volume. This is

done by summing the dose contributions from each irradiated volume element.

D(r) = J r/ T (r7) [H p (r-i7) + H s (r- / ) ] d3 r7 2-10

In practice the convolution can proceed by either (a) integrating the dose deposition

at successive dose deposition points due to terma throughout the irradiated medium or

(b) calculating the dose throughout the medium due to terma at successive interaction

points [24, 2].

The convolution of terma with a kernel to obtain the dose is written as

D = T®K 2-11

Where T is the Terma, K is the kernel and <8> is the convolution operator.

2.6.2 Collapsed cone convolution

Collapsed cone convolution method used uses the polyenergetic terma and kernel,

where the kernel is represented analytically and combines primary and scatter

contributions. The function used to characterize the kernel is of the form

A e~a"'' + B e~v

H(r,8) = ^ + / g g

r

where there are a finite number of polar angles, 9, with respect to the primary

beam along which the function is defined. The interaction site can be considered

to be the apex of asset of radially directed lines spreading out in three

dimensions, where each line has a polar angle of 6 and azimuthal angle of O.

Each of these lines is further considered to be the axis of a cone, where the solid

angle covered by the complete set of cones is 47t. The kernel function along each

21

line is actually the energy deposited within the entire cone at radius r, collapsed

onto the line, hence the name of the method. This means that when the

interaction site is at a point in a cartesian voxel array, fractional energy is

deposited in voxels within the cone. This is not really a problem, however, as

those voxels that do not receive energy are compensated by energy deposited

due to adjacent lines radiating out from a different interaction site. Further more,

close to the interaction site (where most fractional energy is deposited) a

cartesian voxel will probably enclose more than one cone, so no accuracy is lost

[23].

2.6.2.1 Algorithm overview

The collapsed cone convolution algorithm was first proposed by Ahnesj"o [4].

The essence of the collapsed cone approximation can be seen in Fig .2.4 (taken from

[4])-

The "Collapsed Cone" approximation is made as follows:

All energy released into coaxial cones of solid angle D. mn from volume elements on

the axis is rectilinearly transported, attenuated and deposited in elements on that axis

[22,15, and 9]. This allows for accurate computation of the dose deposited to all

voxels while performing many less computations. The dose can now be calculated

using M*N3 calculations, where M is the number of cones (typically between 48 and

384) andN is the number of cartesian voxels along one side (i.e. there are N3 voxels).

As mentioned earlier, the kernel used in this implementation is the exponentially

calculated kernel [5]. The exponentially calculated kernel is composed of two

different components. One component calculates the primary energy (which has a

rather sharp falloff as distance from the interaction point increases) and the other

component calculates the scatter energy (which does not have near as sharp a drop-off

as the primary energy) [18].

22

Figure. 2.4 (a) In the collapsed cone approximation, all energy released from

primary photons at elements on a cone axis of direction (9m, ®n)and flowing

in coaxial cones of solid angle Q. mn, is rectilinearly transported and deposited

on the axis. In order to transport all energy released, several cone axes must

be used, such that X m,n = 4 * 7t, and parallel axes of each direction, must

cover the irradiated volume. In the figure, only one axis is shown. An infinite

number of axis, where Q. mn = 0, yields the continuous case of rectilinear motion, (b)

Close to an interaction point the corresponding cone covers only a fraction of a

cartesian voxel, as indicated by the voxels A and A'. Further away, also off axis

voxels are covered by the cone, as indicated by B and B' . In the collapsed cone

approximation, the energy that should have been deposited in voxel B' from

interactions at the vertex of the lower cone is deposited in voxel B and vice versa. The

relative accuracy of the calculated energy deposition therefore decreases with the

distance from the scattering point. The error is small since the magnitude of

polyenergetic point spread functions decreases rapidly with the distance from the

interaction point

23

CHAPTER 3

MATERIALS AND METHODS

3.1 External beam therapy equipment

Beam data will be obtained from Theratron Equinox-100 teletherapy cobalt unit.

The equipment is licensed to hold Cobalt-60 radionuclide (2cm diameter) contained in

a stainless steel shell. It is designed to load Co-60 source of maximum capacity 555

TBq (15000 Ci). A pneumatic air system, a two-way air cylinder using compressed

air, controls the source drawer. It is used to drive the source from fully shielded

position (Beam OFF condition) to a fully exposed position (Beam ON position).

The Theratron Equinox-100 teletherapy cobalt unit may be operated in fixed of arc

mode. In fixed mode, a stationary treatment is carried out with gantry at fixed angle.

In arc mode, a moving beam treatment is carried out with the gantry moving from a

start to a stop angle for a given arc [36].

3.2 Water phantoms

3.2.1 The blue phantom

The blue phantom is a measuring device for the measurement and analysis of

the radiation field [4]. The blue water phantom is a three-dimensional radiation beam

analyzing system. It has enabled a very quick and precise acquisition of data. Also, it

includes both step by step as well as continuous scanning mode.

The blue water phantom consists of:

• water tank of 48 x 48 x 48 cm dimensions, made of acrylic plastic

(Perspex) and weight 45 kg without water.

• CU500E microprocessor control unit with a build-in dual channel

electrometer with reversible polarity and auto ranging that is fully

software controlled.

• Two single ionization chambers (CC13) to provide scan in three

dimensions.

The phantom tank is placed on a trolley for the convenient moving of the assembly

and water is stored in water reservoir see Fig.3.1. The system is operated through the

OmniPro-Accept software for data acquisition and real-time analysis [31].

24

Figure. 3.1 The blue water phantom

3.2.1.1 OmniPro-Accept system

OmniPro-Accept is a system software utilizing hardware components to measure

radiation dose distribution, consisting of water phantoms, air scanners, film scanners/

digitizers and single or array detectors.

OmniPro-Accept is used to accurately analyze and handle the measured dose

distribution in quality assurance purposes, for calibration of radiation devices, as input

data to Treatment Planning Systems, for acceptance testing, beam tuning and in

research [31,35],

3.2.2 WP34 water phantom

Scanditronix WellhOfer WP34 is water phantom designed for absolute dose

measurements in radiation beams with horizontal beam incidence. Furthermore it is

suitable for the calibration of ionization chambers used in radiation therapy. The

phantom design allows cross calibration of a field ionization chamber against a

calibrated reference chamber at the user facility. The WP34 phantom is not

recommended for acquisition of dosimetry data that are directly used as input for

treatment planning system. This phantom is designed for absolute dose measurements

in radiation beams with horizontal beam incidence. Furthermore it is suitable for the

calibration of ionization chambers used in radiation therapy [30] (see Fig.3.2).

25

3.2.2.ITechnical specifications

The water equivalent thickness of the phantom window should be taken into

account when evaluating the depth at which the chamber is to be positioned (reference

depth) [30].

Figure. 3.2 WP34 water phantom

3.3 Detector systems

Scanditronix-Wellhofer provides a wide range of single detectors for use in the Blue

phantom as show below.

Ionization chambers are recommended for absolute dosimetry and used in relative

dosimetry for the measurement of profiles and depth doses in photon and electron

beams. For continuous scanning they are used in pairs (one field chamber, one

26

reference chamber). The chambers are connected via chamber extension cables to the

CU500E. The CU500E provides a high voltage supply, which is controlled from the

OmniPro- Accept software [30].

3.3.1 Ionization chambers

Different types of ionization chambers can be used during these measurements

(relative and absolute) as follows:

3.3.1.1 Compact Chamber CC13 (serial number 4920 and 4921)

This is a waterproofed chamber, consisting of inner and outer electrode made of

Shonka C552 (see Fig.3.3). The chamber has a leakage current less than ± 4xlO"15A.

The maximum polarizing voltage to be used is +300 Volts. The reference point (point

of positioning the chamber) with out build-up cap is 3.5 mm from the distal end of the

chamber [36].

B

Figure. 3.3 Compact Chamber CC13

Application

• Absolute and relative dosimetry of photon and electron beams in radiotherapy.

• Measurements in air, in solid phantoms or in water phantoms.

• Standard chamber for clinical use in water phantoms [32].

Calibration factor

• Calibration date:

• Radiation quantity: Co-60 y-ray

• Calibration factor for 4921 chamber: ND,W= 2.603 x 1018 Gy/C

• Calibration factor for 4920 chamber: ND,w = 2.611 x 1018 Gy/C

3.3.1.2 FC-65-P, Farmer chamber (serial number 577)

This is a waterproofed chamber, consisting of outer electrode made of POM and inner

electrode made of Aluminum. It has a volume of 0.65 cm3. The leakage current is less

27

than ± 4x10" A. The reference point without build-up cap is 13 mm from the distal

end of the chamber and the reference point with build-up cap is 17 mm from the distal

end of the chamber [34], The maximum polarizing voltage to be used is +300 Volts.

All cylindrical chambers FC-65-P and CC13 are mounted horizontally (i.e.

perpendicular to the beam, radial radiation) [36].

Figure. 3.4 FC-65-P, Farmer chamber

3.3.2 PTW Freiburg 2D-ARRAY

The 2D-ARRAY is a two-dimensional detector array with 256 vented ionization

chambers with size of 8mm x 8mm.

Components of the system:

• 2D-ARRAY Tl0017.

• ARRAY interface T16026.

• RS 232 cable.

• Matrix scan software.

The chambers and amplifiers of the 2D-ARRAY are manufactured with very high

precision. The chamber to chamber variation of the response is in the range < ± 1%

after calibration of PTW [1].

Calibration factor

2D-ARRAY is calibrated versus the absolute dosimeter for the radiation quantity

and measuring depth used. The calibration factor is valid for all channels. The 2D-

ARRAY is calibrated together with the PTW ARRAY interface T16026 [1],

3.4 Therapy electrometer and power supply

3.4.1 The Dose 1 electrometer (serial number of 03/8437)

The Dose 1 electrometer from WellhOfer is a very sophisticated and accurate

measuring device for dose and dose rate in radiation therapy. It has the ability to store

all corrections factors required in the measurements and then to compensate the

corrected reading [36].It is a portable, single channel dosimeter for calibration

dosimetry with ionization chambers and semiconductor detectors in radiotherapy and

radiation protection.

28

The system provides the following libraries: a sensor library containing calibration

information and physical characteristics of the majority of all commonly used

detectors, a library for correction factors, and a library for radioactive check devices.

The communication between the Dosel and the PC software is maintained via a

standard RS-232 serial interface [33].

An ionization chamber is essentially a capacitor in which leakage current or

leakage charge is induced through the action of the radiation beam. The charge or

current that is induced in the chamber is very small and must be measured by a very

sensitive charge or current measuring device (electrometer). The power supply in

ionization chamber/electrometer circuits is either a stand-alone unit or forms an

integral part of the electrometer. In either case it is important that one can change the

magnitude and polarity of the voltage produced by the power supply, so that the ion

collection efficiency of the chamber may be determined for a particular radiation

beam [29].

3.4.2 PTW UNIDOS

UNIDOS is a micro processor controlled universal field dosimeter for

measuring dose and dose rate in radiation beam. It provides several measuring modes:

current and charge-radiological quantities as exposure, photon equivalent dose, air

UNIDOS can store up to 30 of detector types and serial numbers for each detector in a

non-volatile memory, together with the individual calibration factors and other factors

specific for the chamber.

3.5 Dosimetric measurements

3.5.1 Measurements with symmetric fields

3.5.1.1 PDD measurements

Water phantom: Blue phantom

Electrometer: CU500E Control Unit + OmniPro-Accept software

Chamber: CC13 S.N: 4920 ND,W =2.611 x 1018 Gy/C

SSD= 90 cm

Field sizes: 5x5, 10x10, and 15 x 15 cm2

29

3.5.1.2 Beam Profile measurements



Chamber: CC13 S.N: 4920 ND,W = 2.611 x 1018 Gy/C

SSD = 90 cm

Field sizes: 5x5, 10x10, and 15x15 cm2

Depths: 0.75, 5, 10, 20 and 25 cm

The beam profiles were measured in the in-plane and cross-plane directions.

3.5.1.3 Absolute dose measurement

(I) Water phantom: Blue phantom

Electrometer: UNIDOS E

Chamber: FC-65-G S.N: 577 NDjW= 43.8 ± 0.4 mGy/nC

KQ = Ks =

Field size = 10x10 cm2 SSD = 90 cm Z re f=10cm

Temp. = °c press. = kPa KTp =

Measured dose:

(II) Water phantom: WP34

Electrometer: DOSE1

Chamber: FC-65-G S.N: 577 NDj w = 43.8 ± 0.4 mGy/nC

KQ = Ks =

Field size = 1 Ox 10 cm2 SSD = 90 cm Zref = 10 cm

Temp. = 25 °c press. = 967 kPa

Measured dose:

3.5.2 Measurements with motorized wedge

3.5.2.1 PDD measurements:



Chamber: CC13 S.N: 4920 ND,W = 2.611 x 1018 Gy/C

SSD = 90 cm

Field sizes: 3x3, 5x5, 7x7, and 8 x 8 , 10x10, 12x12, 15x15 , 18x18 , 18.5x20 cm 2

30

3.5.2.2 Beam Profile measurements



Chamber: CC13 S.N: 4920 ND, w = 2.611 x 1018 Gy/C

SSD = 90 cm

Field sizes : 3x3 , 5x5 , 7x7 , 8x8 , 10x10 , 12x12 , 15x15 , 18x18 , 18.5x20 cm2

Depths : 0.65, 5, 10, 20 and 25 cm

The beam profiles were measured in the in-plane and cross-plane directions.

3.5.2.3Absolute dose verification

For the particular MW angle and the respective open and MW beam weights, the

dose was measured for a field size of 10 x 10 cm 2 only in a MEDTEC water phantom

at 10 cm depth with a 0.13 cc thimble ion chamber and a NE electrometer.. For

dosimetry, TRS 398 protocol from IAEA was used. Measured dose with ion chamber

was compared with the TPS calculated dose.

3.5.2.4 Absolute dose measurement

(I) Water phantom: Blue phantom

Electrometer: UNIDOS E

Chamber: FC-65-G S.N: 577 N D;W = 43.8 ± 0.4 mGy/nC

Ks = KQ =

Zref=10cm SSD = 90 cm Field size = 10x10 cm2

Temp. = 24 c ° Press. = 967 kPa KTP =

Measured dose:

(II) Water phantom: WP34

Electrometer: DOSE1

Chamber: FC-65-G S.N: 577 NDjW = 43.8 ± 0.4 mGy/nC

Ks = KQ =

Zref=10cm SSD = 90 cm Field size = 10x10 cm2

Temp. =24 c° press. =965 kPa KTP =

Measured dose:

31

CHAPTER 4

RESULTS AND DISCUSSION

l-Water phantom measurements

Table (4.1) shows the motorized 60°-wedge factor for 3, 5, 7, 8, 10, 12, 15, 18

and 18.5 x 20 cm2 field sizes. From 5 x 5 cm2 field size to 15 x 15 cm2 filed size, the

wedge factors were found to be constant with the field size.

Table (4.2) shows the weight factor for MW 15°, 30°, 45° and 60° for 10 x 10 cm 2

field sizes. By using these appropriate beam weights, it is possible to generate isodose

distributions, which exhibit effective wedge angles ranging from 0 to 60°. These

weight factors were achieved by tilting the isodoses at 10 cm depth in a homogeneous

phantom for a desired MW angle. These weight factors can be used for clinical

implementation of MW during treatment planning.

Measured beam profiles for open fields 5x5, 10x10, 20x20 cm were shown in

Fig.4.1. In-plane beam profiles and 4.2. Cross-plane beam profiles at 0.65, 5, 10, 20

and 25cm depths.

Measured CADD for open fields 5x5, 10x10, 20x20 cm2 were shown in figure 4.3

Measured CADD for motorized wedge fields 3, 5, 7,8,10,12,15,18 and 18.5x20cm2

were shown in figure 4.4 at depths ranging from 0 up to 25 cm.

Measured beam profiles for motorized wedge fields 5, 10, 15cm were taken in the in

- plane and cross-plane directions as shown in figures 4.5, 4.6 and 4.7.

2. 2D-ARRAY measurements:

Calculated and measured beam profiles at 5 and 10 cm depths for motorized wedge

fields 5, 10, 15cm in the in-plane and cross-plane directions were compared as shown

in figure 4.8 and 4.9.

2D-ARRAY measurements showed that the shape of the motorized wedge beam

profiles is in all cases almost identical to the calculated beam profiles as can be seen

in the figures mentioned above.

32

3. Absolute dose measurement:-

Absolute dose measurements were carried out to determine the accuracy to

which the Pinnacle model the dose distribution for any wedge angles and fields

sizes.

Absolute dose measurements for the motorized wedge beams at 10 cm depth for MW

15°, 30°, 45° and 60° respectively shows that, there is no significant difference for all

field sizes and wedge angles used and as shown in table (4.3) the percentage deviation

was never larger than 2% which is within the tolerance level for absorbed dose

determination in the AAPM quality control report [29].

33

Table (4.1) Motorized 60° wedge factor for various field sizes at 10 cm depth

Field size ( cm )

3x3

5x5

7x7

8x8

10x10

12x12

15x15

18x18

18.5x20

MWF(60 degree)

0.254

0.254

0.255

0.256

0.258

0.260

0.265

0.270

0.271

Table (4.2) Weight factors for motorized wedge 15°, 30°, 45° and 60° for clinical

implementation

Pinnacle3 calculated

wedge angle(degree)

15

30

45

60

Wedge beam

weight factor

0.155

0.333

0.58

1

Open beam

weight factor

0.845

0.667

0.42

0

34

Table (4.3)Absolute dose verification in machine for motorized wedge 15 ° , 30° ,

45° and 60° at 10cm depth for various field sizes

Field size

(cm2)

5x5

10x10

15x15

MW (degree)

15

30

45

60

15

30

45

60

15

30

45

60

Measured dose

(Gy)

1.924

1.94

1.964

1.99

2.044

2.040

2.034

2.094

2.04

2.03

2.03

1.99

Planned dose

(Gy)

2

2

2

2

2

2

2

2

2

2

2

2

% Variation

1.04

1.03

1.02

1.01

0.98

0.98

0.98

0.95

0.98

0.99

0.99

1.018

35

(a)

(b)

36

(c)

Figure.4.1 In-plane beam profiles at 0.75, 5, 10 and 20 cm depths for symmetric fields

for (a) 5x5 (b) 10x10 and (c) 20x20 cm" field size measured in the blue water

phantom.

(a)

5x5 cross-line symmetry

-10

120

0.75

•5

10

20

10

37

(b)

(c)

Figure. 4.2 Cross-plane beam profiles at 0.75, 5, 10 and 20 cm depths for symmetric

and fields for (a) 5x5 (b) 10x10 and (c) 20x20 cm2 field size measured in the blue

water phantom.

38

PDD symmetry

10 20 30

-5X5 symmetry"

10X10 symmetry'

20X20 symmetry'

Figure. 4.3 The CADD for symmetry field sizes 5x5, 10x10 and 15x15cm

120 i

100-

80-

60-

40-

20-

n

1

' ^ ( f c ^ I X J 3 * S H I

i

3 100

PDD

200 300 400

3x3w

5x5w

7x7w

8x8w

10x10w

12x12w

15x15w

18x18w

[18.5x20w

Figure. 4.4 The CADD for MW taken by WP for 60° at various field sizes.

39

Figure. 4.5(a) In-plane beam profiles at o.65, 5, 10, 20 and 25 cm depths for wedged

field 5x5 cm2

5X5WCROSS-LINE

Figure.4.5 (b) Cross-plane beam profiles at o.65, 5, 10, 20 and 25 cm depths for

wedged field 5x5cm~

40

-15 i

-10

10X10 WIN-PLANE

120 -I

V 40 -

n —~v. 1 U i i

-5 0 5 10 1

15

0.65

5

10

20

25

Figure.4.6 (a) In-plane beam profiles at o.65, 5, 10, 20 and 25 cm depths for wedged

field 10x10 cm2

10X10WCROSS-PLANE

0.65 -5 -10 20 25

Figure.4.6 (b) Cross-plane beam profiles at o.65, 5, 10, 20 and 25 cm depths for

wedged field 10x10 cm

41

-15 -10

15x15w in-line

120

10

— i

15

0.65

5

10

20

25

Figure.4.7 (a) In-plane beam profiles at 0.65, 5, 10, 20 and 25 cm depths for wedged

field 15x15 cm

Figure.4.7 (b) cross-plane beam profiles at o.65, 5, 10, 20 and 25 cm depths for

wedged field 15" 15 cm2

42

5X5W60@5 in-line

Pinnacle

-2D

(a)

5X5W60@10 in-line

-2D

-Pinnacle

(a)

43

10X10W60@5 in-line

90

-15 -10

•Pinnacle

•2D

10 15

(b)

10x10w60@10 in-line

70

-15

• Rnnacle

•2D

15

(b)

44

-15 -10

15X15W60@5 in-line

•Pinnacle

2D

10 15

(c)

-15

15X15W60@10 in-line

-Pinnacle

-2D

—i

15

(c)

Figure. 4.8 In line beam profiles at 5 and 10 cm depths for wedged field sizes for (a)

5x5 (b) 10x10 and (c) 15x15 cm2 field sizes measured by2D-ARRAYwith build up

material, and calculated by Pinnacle.

45

46

(b)

47

-20

15X15W60@5 cross-line

200

-10

Pinnacle

2D

10 20

(c)

-15 -10

15X15W60@10 Cross-line

-Pinnacle

-2D

-5 10 15

(C)

Figure.4.9 Cross beam profiles at 5 and 10 cm depths for wedged fields for (a)

5x5 (b) 10x10 and (c) 15x15 cm2 field size measured with 2D-ARRAYwith build

up material, and calculated by Pinnacle.

48

Discussion

This study was an attempt to describe configuration of unique and novel

treatment device, Theratron Equinox-100 cobalt unit. Usually the telecobalt units are

available with symmetric collimation and individualized wedges. Till now, the

concept of advanced technology with the wedges was used in linear accelerators only.

Installation of a modern telecobalt unit at RICK center provided the opportunity to

investigate the optimal clinical implementation of the MW filter. This is the first

machine in our country and was first installed at RICK center. The comparative data

for MW is not available in the literature as on today.

Our measured data of CADD and profiles for MW were compared between the data

measured by the water phantom (WP) and the 2D-Array. Calculated CADD and

profiles for MW 60° and open beam were compared. Our results from figure4.3 show

the CADD for MW. Thus the results obtained acceptable for clinical implementation.

Table 4.3 shows the weight factor for MW (15°, 30° and 45° and 60°) for 10 x

10cm2 field size. The weight factors were checked for other field sizes (5, 15 and

18.5x20 cm ) also. It was found that the variation in weight factor with field size was

not significant from the clinical point of view and the same weight factor of 10 x 10

cm 2 can be used for the particular MW angle. These weight factors can be used for

clinical implementation and the system gives the treatment time for open and the MW

for a Particular beam weight.

49

CHAPTER 5

Conclusion

A comparison was drawn between measured and calculated dose distribution

using water phantom and 2D-ARRAY with build up materials. No difference was

found for all field sizes.

The Pinnacle3, TPS, uses the same beam data for motorized wedge setting and

models the dose distribution for any wedge angles and field sizes.

The Pinnacle has these advantages and can be used to calculate dose distribution

for any wedge angles and field sizes to the same degree of accuracy as for universal

wedges. The MW provides the capability of modifying the isodose characteristics of

the radiation beam, the same as the universal wedges and can safely be used for

clinical applications. However, to utilize the clinical advantages of MW, accurate

dose calculation and continuous checking are required during the treatment planning

process.

50

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52

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53

Appendix

Appendix 1. Cobalt-60 teletherapy machine. Theratron Equinox, a megavoltage

external beam therapy system using cobalt technology, manufactured by MDS

Nordion, Ottawa, Canada (published with permission from MDS Nordion).

54

Appendix 2. Omnipro Accept Main application window containing one Beam document window

Appendix 3.Diagram of a designed compact ionization chamber, CC13

Appendix 4 Alignment cap mounted on the CC13.

55

Documents

Evaluation of Motorized Wedge for a new generation