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Application note

The 2D-ARRAY seven29A new way of dosimetric verification of IMRT beams

Jörg Bohsung, Klinik für Strahlentherapie Charité Berlin, Standort Mitte

1. Purpose

The purpose of this application note is to demonstrate the possibilities of the 2D-ARRAY seven29from PTW in combination with the VeriSoft analysis software as a dosimetric verification tool ofclinical IMRT fields. For this demonstration, the Varian IMRT system is used, which consists of theIMRT treatment planning system Eclipse Helios V7.2.24 and Varian Clinac accelerators, which areequipped with dynamic multileaf collimators.The reader can use this application note in two ways:• as a demonstration of the possibilities of the PTW 2D-ARRAY for fast and accurate absolute 2D

dose verifications of complex fluence modulated fields.• as systematic instruction (chapter 5) of the whole verification process for a Varian IMRT system.Users of non-Varian IMRT solutions should be easily able to transfer all verification steps to theirIMRT system.

2. Introduction

2.1 Intensity modulated radiotherapy

Intensity modulated radiotherapy (IMRT) is a specialform of three-dimensional conformal radiotherapy(3D-CRT). In contrast to conventional 3D-CRT, whereonly the beam apertures are shaped to the irregularform of the target, IMRT is based on the use of x-raybeams with individually optimized, non-uniformphoton fluencies across the beam area (see Fig. 1).The use of IMRT can improve dose distribution withina patient's body, especially if the target has acomplex three-dimensional shape, e.g., a concavepart that surrounds a critical structure. Fig. 2 shows atypical dose distribution of an IMRT plan for a patientsuffering from a head and neck tumor. Only theprimary tumor and the elective lymph node area aretreated with high doses, while the spinal cord and thecontra lateral parotid gland receive only uncriticaldose levels. It is impossible to create such a complexdose distribution with homogeneous photon fields.

Although the advantages of this new treatment technique are obvious, IMRT is used by only a fewcenters for a relatively small number of patients. IMRT, still in its infancy, has many differentmedical, technical and economic problems that remain to be solved within the next few years.These problems are discussed in detail in [1].

Figure 1: An irregularly shaped field with uniformfluence (left) compared to a fluence-modulatedfield (right).

Figure 2: A typical IMRT dose distribution of a headand neck target volume. The high dose area (red)follows the target shape, while the spinal cord andthe right parotid gland are spared.

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2.2 IMRT delivery techniques

Several IMRT delivery techniques can be found in publications (see, e.g., [1]). Today, most newlyinstalled IMRT delivery systems use modulated beams with fixed beam directions. The fluencemodulations are created by the multileaf collimator (MLC), which runs in one of two special IMRT-modes:

• The multi-segment method approximates the continuously modulated optimized fluence of eachfixed field direction by a stepwise fluence distribution. This is done by combining a set of smallhomogeneous field segments of different weights, which are shaped by the MLC. This methodis often also referred to as step-and-shoot delivery, because the radiation is only turned onwhen the leaves have reached their prescribed positions for each segment.

• A technically more advanced method is the dynamic MLC technique (often also called slidingwindow method). Here the fluence profile along the moving direction of a leaf pair is created bysweeping the leaf pair with different openings over the field while the beam is on all the time.

Both methods result in similar final dose distributions, although the dynamic MLC approach cangenerate a better approximation of the optimized fluence and allows a faster delivery. On the otherhand, the multi-segment method is easier to implement and needs a slightly lower number ofmonitor units.In contrast to conventional homogeneous fields, the number of monitor units for both methods asgiven by the treatment planning system no longer has a simple relation to the delivered dose ofthat field. Therefore, many groups feel it necessary to verify each single treatment fielddosimetrically before its clinical use.

2.3 IMRT and quality assurance

IMRT is a very complex treatment modality. Therefore, new quality assurance procedures must beimplemented throughout the complete clinical process [1]. Here we will only discuss sometechnical aspects. Typically, the clinical implementation of IMRT passes through at least threesteps:

1. The installation and commissioning phase of the treatment planning and the delivery system2. The clinical starting phase, where each treatment is tested by an individual verification

procedure, which typically is extremely time-consuming and not very standardized3. The routine phase, where standardized QA procedures are used to guarantee safe and reliable

IMRT deliveries for a large number of patients

Most groups focus only on the first two phases. However, the third phase is by far the most criticalone for a stable IMRT routine delivery. The routine use of IMRT for large patient numbers is onlypossible if standardized QA procedures have been carefully defined and easy-to-use measuringequipment together with user-friendly analysis software is available.For both MLC modulation methods, the delivered doses have a complex, non-intuitive relationshipto the number of monitor units. It is also impossible to predict the exact combination of fieldsegments or the leaf motion patterns. Therefore, all IMRT groups, which are using the MLC for thecreation of fluence modulations, must establish a precise and reliable method for the dosimetricverification of IMRT plans. Especially during the starting period, each plan must be dosimetricallyverified, but dosimetric verification is also recommended after software upgrades or the expansionto new tumor entities.

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3. Dosimetric verification of treatment plans

3.1 The phantom substitution method

Because a verification of dose distributions within a real patient is not possible, the phantomsubstitution method is often used, which works as follows:

• Within the treatment planning system, the patient plan is transferred - either all together or field-by-field - to a special phantom.

• The dose distribution is recalculated within that phantom without changing any dosimetricallyrelevant treatment parameter.

• At the treatment machine, the phantom – equipped with appropriate dosimeters – is irradiatedusing the IMRT fields of the real patient plan, again without changing any dosimetrically relevanttreatment parameters.

• The measurements are compared against the calculated dose values.A large variety of phantoms and dosimeters are described in publications and are used clinically(see for example [2]). The phantom substitution method can be performed in two different ways,either plan-related or field-related.

3.2 The plan-related approach

For the plan-related approach, the whole plan (i.e., allfields with their correct beam entry directions) istransferred within the treatment planning system to averification phantom and the dose distribution iscalculated.A phantom, which is suitable for the plan-relatedapproach typically has a cylindrical, elliptical orspherical shape and is often made of slabs, whichallow the filling of the phantom with dosimetric films(see Fig. 3). Additionally, it is often possible, to equipthe phantom with ion chambers for absolute point dosemeasurements. PTW offers several specializedphantoms for the plan-related approach:

• The cylindrical Head/Neck Verification Phantom T40015 for film dosimetry;• The Head/Neck Verification Phantom T40014, which accommodates the PTW linear array

LA48;• The Matrix IMRT Verification Phantom T40026 for the use of up to 25 ion chambers, which can

be individually placed throughout the entire height of the phantom.

The advantages of the plan-related approach are obvious:

• The entire plan can be verified within one run.• All treatment parameters including the beam entry directions are identical to the real patient

treatment. Therefore, also critical points of the setup such as influence of the treatment couch,etc., can be detected.

• Because the complete treatment plan is verified, a connection between the measured dosedistribution and the real anatomical situation can be made. It is therefore easy to decide if adetected local disagreement between calculation and measurement is nevertheless clinicallyacceptable or not.

Figure 3: A verification phantom for the plan-related approach. The arrows illustrate the entrydirections of a typical IMRT plan.

Dosimetricfilm

Phantom

Hole for ionchamber

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However, the plan-related approach also has several practical disadvantages, which make itsroutine use difficult:

• Real 3D dose measurements are extremely time consuming and not available under clinicalconditions. A plan-related verification is therefore normally restricted to only a few 1D or 2D cutsout of the 3D dose cube. Therefore delivery problems (e.g., misaligned leafs) outside themeasuring zone may not be detected.

• Depending on the used measuring system, the plan-related approach may be accompanied bysevere dosimetric problems. A typical example is the film calibration, which is not trivial for films,which are oriented parallel to the beam entrance direction.

• Because all fields are treated at once, it is often difficult to locate the reason for a detecteddosimetric discrepancy.

• The phantom-preparation is time consuming.

3.3 The field-related approach

For the field-related approach, each single treatmentfield is transferred separately to a verification phantom.All treatment parameters are the same as for the realpatient plan, except the gantry angle, which is normallyset to 0° for all beams. The field-related approachrequires only a very simple rectangular phantom,which is able to carry a dosimetric film or anotherplanar dosimeter at a plane perpendicular to the beamentrance direction (see Fig. 4). Sometimes thephantom has additional holes for ion chambermeasurements. PTW provides the Universal IMRTVerification Phantom T40020 for the field-relatedapproach.

The field-related approach is often criticized, because it does not reflect the real treatment as wellas the plan-related approach. On the other hand, the field-related approach has a number ofadvantages, which are especially valuable if IMRT is operated under routine conditions.

• The verification process covers the complete modulated area of each field. All potential deliveryproblems are therefore safely detected.

• Dose measurements are always performed in a plane perpendicular to the central axis; hence,dosimetric problems (e.g., film calibration) are less critical than for the plan-related approach.

• A detected dosimetric discrepancy can easily be traced back to its reason, e.g., an improperlyadjusted leaf.

• The preparation and setup of the phantom is easy and not very time-consuming.• The field-related approach is perfectly adapted to the use of various electronic 2D measuring

devices such as the 2D-ARRAY seven29.

For groups which have either no access to film dosimetry or want to avoid it because of itscomplexity and cost, the latter point is an important argument in favor of the field-related approach.

Figure 4: A verification phantom for the plan-related approach.

Dosimetricfilm

Phantom

Hole for ionchamber

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4. The 2D-ARRAY seven29 and the analysis software VeriSoft

4.1 2D-ARRAY seven29

The 2D-ARRAY seven29 consists of a plane matrix of 27 x 27 air-filled ion chambers. The ventedplane-parallel ion chambers are 5 mm x 5 mm x 5 mm in size and the center-to-center spacing is10 mm. The 729 chambers cover a maximum field size of 27 cm x 27 cm. The surroundingmaterial is acrylic (PMMA). The package includes an interface for fast data acquisition. The displaycycle can be selected between 400 and 1000 ms.The 2D-ARRAY seven29 allows absolute dose and dose rate measurements of high-energyphoton fields. On-site calibration is not necessary.The data acquisition software MatrixScan is used to acquire dose or dose rate data; it displays 3Dgraphics and transfers the acquired data to the software packages VeriSoft, MultiCheck orMEPHYSTO.

4.2 VeriSoft

VeriSoft assists physicists in comparing dose distributions measured in an IMRT verificationphantom with dose distributions computed by a radiotherapy treatment planning system. Matricesof measured and calculated points of an IMRT beam are compared by subtracting the matricesand visualizing the result. The gamma evaluation method [3] is supported, hot and cold spots caneasily be located, and the maximum and average deviation between treatment plan and measuredbeam is determined.VeriSoft allows to import data from various treatment planning systems.

5. Dosimetric verification of IMRT fields with the 2D-ARRAY -a step-by-step instruction for the Varian IMRT solution

In this chapter, the field-related dosimetric verification of IMRT fields with the 2D-ARRAY seven29,as it is performed at the Charité Hospital Berlin, is demonstrated systematically.

5.1 The IMRT system

IMRT treatment planning is done with the Eclipse Helios treatment planning system V7.2.24(Varian Medical Systems). Within Eclipse, the dynamic leaf motion instructions are calculated,which are necessary to generate the optimized fluences at the treatment machine.The modulated photon fields are delivered by Varian Clinac linear accelerators, which areequipped with different dynamic multi leaf collimators (MLC-120, MLC-80 and MLC-52).All necessary treatment parameters, i.e., monitor units, field sizes, gantry angles and leaf motioninstructions, are stored in the database of the record-and-verify system VARiS Vision 6.2 andprovided for patient treatment. In Berlin, we perform not only patient treatments but also all IMRTverifications under the control of VARiS Vision.

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5.2 Phantom setup, CT scanning and preparation within Eclipse

For the field-related verification process, no special phantom is necessary; it is convenient toarrange the array between plates of water equivalent material. We use a sandwich setup of waterequivalent PTW RW3 plates with a stack of 3 cm below and 5 cm above the array (see Fig. 5).The phantom arrangement is CT scanned then in exactly the same way as it is later used for theverification measurements. To achieve an adequate spatial resolution during the followingverification dose calculations, it is essential to scan the phantom with a sufficiently small slicethickness. We have scanned the phantom with a slice thickness of 2 mm (Fig. 6). The scannedphantom is imported via DICOM to Eclipse. Directly after import, it is convenient to define a userorigin within Eclipse exactly at the effective measuring point of the central ion chamber of thearray.

The effective measuring point is located in the middle of the chamber area and 5 mm below thesurface of the 2D-ARRAY. By that, all fields, which are verified afterwards, are automaticallypositioned correctly with their isocenter at the effective measuring point of the central ion chamber.Note that it is necessary to perform the scanning and import process only once. Within Eclipse, thephantom with the correctly placed user origin can then be defined as the default phantom, which isalways used for verification.

5.3 Transfer of the patient treatment fields and dose calculation

Eclipse supports both the plan-related and the field-related approach for the verification of IMRTplans. After an IMRT plan is accepted for treatment, the planner has to use the function “createverification plan”, to start the verification process. Here, the user can select between severalpossibilities (Fig. 7). For the field-related approach with the 2D-ARRAY, the following selectionsare necessary (Fig. 7a):• Each patient field must be placed into separate verification plans• The couch, gantry and collimator angles must be reset to 0°Then the verification method has to be defined (Fig. 7b), which is “Phantom” in our case. The usercan then select the correct verification phantom, which then can be set as default.

Figure 5: For the field-related verification the 2D-ARRAY is simply located between RW3 plates. 3 cmRW3 material are below, 5 cm above the array.

Figure 6: The scanned phantom arrangementafter import to Eclipse. The phantom wasscanned with 2 mm slice thickness. The userorigin is marked by a red cross.

2D array

RW3

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For each field of the patient plan, a new verificationplan is now created. The field isocenters areautomatically positioned at the correct place if theuser origin was properly defined (see 5.2). Thecreated verification fields have exactly the samedosimetrically relevant treatment parameters as thereal patient fields. Any changes are suspended byEclipse. Thus, it is guaranteed that the verificationfields really reflect the behavior of the real patienttreatments.After the correct transfer of the treatment fields to theverification phantom, the planner can calculate the3D dose distribution of each field. In Fig. 8, the dosedistribution of an IMRT field of a complex head andneck treatment plan within the verification phantomis depicted. Note that the frontal view already showsthe 2D dose distribution within the effectivemeasuring plane of the 2D-ARRAY.

5.4 Export of the verification dose plane

The dose comparison in VeriSoft requires a DICOM dose export of the 2D dose distribution withinthe effective measuring plane of the ion chambers. To do that, the following steps are necessary:• The frontal dose view in Eclipse must be adjusted in such a way that it shows the correct

measuring plane. If the user origin was defined as discussed in section 5.2, the y-coordinate ofthat plane is y=0.

• The frontal dose view must have the focus. The easiest way to guarantee for that is to enlargethe frontal dose view (Fig. 9a).

• The plane export is started by a right-click (Fig. 9a).• Within the dose export wizard, the “absolute” dose export must be selected and the dimensions

of the dose plane must be correctly set (Fig. 9b).The Eclipse dose export wizard automatically positions the center of the exported dose matrix atthe field isocenter (see Fig. 9b). This function is important for the later dose comparison withinVeriSoft as discussed in section 5.7 and was not available in earlier Eclipse versions.To avoid confusion with the cryptic DICOM file names, it is convenient to save all DICOM planedose files within a special folder, which is given the patient’s name, for example.

Figure 7: Creation of the verification plan (both plan- and field-related) within Eclipse. The correct options for the field-related approach with the 2D-ARRAY are selected.

Figure 8: Dose distribution of an IMRT field within theverification phantom. The dosimetrically relevanttreatment parameters are identical to the patient planand cannot be changed by the user.

a b

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5.5 Phantom setup and calibration measurement

The Phantom setup for the measurements is shown in Fig. 10. The gantry and collimator anglesare set to 0°. The 2D-ARRAY is located on top of a package of 3 cm RW3 material and adjusted ina way that the isocenter is positioned at the effective measuring point of the central ion chamber,i.e., 5 mm below the front plate of the array (Fig. 10a). Then another stack of 5 cm RW3 material isadded above the array (Fig. 10b). Finally, the SSD of this arrangement should be 94.5 cm.

Although the 2D-ARRAY is already calibrated in absorbed dose to water, normally eachmeasurement must be corrected for different air pressure and temperature, for the used photonquality and for possible non-water equivalent properties of the phantom. To avoid thesecorrections, which are time-consuming and susceptible to errors, we use a simple calibration fieldwith a known dose, which is delivered before each verification measurement:

• Within the treatment planning system, a simple “calibration plan” with only one homogeneousfield with a field size of 10x10 cm2 and a gantry angle of 0° is created. Beam quality andisocenter position are identical to those of the IMRT fields.

• The monitor units of that field are adjusted to an isocenter dose Dcal of e.g., 2.00 Gy.• After positioning of the phantom (Fig. 10), this calibration field is always treated as the first field

of a verification session and the dose is measured with the 2D-ARRAY using MatrixScan.

Figure 9: DICOM export of the correct dose plane with y=0. Note, that the center of the exported dose matrix isautomatically positioned at the isocenter.

Figure 10: Phantom setup for the verification measurements. The isocenter is adjusted to the effective measuring pointof the central ion chamber of the 2D-ARRAY.

a b

a b

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• The readout M of the central chamber is noted. This chamber can be selected in the “options”menu of MatrixScan (chamber coordinates [14,14]). The dose is then visible at the status bar ofMatrixScan.

• A calibration factor f = Dcal/M is calculated, which is used for the correction of all later verificationmeasurements (see Section 5.7).

This calibration method has the additional advantage that it also corrects for possible deviations inthe output calibration of the linear accelerator.

5.6 Verification measurements

In Berlin, all verification measurements areperformed in the clinical mode under the controlof VARiS Vision. The verification fields areprepared within VARiS Vision in its owntreatment course, which is linked to the patientfor whom the verification is performed. Unlike thereal treatment fields, however, the verificationfields do not add up any dose to the patientwithin VARiS Vision.Immediately after the delivery of the calibrationfield (see section 5.5), the IMRT fields areirradiated one after the other and measured withthe acquisition software MatrixScan. In Fig. 11, ascreenshot of MatrixScan during themeasurement of an IMRT field is depicted.To block as little machine time as possible, onlythe raw data are saved and no further corrections are performed during the measurements. If thecalculated DICOM dose plane data were already saved in a special patient folder as proposed insection 5.4, it makes sense to store the measurements within the same folder.

5.7 Analysis of the measurements using VeriSoft and documentation of theresults

To guarantee a smooth and fast data analysis process,we suggest using the following procedure for eachcalculated and measured field pair:

• The calculated data set is read into matrix A. Forsmall fields, it is worth defining an appropriate regionof interest to avoid too many data points outside theinteresting area.

• The corresponding measured data set is read intomatrix B.

• The measured data set is calibrated by pushing the“Calibrate” button. Within the calibration window(Fig. 12), the measured data set is now multiplied bythe determined correction factor f = Dcal/M withDcal = 2.00 Gy (see section 5.5).

In the example shown in Fig. 12, the measured central chamber dose M of the calibration fieldwas M = 2.02 Gy.

Figure 11: Screen shot of MatrixScan duringa measurement of an IMRT field.

Figure 12: Calibration procedure. In thisexample, the measured data set iscorrected by a factor 2.00/2.02 = 0.99.

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• By default, VeriSoft normalizes each matrix individually to its maximum dose. This is notmeaningful for absolute dose measurements with the 2D-ARRAY. Therefore, both data setshave to be normalized to the same meaningful value (e.g., the dose at the isocenter).

• The correct positioning of the two data sets is checked using the “Profiles” option. Normally, noalignment should be required. However, positional corrections of ±1 mm in both x and ydirections are sometimes necessary due to slight misalignments of the phantom during themeasurements.

Fig. 13a shows the isodose overlay, which already gives a first impression of the concurrence ofthe two data sets. This concurrence is quantitatively confirmed by vertical and horizontal line scansthrough the isocenter (Fig. 13b).

Line scans have the disadvantage that an investigation of the complete area is quite time-consuming.

Therefore, VeriSoft provides additional quantitative 2D compare modes.

In Fig. 14, a difference plot is shown, where each measured chamber dose Dm(i,j) is compared tothe averaged calculated dose <Dc(i,j)> according to the following formula:

%100*)j,i(D

)j,i(D)j,i(DD

)j,i(Dc

cm −=

∆

Note that VeriSoft provides also a percentage difference mode, which is relative to thenormalization dose of matrix A. This mode however is not as sensitive to the low dose.

Figure 13a: VeriSoft isodose overlay of a typical IMRThead and neck field. The measured isodoses are depictedas dotted lines.

Figure 13b: Horizontal (above) and vertical (below)line scans through the isocenter of the IMRT field of Fig. 13a.Calculations are shown as blue lines and the measuredchamber values are depicted as green bars.

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While dose difference plots work very good within low-gradient areas of the analyzed dosedistribution, they may show artificially large deviations in high-gradient parts (Fig. 14). To overcomethat problem, which is especially critical for highly modulated IMRT fields, Low et al. developed thegamma analysis method [3]. The gamma method judges the concurrence of 2D dose distributionsusing two criteria: the local dose difference within low-gradient areas and the distance-to-agreement (i.e., the distance of two points, which have the same dose). The user has to define twoacceptance values: The maximum acceptable local dose difference (in %) and the maximumacceptable distance to concurrence (in mm). In VeriSoft, the gamma method is implementedaccording to the method of Depuydt et al. [4], which adapts the Low algorithm to the use of discretedata sets. For our analyses, we use always 3% dose difference and 2 mm distance to agreement.Calculated dose value are not analyzed for dose values below 5% of the maximum.Fig. 15 shows the resulting gamma evaluation. The acceptance criteria are not met for only a fewpoints below the 10% isodose. This is a typical result. Two factors are responsible for higherdeviations with very low doses:

• The measuring device has a higher relative measuring error.• For very low doses, the dose of a dynamically collimated IMRT field is mainly determined by leaf

transmission, which is not calculated as precise as the dose within the open part of the field bythe treatment planning system.

We normally accept these dose deviations, if they only show up in low dose areas. In chapter 6,more verification examples of IMRT fields are discussed

Figure 14: Difference plot of the dose distribution ofFig. 13a. Most differences are below 3% of the localmeasured dose. Higher deviations are only found at high-gradient regions and at very low doses.

Figure 15: Gamma index plot of the dose distribution ofFig. 13a. The red dots indicate points where theacceptance criteria (3%, 3 mm) are not reached. Allthese points are located within low-dose regions.

a

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Fig. 16 finally shows the printout of the IMRT field, as it is provided by VeriSoft and as we use it fordocumentation.

Figure 16: Documentation of the result

5.8 Time requirement

The verification process with the 2D-ARRAY seven29 is highly standardized. Therefore, the entireprocedure can be performed within an acceptable time. In Tab. 1, the time requirements for thedifferent verification steps for a five-field IMRT plan are collected.Future versions of VeriSoft will allow an even more automated data analysis, so that a further timereduction can be expected.

Procedure Section Average time[min]

Transfer of the patient treatment fields and dose calculation 5.3 10

Export of the verification dose plane 5.4 5

Verification measurements including setup 5.5 25

Analysis of the measurements with VeriSoft 5.6 20

Total 60

Table 1: Time requirement for the different steps of the verification procedure.

The time requirement listed above is for the verification process with the 2D-ARRAY of one patientplan. However, if plans of more than one patient have to be verified, the same hardware setup forthe measurements is used, resulting in time savings. The verification process itself is also moreefficient if all fields of all patients are checked against the plans in one session.

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6. Some clinical examples

In Berlin, we use IMRT as a routine treatment option mainly for treatments of the prostate, headand neck tumors and for irradiations of the breast if the internal mammary lymph node chain has tobe included into the target volume. In the next sections, some examples of real patient IMRT plansand their verifications are shown.

6.1 Prostate

For prostate IMRT treatments we use a simultaneousintegrated boost (SIB) technique, where two different dosesare simultaneously delivered to different parts of the targetvolume. The SIB technique provides two advantages:Firstly, it allows an increased dose within a specific targetregion (e.g., prostate) and a reduced dose to the peripheralregion (e.g., periprostatic area). Secondly, it keeps theoverall treatment time constant while increasing the targetdose. For our patients, the CTV is irradiated with a singledose of 2.0 Gy per fraction whereas the PTV is treatedsimultaneously with 1.8 Gy. In Fig. 17, a five-field IMRT SIBtreatment plan for a prostate patient is shown. With the SIBtechnique, we are able to increase the therapeutic dosesafely up to 82.0 Gy without increasing the doses to theorgans at risk, especially the rectum [5].In Fig. 18 the isodose overlays and gamma distributions for all five prostate IMRT fields of thetreatment plan of Fig. 17 are put together. Typically, prostate IMRT fields are not modulated verymuch. Therefore, no critical areas are normally found during the verifications.

Figure 17: Dose distribution of an IMRT SIBprostate treatment plan with five fields.

Figure 18: Isodose overlays and gamma distributions of all five IMRT fields of a prostate treatment plan (measuredisodose lines are dotted). Nearly all gamma values are < 1 in all fields.

Field 1 Field 2 Field 3 Field 4 Field 5

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6.2 Head and neck

For postoperative head and neck patients, we also use an SIB IMRT technique with seventreatment fields. The former tumor volume is irradiated simultaneously with a higher dose levelthan the elective lymph nodes. The spinal cord and the contralateral parotid gland are spared. Anexample for a head and neck treatment plan was already shown in Fig. 2.The verification results for all seven fields of that plan are depicted in Fig. 19. As already discussedin Section 5.7, it is normal for such highly modulated fields that some small areas within the low-dose part of the dose distributions show gamma values >1.

Figure 19: Isodose overlays and gamma distributions of all seven IMRT fields of a head and neck treatment plan (measuredisodose lines are dotted). Because of the more complex modulation, some gamma values within the low-dose regions are>1 (see discussion in Section 5.7).

Field 7Field 6Field 5

Field 1 Field 2 Field 3 Field 4

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6.3 Breast

The gain of IMRT for breast treatments is not asclear as for the other discussed tumor sites. Ifthe breast alone is the target volume,conventional dose distributions cannot beimproved very much by IMRT. However, if theinternal mammary lymph node chain has to beincluded, the target volume is concavely shapedaround the chest wall and all conventionaltechniques suffer from a quite high dose burdento the ipsilateral lung. For this special case,IMRT can improve the conformity of the dosedistribution. Fig. 20 shows a typical seven fieldIMRT plan for a breast treatment with includedinternal mammary lymph node chain.

Figure 20: Axial and frontal dose distribution of abreast IMRT with included internal mammary lymphnode chain.

Figure 21: Isodose overlays and gamma distributions of all seven IMRT fields of a breast treatment with included internalmammary chain (measured isodose lines are dotted). Because of the extremely complex modulation, some gamma valueswithin the low-dose regions are >1 (see discussion in Section 5.7).

Field 7Field 6Field 5

Field 1 Field 2 Field 3 Field 4

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Fig. 21 shows again the verification results. The typical fields of breast IMRT plans are extremelymodulated with large central low-dose areas to spare the lung. Therefore, some parts of gamma>1 have to be accepted again. Note that the spatial resolution of the 2D-ARRAY seven29 is goodenough, even for such highly modulated IMRT fields.

7. Conclusion

The 2D-ARRAY seven29 is a very reliable tool for the fast and precise verification of IMRT fields.Together with the analysis software VeriSoft, even complex IMRT treatment plans can be verifiedand documented within approximately one hour. Because air-filled ion chambers are used, nocomplicated calibration procedures are necessary; the system is always ready for high-precisionmeasurements. The effective spatial resolution of the 2D-ARRAY is sufficiently high to obtainreliable results even for highly modulated IMRT fields.

The dosimetric verification of IMRT fields is an important part of the routine quality assurancepackage for IMRT treatments. The 2D-ARRAY and VeriSoft provide a reliable and easy-to-use toolfor that purpose.

Additionally the 2D-ARRAY, in combination with the software MultiCheck, can be used for dailycheck of the constancy of main beam parameters of the linear accelerator.

8. Literature

[1] Intensity Modulated Radiation Collaborative Working Group, Intensity-modulatedradiotherapy: current status and issues of interest. Int. J. Radiation Oncology Biol. Phys.51: 880-914 (2001).

[2] Van Esch A., Bohsung J., Sorvari P., Tenhunen M., Paiusco M., Iori M., Engström P,Nyström H, Huyskens D. Acceptance tests and quality control (QC) procedures for theclinical implementation of intensity modulated radiotherapy (IMRT) using inverseplanning and the sliding window technique: experience from five radiotherapydepartments. Radiother. Oncol. 65 (2002) 53-70.

[3] Low, D., Harms, W., Mutic, S., Purdy, J. A technique for the quantitative evaluation ofdose distributions, Med. Phys. 25 (1998) 656-661.

[4] Depuydt, T., Van Esch, A., Huyskens, D. A quantitative evaluation of IMRT dosedistributions: refinement and clinical assessment of the gamma evaluation. Radiother.Oncol. 62 (2002) 309-319.

[5] Böhmer, D., Bohsung, J., Eichwurzel, I., Moys, A., Budach, V. Clinical and physicalquality assurance for intensity modulated radiotherapy of prostate cancer. Radiother.Oncol. 71 (2004) 319-325.

Text and pictures by courtesy ofDr. Jörg BohsungCharité UniversitätsklinikumSchumannstr. 20/2110117 Berlin

Reprint by PTW-FreiburgLörracher Straße 779115 Freiburg, GermanyPhone: +49 761 490 55-0info@ptw.de

D742.208.0/0 2004-06

Eclipse, Helios, and Millennium are trademarks of Varian Medical Systems, Inc.CadPlan, Clinac, Varian, Varian Medical Systems, and VARiS are registered trademarks of Varian Medical Systems, Inc.VeriSoft and MEPHYSTO are registered trademarks of PTW-Freiburg

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