AimTo study of 2 Dimensional ion chamber array for angular response and its utility for quality assurance of dynamic multileaf collimator and pretreatment intensity modulated radiotherapy plans.Materials and MethodsThe MLC QA test patterns and IMRT plans were executed on 2D ion chamber array having 1020 vented pixel ionization chambers. The dynamic MLC QA test patterns were chair test, x–wedge, pyramid, open swipe field, garden fence and picket fence. Performance of Dynamic wedges was compared with physical wedges. For IMRT verification, five patients with localized prostate carcinoma were planned using dynamic IMRT technique. Angular response of MatriXX was measured by exposing the system from different gantry angles.ResultsDynamic MLC QA tests such as chair, x-wedge, pyramid, and open swipe field were successfully verified. MatriXX was not able to recognize the bar pattern of picket test and garden fence test. The response of MatriXX gradually decreases from 0° to 180° angles and it was 7.7% less at 180° angle. The dynamic wedge profiles were matching with corresponding physical wedge profiles. For pretreatment IMRT QA, the average dose difference between planned and measured dose was 1.26% with standard deviation of 1.06.ConclusionI'mRT MatriXX can be used for routine dynamic MLC and IMRT pretreatment QA but care should be taken while taking measurements in penumbra region because of its limited spatial resolution

BHARDWAJ, Anup Kumar

RANA, Bhupendra

SHARMA, Suresh Chander

SHUKLA, Arvind

Via Medica Journals

Anup Kumar Bhardwaj et al • Study of 2D ion chamber array for angular response and QA of dynamic MLC...89Rep pRact oncol RadiotheR • 2009 • 14/3/: 89–94Study of 2D ion chamber array for angular response and QA of dynamic MLC and pre-treatment IMRT plansAnup Kumar BHARDWAJ, Suresh Chander SHARMA, Bhupendra RAnA,  Arvind SHuKLAAbstrActAIM: To study of 2 Dimensional ion chamber array for angular response and its utility for quality assur-ance of dynamic multileaf collimator and pretreatment intensity modulated radiotherapy plans.MAtErIALs And MEtHOds: The MLC QA test patterns and IMRT plans were executed on 2D ion chamber array having 1020 vented pixel ionization chambers. The dynamic MLC QA test patterns were chair test, x–wedge, pyramid, open swipe field, garden fence and picket fence. Performance of Dy-namic wedges was compared with physical wedges. For IMRT verification, five patients with localized prostate carcinoma were planned using dynamic IMRT technique. Angular response of MatriXX was measured by exposing the system from different gantry angles.rEsULts: Dynamic MLC QA tests such as chair, x-wedge, pyramid, and open swipe field were suc-cessfully verified. MatriXX was not able to recognize the bar pattern of picket test and garden fence test. The response of MatriXX gradually decreases from 0° to 180° angles and it was 7.7% less at 180° angle. The dynamic wedge profiles were matching with corresponding physical wedge profiles. For pretreatment IMRT QA, the average dose difference between planned and measured dose was 1.26% with standard deviation of 1.06.cOncLUsIOn: I’mRT MatriXX can be used for routine dynamic MLC and IMRT pretreatment QA but care should be taken while taking measurements in penumbra region because of its limited spatial resolution.KEy wOrds: IMRT QA; Angular response of array; I’mRT MatriXXReceived: 21.05.2009Accepted: 28.05.2009Subject: original articleDepartment of Radiotherapy  Post Graduate Institute of Medical Education and ResearchAddress for correspondence: Dr Anup Kumar BhardwajDepartment of Radiotherapy  Post Graduate Institute of Medical Education and ResearchChandigarh 160012ChandigarhIndiae-mail: anoop401@rediffmail.combAcKGrOUndIntensity modulated radiotherapy (IMRT) is the advance form of 3 dimensional conformal radiotherapy (3DCRT). The clinical imple-mentation of IMRT requires special qual-ity assurance (QA) procedures for multileaf collimator (MLC) as well as for treatment machine commissioning, including patient-related routine QA beyond the ones currently performed for 3DCRT with the MLC [1–7]. IMRT allows the clinical implementation of highly conformal non-convex dose distribu-tions with better surrounding critical organ sparing. But it involves the complex move-ment of multileaf collimator leaves, so pre-treatment plan verification is necessary. Ab-solute dose verification is usually carried out with the help of free air type ionization cham-bers along central axis or at any other refer-ence point. But absolute dose verification at one or two points does not suffice the purpose of plan verification because dose delivery is not uniform through out the field. Hence planned dose fluence is compared with deliv-erable dose fluence usually using film dosim-etry. Now a days many electronic systems are available in the market to overcome the film dosimetry related problems. These systems mostly include the electronic portal imaging devices (EPID) or closely spaced 2 dimen-sional arrays of vented ionization chambers or diodes. In this study we have investigated the applications of one such system contain-90 Rep pRact oncol RadiotheR • 2009 • 14/3/: 89–94ORIGInAL ARTICeLSing the 2 dimensional arrays of vented ioniza-tion chambers.AIMTo study of 2 Dimensional ion chamber array for angular response and utility of ion cham-ber array for quality assurance of dynamic multileaf collimator and pretreatment inten-sity modulated radiotherapy plans.MEtHOd And MAtErIALsLinear Accelerator Clinac DHX (Varian Medical Systems Inc., Palo Alto, CA), high energy linear accelera-tor (Linac) with photon energies 6 MV & 15 MV and fitted with Millennium 80 multileaf collimator was used in this study. Millennium 80 MLC system has 40 pairs of leaves in each bank and MLC leaf width projected at isoce-ntre is 1 cm. The MLC leaf ends are rounded. The inter leaf leakage is minimized by tongue and groove arrangement. The maximum leaf speed is 3 cm/sec at isocentre. Eclipse Planning system: 3 dimensional treatment planning system Eclipse version 7.5.51 (Varian Medical Sys-tems Inc., Palo Alto, CA) with inverse plan optimization, was used for creating different treatment fields using 6 MV photon beam. The optimal fluence profiles calculated by inverse plan optimization were used by the leaf mo-tion calculator program to produce MLC mo-tion patterns. The dose calculation was done by Pencil Beam Convolution (PBC) algorithm incorporated in the 3D-TPS. The MLC work-station records the system status (MLC leaf positions) during dMLC treatment delivery every 50 ms to calculate leaf deviation statis-tics and histogram data. All the systems were networked through networking system ARIA (Varian Medical Systems Inc., Palo Alto, CA).I’Mrt MatriXX ArrayThe relative dosimetry was performed using OmniPro I’mRT software version 1.4.3.3 (IBA Dosimetry AB, Uppsala, Sweden) with 2D ion-ization chamber array, I’mRT MatriXXTM (IBA Dosimetry AB, Uppsala, Sweden). 2D ion chamber array, I’mRT MartiXX has 24.4 x 24.4 cm2 active area and 1020 vented pixel ionization chambers with center-to-center distance of 7.62 mm (Fig. 1). The ionization chambers are cylindrical in shape with 4.5 mm diameter and 5 mm height, having total volume 0.08 cc. They are placed in a square pattern of 32 x 32 matrix. 4.5 cm and 4 cm sol-id water phantom slabs (SP34, Gammex Inc., Middleton, WI) were kept above and below the MatriXX, respectively. By putting 4.5 cm solid water phantom thickness, the effective point of measurement was at 5 cm from the top surface. Although MatriXX has sufficient inbuilt backscatter thickness (2.2 cm) for dose measurement from front side, but we kept ad-ditional 4 cm solid water phantom slabs below MatriXX for measuring its response from all directions. Radiation dose of 200 cGy was planned at iso-center from different gantry angles at an interval of 5° and corresponding reading were measured. This we carried out to check the feasibility of pretreatment IMRT QA and dose verification at actual treatment angles in spite of dose verification at gantry angle 0°. For warm up of MatriXX, 15 Gy ra-diation bath was given over 24 x 24 cm2 active area. MatriXX calibration was performed for absolute dose measurement using 0.6 cc PTW UNIDOS (PTW-FREIBURG, Germany) free air type cylindrical ionization chamber. Ap-propriate temperature and pressure correc-tion factors were also applied during absolute dose measurement. As in dynamic IMRT both dose and dose rate changes in space and in time, so MatriXX response was tested for dose linearity and dose rate dependence. The sampling time was kept 150 mSec during all measurements.Fig. 1. Transverse and coronal view of MatriXX on CT slices.Anup Kumar Bhardwaj et al • Study of 2D ion chamber array for angular response and QA of dynamic MLC...91Rep pRact oncol RadiotheR • 2009 • 14/3/: 89–94MLc and Linac QA Procedures:The MLC QA test patterns were designed for millennium 80 multileaf collimator using MLC shaper. The dynamic MLC QA test pat-terns were chair test, x–wedge, pyramid, peak test, open swipe field, garden fence and picket fence [8–10]. X-wedge test was designed us-ing five different intensity levels, in 2 cm dis-tance each. Pyramid test was designed using constant movement of opposing leaf pairs to-wards central axis. Peak test was designed using single leaf pair movement at 2.5, 5, 7.5, 10, and 15 mm gaps. Open swipe field was designed using 10 mm leaf gap with uniform MLC speed over 15 cm field width. The picket test was designed using the 15 leaf movements of 1 cm width at 1 cm interval. Garden fence test was designed using 2 mm leaf gap with 2 cm interval. Other than these MLC QA tests we also checked the performance of Dynamic wedg-es of 15°, 30°, 45° and 60° with correspond-ing physical wedges. Percentage depth dose (PDD) profile was also measured for 10 x 10 cm2 field size for 6 MV photon beam using Ma-triXX and solid water phantom slabs. The PDD values were compared with depth dose values obtained with an ion chamber RK (0.12 cc, IBA Dosimetry AB, Uppsala, Sweden) in 3D water phantom RFA-300. For all these measure-ments the gantry angle was kept 0°. For IMRT verification, five patients (7 fields each, total 35 fields) with localized prostate carcinoma were planned using dynamic IMRT technique on Eclipse treatment planning system. All test patterns and planned IMRT photon fluence were exported to CT scanned solid water phan-tom and MatriXX combination at 5 cm depth keeping SSD 95 cm. Dose calculation was done using 2.5 mm grid spacing. The MLC QA test patterns and IMRT plans were executed on 2D ion chamber array, I’mRT MartiXX.To check the angular response of MatriXX, radiation dose of 2 Gy was planned at the iso-centre from different gantry angles and cor-responding angular response of MatriXXwas checked. The treatment couch top was tennis racket type so no attenuation factor was ap-plied for radiation beams through couch.Gamma methodGamma method [11] was used to compare the planned dose distribution in TPS and deliv-ered dose distribution on MatriXX for all test patterns. Gamma (g) method is useful in mea-suring distance to agreement (DTA) in high gradient region, and sensitive to dose differ-ences between calculated and delivered plan. The value of g calculated by OmniPro I’mRT software was used for plan acceptance crite-ria. The plan was accepted only if more than 95% pixels having the value of g ≤ 1 in planned active area. The delta dose difference 3% and delta distance (DTA) 3 mm were taken for calculating gamma value.rEsULts And dIscUssIOnMatriXX dosimetric characteristicsDose response of MatriXX was linear in 0.2 Gy to 8 Gy with maximum variation of 0.01% (Fig. 2). MatriXX shows over response at low dose rates (100 MU/min) and less response at higher dose rates (600 MU/min). But this dose rate dependence was with in 0.20% in range of 1-6 Gy/min. This dependence on dose rate may be because of definite sampling time. Amerio et al (12) also checked the dose and dose rate dependence of similar system and the response of pixel ionization chambers was linear in the dose range from 0.1 to 10 Gy. PDD measurement results were quite simi-lar to RFA measurement. Maximum variation was 1.9% for 3 mm depth. On other depths the variation was less than 1% (Fig. 3).MLc QA resultsDynamic MLC QA tests such as chair, x-wedge, pyramid, and open swipe field were successfully verified. For chair test, the dose difference between TPS calculated dose and MatriXX measured dose was 0.6% at pre-scription reference point along central axis (Fig. 4). 96.2% pixels passed the gamma analysis criterion in region of interest (ROI). In the peak test, the peaks, other than 2.5, 5 and 7.5 mm were successfully detected with dose difference less than 2.3%. But dose dif-ference was 6.5% for 7.5 mm, 16.3% for 5 mm, and 39% for 2.5 mm peak. MatriXX was not able to recognize the bar pattern of picket test and garden fence test (Fig. 5). For all dynamic wedges, there was good agreement between TPS calculated and measured dose profiles. The dynamic wedge profiles were matching 92 Rep pRact oncol RadiotheR • 2009 • 14/3/: 89–94ORIGInAL ARTICeLSwith corresponding physical wedge profiles (Fig. 6). Table 1 shows the results of other QA tests and IMRT fields.Most of test patterns successfully passed the gamma analysis criterion with more than 95% pixels in defined field size. But because of spatial resolution of 7.62 mm, MatriXX was not able to recognize garden fence, picket 02004006008000 100 200 300 400 500 600 700 800Planned dose (cGy)Measued dose (cGy)Figure 2. Dose response of MatriXX over dose range 20 to 800 cGy.FigureFig. 2. Dose response of MatriXX over dose range 20 to 800 cGy.0204060801000 50 100 150 200 250 300Depth (mm)PDDRFA measuedMatriXX measuedFigure 3. PDD measurement comparison between RFA-300 and MatriXXFigureFig. 3. P  measurement comparison between RFA-300  and MatriXXFig. 4. Gamma Analysis and profile matching along X and Y direction for chair test.Fig. 5. Profile matching between TPS and MatriXX measurement along X direction for Picket and Garden fence test.Fig. 6. Gamma Analysis and profile matching along X and Y direction for 45° wedge.Anup Kumar Bhardwaj et al • Study of 2D ion chamber array for angular response and QA of dynamic MLC...93Rep pRact oncol RadiotheR • 2009 • 14/3/: 89–94fence, and 2.5 mm pattern in peak test, and was not able to reproduce the steep dose gra-dient in penumbra region. The measured dose difference was more than 3% in penumbra re-gion especially in Y direction (perpendicular to MLC leaf movement) because dose distri-bution can not be faithfully reflected by linear interpolation in high dose gradient region by MatriXX. Other than penumbra region, all pixels successfully passed the gamma analy-sis.Angular response of MatriXXThe MatriXX response shows dependence on angle of irradiation. The response of MatriXX gradually decreases from 0° to 180° angles and it was 7.7% less at 180° angle. Two sharp peaks shown in figure 7, represents the de-crease in response due to radiation beam at-tenuation through carbon fiber side railings at these angles.IMrt QA resultsFor pretreatment IMRT plan QA, the aver-age dose difference between planned and measured dose was 1.26% with standard de-viation of 1.06. On an average 95.12% pixels with SD 2.16, passed the gamma analysis test. Figure 8 shows the TPS calculated dose fluence and MatriXX measured dose fluence with gamma analysis and profile matching for one field of IMRT treatment. We also tried to perform the pretreatment IMRT QA on ac-tual treatment angles but because of angu-Type of MLC QA test % dose difference between TPS calculated and MatriXX measured dose along CAX% of pixels passing gamma criterion in defined field sizeChair 0.60 96.20x-wedge 0.50 98.28Pyramid 0.45 99.20Open swipe field 0.96 97.60Dynamic wedge 15° 0.25 98.26Dynamic wedge 30° 0.31 97.15Dynamic wedge 45° 0.51 96.89Dynamic wedge 60° 0.41 96.21IMRT fields (Total = 35)average =1.26 (SD = 1.06)average = 95.12 (SD = 2.16)Table 1. Results of different QA tests and IMRT fieldsFig. 7. Response of MatriXX with angle of irradiation0.90.920.940.960.9810 100 200 300 400Gantry angleRelative responseMatriXXresponseFigure 7. Response of MatriXX with angle of irradiationFigureFig. 8. TPS calculated and MatriXX measured dose profiles with Gamma analysis results with profile matching for one IMRT field94 Rep pRact oncol RadiotheR • 2009 • 14/3/: 89–94ORIGInAL ARTICeLSlar dependence of response of MatriXX, the variation between measured and calculated dose were more than 5%. The gamma analy-sis tests were not satisfied even for delta dose 5% and DTA 3 mm.cOncLUsIOnI’mRT MatriXX is able to measure absolute dose and dose distributions simultaneously. It can be used for routine dynamic MLC and IMRT pretreatment QA. As we have seen, its response was independent of dose and dose rate, so it has advantages in high dose regions over films. The MatriXX response is depen-dent on angle of irradiation, so it is not suitable for pretreatment IMRT QA at actual treatment angles and in rotational beam therapy such as tomotherapy. Because of its limited spatial resolution, care should be taken while taking measurements in penumbra region.references1. 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Study of 2D ion chamber array for angular response and QA of dynamic MLC and pretreatment IMRT plans

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Study of 2D ion chamber array for angular response and QA of dynamic MLC and pretreatment IMRT plans

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