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Medical Dosimetry 37 (2012) 53-60

Medical Dosimetry

journal homepage: www.meddos.org

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Characterization of responses of 2d array seven29 detector and its combined usewith octavius phantom for the patient-specific quality assurance in rapidarctreatment delivery

S. A. Syamkumar, M.Sc., Sriram Padmanabhan, M.Sc., Prabakar Sukumar, M.Sc., andVivekanandan Nagarajan, Ph.D.Department of Medical Physics, Cancer Institute (WIA), Chennai, India

A R T I C L E I N F O

Article history:

A B S T R A C T

A commercial 2D array seven29 detector has been characterized and its performance has been

Received 5 July 2010Accepted 24 December 2010

evaluated. 2D array ionization chamber equipped with 729 ionization chambers uniformly arranged ina 27 27 matrix with an active area of 27 27 cm2 was used for the study. An octagon-shapedphantom (Octavius Phantom) with a central cavity is used to insert the 2D ion chamber array. All

measurements were done with a linear accelerator. The detector dose linearity, reproducibility, outputfactors, dose rate, source to surface distance (SSD), and directional dependency has been studied. Theperformance of the 2D array, when measuring clinical dose maps, was also investigated. Forpretreatment quality assurance, 10 different RapidArc plans conforming to the clinical standards wereselected. The 2D array demonstrates an excellent short-term output reproducibility. The long-termreproducibility was found to be within 1% over a period of 5 months. Output factor measurementsfor the central chamber of the array showed no considerable deviation from ion chambermeasurements. We found that the 2D array exhibits directional dependency for static fields.Measurement of beam profiles and wedge-modulated fields with the 2D array matched very well withthe ion chamber measurements in the water phantom. The study shows that 2D array seven29 is areliable and accurate dosimeter and a useful tool for quality assurance. The combination of the 2Darray with the Octavius phantom proved to be a fast and reliable method for pretreatment verificationof rotational treatments.

2012 American Association of Medical Dosimetrists.

Keywords:2D Seven29 arrayRapidArcQuality assuranceIntroduction

The verification of radiotherapy treatment plans is a very impor-tant step in complex radiotherapy techniques because the primarygoal of radiation therapy is to deliver doses of ionizing radiation to atarget volume while minimizing the dose to critical organs andhealthy tissues. Ionization chamber array has become the standarddevice for quality assurance measurements in modern radiotherapy.Although radiographic film proved overall to be the most practicaland cost-effective method, there are some difficulties with using theverification film. The film is affected by the processor characteristicsat development time, creating the need for producing a film calibra-tion curve for each quality assurance to be performed, even for filmsfrom the same batch. The verification film response is energy depen-

Reprint requests to: S. A. Syamkumar,M.Sc., Department ofMedical Physics, Cancer

nstitute (WIA), Sardar Patel Road, Adyar, Chennai, Tamil Nadu 600036, India.

E-mail: skppm@rediffmail.com

0958-3947/$ see front matter Copyright 2012 American Association of Medical Dosimetroi:10.1016/j.meddos.2010.12.013dent and can cause dosimetry errors in measuring changing energyspectrum fields. Also film-based measurements consume more timecompared with 2D array measurements. The 2D array ionizationchamber devices are easy to use and provide quality assurance resultswhile measurements are being performed.

The complexity of treatment delivery has been increased re-cently by the implementation of intensity-modulated radiation ther-apy (IMRT), intensity-modulated arc therapy, and tomotherapy.15 It isnow possible to produce small, irregular fields and dose shaping bythe use ofmultileaf collimators (MLCs). Studies by Zelefsky et al., Kamet al., and Krueger et al. have shown clinical advantages in these newplanning and delivery techniques. Various studies have been per-formed with the 2D array seven29 for measuring IMRT delivery veri-fication.610 This complexity of clinical treatment planning and deliv-ery raises the need for more accurate dose measurement andverification systems.

In this study, we aimed to characterize the 2D array seven29 de-

tector for its response, such as linearity, reproducibility, output fac-

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S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-6054tors dependency, dose-rate dependency, and sensitivity for photonbeams. In particular, we compared the 2D array responses for static aswell as rotational deliveries. Also, the performance of the 2D array ionchamber formeasuring clinical dosemaps has been studied. Pretreat-ment patient-specific quality assurance for 10 different RapidArc(Varian Medical Systems, Palo Alto, CA) cases were done using a 2Darray combined with Octavius phantom and analyzed using the PTWVerisoft software (Freiburg, Germany).

Materials and Methods

All measurements were done on Varian Clinac 2100 C/D (Varian Medical Systems,Palo, Alto, CA) with 6- and 15-MV photons. The radiation detector, 2D seven29 ionchamber array (T10024) model (PTW) was used in this work. The 2D array consists of729 air-vented cubic ionization chambers uniformly arranged in a 27 27matrix withan active area of 27 cm2.

The ionization chambers in the 2D array are separated by 0.5 cm and the center-o-center distance between 2 adjacent chambers is 1 cm. Each chamber has a vol-me of 0.125 cm3. The linear dimensions of the 2D array are 2.2 30.0 42.0 cm3.

The detector is a pixel-segmented ionization chamber whose main features are 2Dread-out capability, large detection area, good homogeneity, and dead time-freeread-out. The 2D arrays are operated at a chamber voltage of 400 V. The referencepoint of the detector is located at 0.5 cm behind the 2D array surface. The wallmaterial is made up of graphite and the material surrounding the vented ionizationchamber is polymethyl methacrylate. Dose and dose rate mode is available for themeasurements. The measurement ranges for absolute dose is 200 mGy to 1000 Gy,

Fig. 1. Standard measurement setup for the 2D ion chamber arrayFig. 2. Verification plan window in Eclipse treatment planning system (version 8.6) for thand for dose rate measurements from 500mGy/min to 10 Gy/min as specified by themanufacturer. The 2D array is calibrated for absolute dosimetry in a Co60 photonbeam at the PTW secondary standard dosimetry laboratory. Throughout this work,the detector array was used in absolute dose measuring mode. The dose rate of themachine is kept at 300 MU/min, which is the pulse rate modemostly used in clinicalpractice, except for verification of the dose rate dependency study. PTWmatrix scansoftware was used to acquire the data from the 2D array detector. Before all mea-surements, the 2D array was calibrated by delivering a known dose of radiation fora 10 10-cm2 field size under reference conditions.

To account for the buildup and backscatter, the 2D array ion chamber array isandwiched between the virtual water phantom (Medtec Inc., Orange City, IA). Theffective point ofmeasurement of the 2D arraywas kept at 5 cmdepth from the surfacef the virtual water phantom and the source-to-surface distance (SSD) at 95 cm (Fig. 1).he detector is placed so the central axis of beam passes through the central ion cham-er (14 14). All results were compared with independent measurements using ion-zation chambers.

haracterization of 2D array seven29

erification of linearityThe dose linearity was evaluated by irradiating the 2D array for a field size of 10

0 cm2 for 6-MV and 15-MV photon beams. The output of 2D array for various monitorunits (MUs) was determined.

Verification of reproducibilityThe reproducibility is the percentage difference between consecutive measure-

ments for the same radiation dose. The performance of 2D array seven29wasmeasured

rray is sandwiched between a build-up and backscatter material.e pretreatment patient quality assurance with the 2D array and Octavius phantom.

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S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-60 55to verify short-term, over a period of hours, and long-term reproducibility, over a pe-riod of 5months. The outputwasmeasured by delivering 100MU for a fixed field size of15 15 cm2. The measurement was repeated for 10 readings.

erification of output factor dependencyThe response of a small volume ionization chamber for small field size is of greater

mportance because of its potential applications for the verification of IMRT plans.11

The performance of 2D array for radiation output was measured by delivering 100 MUfor various field sizes ranging from 2 2 cm2 to 27 27 cm2 for 6-MV and 15-MVhoton beams, respectively.

erification of dose rate dependencyThe response of 2D array toward dose rate was measured and compared for 6-MV

nd 15-MVX-rays. The detectorwas irradiated by delivering 200 cGy for a 10 10-cm2

field size at various dose rates (100, 200, 300, 400, 500, and 600 MU/min).

Verification of SSD dependencyThe SSD dependency was studied for 6-MV and 15-MV photon beams. For 100

MU and 10 10-cm2 field size, the doses were measured for different source-to-etector distance.

erification of directional dependencyThe directional dependence of the 2D array detectorwasmeasured as a function

f beam angles. An octagon-shaped phantom (Octavius phantom) having a width of 32m and a length of 32 cm made of polystyrene (physical density 1.04 g/cm3, relativelectron density 1.00) with a central cavity (30 30 2.2 cm3) was used to insert the

2D ion chamber array for the verification of directional dependency. The source todetector effective point of measurement was kept at 100 cm and the SSD at 84 cm.

The directional dependencywas studied for static delivery by keeping the 2D arrayvertically inside the Octavius phantom to avoid the radiation beampassing through thecouch. For the field size of 15 15 cm2, the readings were acquired by delivering 100U for gantry angles from 90270, with a gantry angle difference of 15. Therefore,

Fig. 3. Setup for the pretreatment quality assurance tests for RapidArc treatmentdelivery. 2D array inserted inside the Octavius phantom. SSD has been kept at 84 cm.Fig. 4. Linearity test for 6-MV and 15-MV photon beams. Values correspond to thecentral axis ionization chamber dose.in this study gantry 90 corresponds to the orthogonal beam incidence from thefront side of the array and gantry 270 corresponds to the irradiation through therear of the array. Also open dynamic arcs (gantry angle from 240120 in clockwisedirection) for 10 10-cm2 and 20 20-cm2 field sizes were delivered to check thedirectional dependency for rotational delivery.

Clinical applications

The open (20 20 cm2) and wedge field profiles were measured with a 2D array.lso, complex MLC test patterns, such as chair pattern, sweeping field, and split fields,ere done. The 2Darraymeasuredfluencewas comparedwith the treatment planningystem (TPS) calculated using the gamma analysis method.

retreatment quality assurance of RapidArc using 2D array and Octavius phantomVolumetric arc modulation using RapidArc (Varian Medical Systems) is a

ethod for delivering IMRT precisely using rotational beams within a very shorteriod than the conventional IMRT.12,13 RapidArc rotates 360 around the patient,

enabling very small beams with varying intensity to be aimed at the tumor frommultiple angles. Unlike helical IMRT treatments or other forms of radiation therapy,with RapidArc the radiation treatment delivered to the patient can be modulatedcontinuously throughout treatment because the beam is on even when the gantry ismoving.4,5

In the RapidArc approach, both the treatment planning and linac systems incorpo-rate the following capabilities: variable dose rate, variable gantry speed, DynamicMulti Leaf Collimator (DMLC) movement. Because the RapidArc delivery involvescomplex treatment delivery procedures like variable dose rate and variable gantryspeed, the patient-specific quality assurance for each patient should be done.

RapidArc treatment planningThe ability of the detector array formeasuring planar dose distributions has been

evaluated for different RapidArc cases. Ten different RapidArc plans (head and neck,esophagus, cervix) conforming to the clinical standardswere selected for this study. Allplanning was done using the Eclipse planning system (version 8.6, Varian Medica Sys-tems) using the AAA algorithm14,15 (analytical anisotropic algorithm). The optimiza-tion is based on the so-called progressive resolution optimization (PRO) algorithm.

Verification plans were done for all the cases with the 2D array and the Octaviusphantom (Fig. 2). The 2D isocenter dose planes calculated in the TPS were exported tothe Verisoft software for evaluation using the Gamma analysis method proposed byLow et al.16 The acceptance criteria of 3 mm for the distance to agreement (DTA) andose difference tolerance level of 3%were chosen. Also the percentage of the evaluatedose points passing the gamma indexwas kept at a limit 95%. The experimental setups shown in Fig. 3. The values acquired using matrix scan software correspond to theose in Gy for each ionization chamber. Verisoft software averages the calculated doseistribution automatically if a 2D array data file is loaded.

Results

Verification of linearity

From the linear response curve (Fig. 4), the results show that thedetector has a high degree of linearity within the range of 2500 MUwith a linearity coefficient R of 1.0 for both energies. Also the leastsquares fit shows that the linear relationship betweenMU and ioniza-

Fig. 5. Output factor comparison between 2D array and pinpoint chamber for 6-MVand 15-MV photons.tion chamber response is good for the lowest delivered doses.

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S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-6056Verification of reproducibility

The reproducibility of the measurements within each set is ex-cellent. Also the variation of response from chamber to chamber is1%. The 2D array demonstrates excellent short-term output re-producibility with a maximum standard deviation of 0.1%. Thelong-term reproducibility was found to be within 1% (standarddeviation) over a period of 5 months for both 6-MV and 15-MVenergies. This agrees with the results published by Spezi et al.17 Alleasurements provide a record of the relative sensitivity of each

onization chamber with respect to the central ionization chambern the array, presuming that the beam flatness and symmetry arenchanged during this period.

erification of output factor dependency

Figure 5 shows the field sizedependent output factor curve of theentral ionization chamber of the 2D array for 6-MV and 15-MV pho-on energy. The point doses were verifiedwith an ionization chambern a water phantom for the same measurement setup. Output factoreasurements for the central chamber of the array showed no con-iderable deviation from ion chamber measurements for bigger fieldizes. The small deviation observed in the higher field size is becausef the difference in the phantom used for measurements. Maximum

Fig. 6. Dose rate response curveFig. 7. SSD response curve for 6-MV and 15-MV. 2D array valueariation for chamber and 2D arraymeasurements is found to be 1.3%or 6 MV and 1.2% for 15 MV.

For field sizes below 3 3 cm2, pinpoint chambers show the cor-ect values, whereas the 2D array ionization chambers, because of theolume effects,18,19 tend to slightly underestimate the true outputactor. Thus it can be concluded that the sensitivity of the ionizationhambers in the 2D array does not have any increased energy depen-ence that could increase their response to scattered photons. Alsohe scatter property of the 2D array is nearly equal to that of water.

erification of dose rate dependency

Figure6showsthedoseratedependencycurve from100600MU/minor 6-MV and 15-MV photon energies. The results show that the 2D arrayave high dose rate independent response for the dose rates ranging from00600MU/minwith a standard deviation of0.7% for 6-MVand0.5%or15-MVphotonenergiesbetween themeasuredvalues. The resultswereompared with a 0.6-cc ionization chamber with a maximum variation of.5%with 6MVand0.9%with 15MV.

erification of SSD dependency

The responses of the detectors as a function of SSD for 6-MV and5-MV photon beams are displayed in Fig. 7. The results were com-

MV and 15-MV photon energy.s have been compared with ion chamber measurements.

6-MV

S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-60 57pared with those obtained using a farmer chamber for the samemea-surement setup. The 2D array agrees with the ionization chambermeasurementwithin 1% for the range of SSDs performed in this study.

Verification of directional dependency

The detector array shows some angular dependency when it isirradiated from the lateral as well as the bottom side. The percentagevariation for TPS calculated and 2D arraymeasured static fields wasfound to be 0.7% when the array is irradiated from the front side for6-MV photon energy and 1.49% deviation for 15-MV photon energy.Also 5.12% and 6.24% was observed for 6MV and 15MV, respectively,whereas the 2D array was irradiated parallel to the beam axis. The 2Darray predicts slightly less dose when it is irradiated through the bot-tom side. The deviation was found to be 4.9% for 6-MVand 5.41% for15-MV photon energies.

Figure 8 shows the directional dependency of 2D array vs. semi-flex ionization chamber measured for static delivery. The deviationis found to be 0.7% and 1.17% for 6 MV and 15 MV, respectively,when the array is irradiated from the front side. A maximum vari-ation of 4.65% for 6 MV and 5.14% for 15 MV were observed whenthe 2D array was irradiated parallel to the beam axis. An absolutedose deviation of 1.02% for 6 MV and 1.87% for 15 MVwas observedwhile the beam incident passing through the posterior side of thearray. Results are comparable with the published data by Van Eschet al.13

The results for directional dependency for 6-MV and 15-MV arcdeliveries are shown in Fig. 9. The measured open arcs were ana-lyzed with the TPS planned using the Gamma analysis method. Forall the arc deliveries, the TPS and the measured value agree wellwithin 3-mm DTA, 3% dose difference tolerance level (DD), for 95%of the evaluated dose points using the gamma analysis method. Thepoints where the gamma analysis fails was mainly observed in thepenumbral region.

Clinical applications

Figure 10 shows the profiles of open and different wedge-modu-

Fig. 8. Directional dependency forlated fields for 6-MV photon beams. The results have been compared twith a semiflex ionization chamber. All measured data were normal-ized to the central axis. The open beamprofile comparison shows verygood agreement with ionization chamber measurements. Similarly,wedge field profile results match very well with the ion chambermeasurements for all 4 wedges (15, 30, 45, 60).

Also chair pattern and split-fields test for MLC positions andsweep-field tests for MLC performance shows good agreement withthe calculated fluence using TPS. Figure 11, a and b shows the gammaanalysis for MLC position check and performance test, respectively.Themismatch in the gammaanalysiswas observedonly in theperiph-eral regions.

Pretreatment RapidArc quality assurance using 2D array and Octaviusphantom

Table 1 shows the result for the 10 different RapidArc plans. Theresults indicate that the measured value agrees well with that of thevalue calculated by TPS in the treated volume region. The percentagedose points failed the gammacriteria is5%, except for the 2head andneck cases, for which planning was done with double arc using theavoidance sector. Also the higher variation observed, particularly inthe low dose regions outside the treatment volume,may be caused bycomplex MLC movements with small effective openings, which willmake it more challenging to deliver the radiation.

Discussion

Characterizations of the response are very much essential for an ionchamberbased detector before their clinical use. Ion chamberbaseddetector arrays are known to have insignificant energy and dose ratedependence formegavoltage photon beams but require a large sensitivevolumewith a diameter of 5 mm for each chamber to gain a signal, andthey will therefore exhibit a volume averaging effect in steep dose gra-dient regions.18

2D array shows a very high response on short-term and long-term reproducibility, which should be essential for a dose measur-ing system. Also longer warm-up time seems to be needed toachieve more accurate results.9 Studies indicate a slight increase of

and 15-MV static photon beams.he 2D array readings as the field size is increased. This may be

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S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-6058because the amount of low-energy contamination increases asthe field size increases at a constant SSD. Furthermore, the meanphoton energy of the primary radiation beam is reduced, with in-creasing field size as the proportion of the radiation incident on thephantom caused by scatter from the beam-defining system in-creases.

2D array exhibits a considerable percentage of directional depen-dency for static fields as the array is irradiated from the bottom side andparallel to thebeamaxis. So, inpractice a correction factor for directionaldependency should be included in the measurements. For all calcula-tions in the TPS, the AAA algorithmhas been used, which provides suffi-cient accuracy by accounting for the heterogeneity correction.20,21

The advantage of air-filled detectors is their insensitivity to ra-diation damage, which is explained by Spezi et al.17 Another advan-tage of air-filled detectors surrounded by materials of low atomicnumber is that they are free from any enhanced sensitivity to low-

Fig. 9. Directional dependency for 6-MV and 15-MV arc deliveries. Gamma analysis rnalysis fails.

2Fig. 10. Dose profiles for 20 20-cm open fields for 6 MV and different wedge-modulatedith the semiflex ionization chamber measurements done in a water phantom at the same menergy scattered photons because of the photoelectric effect inmaterials of the array. The main limitations of the 2D array type10024 are the geometrical resolution of the detector, the size of thesingle detector, and the center-to-center distance between the de-tectors. This efficiency problem is common to other planar detec-tors presently available.

Themain advantage of using the 2D array for RapidArc quality assur-ance measurements is the ability to perform absolute dose compari-sons for hundreds ofmeasurement positions using only a single-beamdelivery, compared with the multiple, absolute point dose measure-ments to be done with a single ionization chamber. As our resultshows, the 2D array seven29 detector can be used for pretreatmentverification of RapidArc plans. Also it is important to normalize thedose distribution in the high-dose region and exclude the low-dosearea. Quality assurance for the same patients was also done usingother available phantoms with ion chambers and films. The direct

for cross-plane and in-plane profiles. Red color indicates the area where the gammaprofiles. (15, 30, 45, 60) for 6MV photon beams. Measured dose profiles comparedeasurement setup. Profiles are normalized to the central axis beam.

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S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-60 59comparison of gamma analysis with the other conventional methodswas not meaningful because of the difference in the size and shape ofthe phantom. The other conventional methods were only used todemonstrate whether the RapidArc plans were calculated and deliv-ered correctly. Also the measured and calculated profiles were com-

Fig. 11. (a). MLC quality assurance test using 2D array. Gamma analysis for split fieldserformance.pared qualitatively.To compensate for the anisotropic behavior of the 2D array duringarc measurements with the Octavius phantom, a 2-cm compensationcavity has been provided in the bottom part for the Octavius linacphantom. This offers a better compensation for the directional depen-dency when the array is used for the RapidArc pretreatment patient-

hair pattern test for MLC positions. (b) Gamma analysis for sweep-field tests for MLCspecific quality assurance. The reduced charge collection will be bal-

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S. A. Syamkumar et al. / Medical Dosimetry 37 (2012) 53-6060anced by the removal of the adequate phantom material. Thecompensating cavity should extend up to the sides of the array. Becauseof practical difficulties, the cavity is limited only to the bottom side.

We found from our studies that the 2D array with the Octaviusphantom can be used for online verification of the RapidArc qualityassurance, which can be done within a short period, when comparedwith the film analysis or Electronic Portal Imaging Device (EPID)mea-surements. The spatial resolution of film and EPID will be superiorwhen comparedwith the 2D arraymeasurements.22,23 However, EPIDequires correction factors for energy dependence.24 The spatial res-olution problem can be overcome in 2D array by merging the imagesin each of the measurements. Also film requires an expensive 2D filmdensity scanner and software for converting optical density to doseand comparing the 2D dose distribution to the dose distribution pro-vided by the TPS.25 Setting, acquiring data, and analyzing will takeabout 30 minutes using the 2D array.

Conclusion

On the basis of the studies performed, it can be concluded that the2D array seven29 has the necessary characteristics and can be usedefficiently in a clinical setting. The 2D array provides an overall accu-racy when compared with single ionization chamber measurementsfor static and rotational delivery. Moreover, the dose calibration forthe 2D array is easy and stable. Also our studies have shown that 2Dseven29 arraywith Octavius phantom is an efficientmethod for Rapi-dArc patient-specific quality assurance with a satisfactory accuracyfor clinical practice.

References

Table 1Gamma analysis results for pretreatment quality assurance of 10 RapidArc cases

SitesPatientNumber

Gamma Analysis for 3 mmDTA, 3% DD. % Points

Failed

Head and neck 1 4.722 2.853 5.884 7.11

Esophagus 5 1.396 1.31

Cervix 7 3.538 3.009 4.31

10 2.89

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17. Spezi, E.; Angeline, A.L.; Romani, F.; et al.Characterization of a 2D ion chamber arrayfor the verification of radiotherapy treatments. Phys. Med. Biol. 50:336173; 2005.

18. Low, D.A.; Parikh, P.; Dempsey, J.F.; et al. Ionization chamber volume averagingeffects in dynamic intensity modulated radiation therapy beams. Med. Phys. 30:170611; 2003.

19. Laub, W.U., Wong, T. The volume effect of detectors in the dosimetry of small fieldsused in IMRT.Med. Phys. 30:3417; 2003.

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Characterization of responses of 2d array seven29 detector and its combined use with octavius ph ...IntroductionMaterials and MethodsCharacterization of 2D array seven29Verification of linearityVerification of reproducibilityVerification of output factor dependencyVerification of dose rate dependencyVerification of SSD dependencyVerification of directional dependency

Clinical applicationsPretreatment quality assurance of RapidArc using 2D array and Octavius phantomRapidArc treatment planning

ResultsVerification of linearityVerification of reproducibilityVerification of output factor dependencyVerification of dose rate dependencyVerification of SSD dependencyVerification of directional dependencyClinical applicationsPretreatment RapidArc quality assurance using 2D array and Octavius phantom

DiscussionConclusionReferences