Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance

PurposeIntraoperative radiation therapy (IORT) using electron beam is commonly done by mobile dedicated linacs that have a variable range of electron energies. This paper focuses on the evaluation of the EBT2 film response in the green and red colour channels for IORT quality assurance (QA).MethodsThe calibration of the EBT2 films was done in two ranges; 0–8 Gy for machine QA by red channel and 8–24 Gy for patient-specific QA by green channel analysis. Irradiation of calibration films and relative dosimetries were performed in a water phantom. To evaluate the accuracy of the film response in relative dosimetry, gamma analysis was used to compare the results of the Monte Carlo simulation and ionometric dosimetry. Ten patients with early stage breast cancer were selected for in-vivo dosimetry using the green channel of the EBT2 film.ResultsThe calibration curves were obtained by linear fitting of the green channel and a third-order polynomial function in the red channel (R2 = 0.99). The total dose uncertainty was up to 4.2% and 4.7% for the red and green channels, respectively. There was a good agreement between the relative dosimetries of films by the red channel, Monte Carlo simulations and ionometric values. The mean dose difference of the in-vivo dosimetry by green channel of this film and the expected values was about 1.98% ± 0.75.ConclusionThe results of this study showed that EBT2 film can be considered as an appropriate tool for machine and patient-specific QA in IORT.     

Dosimetric evaluation of Gafchromic EBT2 film for breast intraoperative electron radiotherapy verification

PurposeQuality assurance (QA) is one of the most important issues that should be addressed for intraoperative electron radiotherapy (IOERT), which is not benefiting from image-based treatment planning system. The aim of this study is to evaluate the dosimetric characteristics of Gafchromic EBT2 film for breast IOERT QA procedure.MethodsDue to the fact that some dedicated accelerators are being used for IOERT, dependence of the film response to energy, field size, dose rate and incidence angle of electron beam from the LIAC IOERT accelerator was studied. Then, film response curve to breast IOERT doses was obtained and its accuracy was evaluated and justified through comparison to the results of ionometric dosimetry.ResultsThe results of this study indicated that there are no significant differences between the film responses at different energies of 6, 8, 10 and 12 MeV (P-value = 0.99). Similarly, no field size dependency was found when evaluating the response of the film to different field sizes ranging from 4 to 10 cm (P-value = 0.94). Film response was found to be independent of the dose rate of intraoperative electron beam (P-value = 0.12). Film response variations with changing the beam incidence angle were not significant (P-value > 0.8). Calibration curve at the dose range of 8–24 Gy had an acceptable accuracy. The difference between the results of film dosimetry and ionometric dosimetry was around 5% which was in agreement with the results of dose uncertainty estimation.ConclusionThe EBT2 film was found to be a potentially appropriate tool for breast IOERT verification.     

Time and frequency to observe fiducial markers in MLC-modulated fields during prostate IMRT/VMAT beam delivery

ObjectiveThis work investigates the time and frequency to observe fiducial markers in MLC-modulated fields during intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) beam delivery for real-time prostate localization.MethodsThirty seven prostate patients treated with IMRT or VMAT were included in this retrospective study. DRR images were generated for all MLC segments/control points using the TPS. The MLC leaf pattern of each control point was overlaid on the DRR, and the number of fiducials within the MLC opening was analyzed. EPID images of fiducials in a pelvic phantom were obtained to demonstrate the fiducial visibility during modulated beam delivery.ResultsGold fiducials were visible on EPID images. The probability of seeing a number of fiducials within the MLC opening was analyzed. At least one fiducial was visible during 42 ± 2% and 52 ± 2% beam-on time for IMRT of the prostate with and without lymph nodes, and during 81 ± 4% and 80 ± 5% beam-on time for VMAT of the prostate with and without lymph nodes, respectively. The mean time interval to observe at least one fiducial was 8.4 ± 0.7 and 5.9 ± 0.5 s for IMRT of the prostate with and without the lymph nodes, respectively, and 1.6 ± 0.1 s for VMAT prostate patients. The estimated potential dosimetric uncertainty was 7% and 2% for IMRT and VMAT, respectively.ConclusionsOur results demonstrated that the time and frequency to observe fiducial markers in MLC-modulated fields during IMRT/VMAT beam delivery were adequate for real-time prostate localization. The beam’s eye view fiducial positions could be used for intrafractional target monitoring and motion correction in prostate radiotherapy.     

Fast protocol for radiochromic film dosimetry using a cloud computing web application

PurposeTo investigate the feasibility of a fast protocol for radiochromic film dosimetry to verify intensity-modulated radiotherapy (IMRT) plans.Method and materialsEBT3 film dosimetry was conducted in this study using the triple-channel method implemented in the cloud computing application (Radiochromic.com). We described a fast protocol for radiochromic film dosimetry to obtain measurement results within 1 h.Ten IMRT plans were delivered to evaluate the feasibility of the fast protocol. The dose distribution of the verification film was derived at 15, 30, 45 min using the fast protocol and also at 24 h after completing the irradiation. The four dose maps obtained per plan were compared using global and local gamma index (5%/3 mm) with the calculated one by the treatment planning system. Gamma passing rates obtained for 15, 30 and 45 min post-exposure were compared with those obtained after 24 h.ResultsSmall differences respect to the 24 h protocol were found in the gamma passing rates obtained for films digitized at 15 min (global: 99.6% ± 0.9% vs. 99.7% ± 0.5%; local: 96.3% ± 3.4% vs. 96.3% ± 3.8%), at 30 min (global: 99.5% ± 0.9% vs. 99.7% ± 0.5%; local: 96.5% ± 3.2% vs. 96.3 ± 3.8%) and at 45 min (global: 99.2% ± 1.5% vs. 99.7% ± 0.5%; local: 96.1% ± 3.8% vs. 96.3 ± 3.8%).ConclusionsThe fast protocol permits dosimetric results within 1 h when IMRT plans are verified, with similar results as those reported by the standard 24 h protocol.     

A protocol for absolute dose verification of SBRT/SRS treatment plans using Gafchromic™ EBT-XD films

PurposeTo provide a practical protocol for absolute dose verification of stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) treatment plans, based on our clinical experience. It aims to be a concise summary of the main aspects to be considered when establishing an accurate film dosimetry system.MethodsProcedures for film calibration and conversion to dose are described for a dosimetry system composed of Gafchromic™ EBT-XD films and a flatbed document scanner. Factors that affect the film-scanner response are also reviewed and accounted for. The accuracy of the proposed methodology was assessed by taking a set of strips irradiated to known doses and its applicability is illustrated for ten SBRT/SRS treatment plans. The film response was converted to dose using red and triple channel dosimetry. The agreement between the planned and measured dose distributions was evaluated using global gamma analysis with criteria of 3%/2mm 10% threshold (TH), 2%/2mm 10% TH, and 2%/2mm 20% TH.ResultsThe differences between the expected and determined doses from the strips analysis were 0.9 ± 0.6% for the red channel and 1.1 ± 0.7% for the triple channel method. Regarding the SBRT/SRS plans verification, the mean gamma passing rates were 99.5 ± 1.0% vs 99.6 ± 1.0% (3%/2mm 10% TH), 96.9 ± 3.5% vs 99.1 ± 1.3% (2%/2mm 10% TH) and 98.4 ± 1.8% vs 98.8 ± 1.5% (2%/2mm 20% TH) for red and triple channel dosimetry, respectively.ConclusionsThe proposed protocol allows for accurate absolute dose verification of SBRT/SRS treatment plans, applying both single and triple channel methods. It may work as a guide for users that intend to implement a film dosimetry system.     

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

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.     

Measuring the dose in bone for spine stereotactic body radiotherapy

PurposeCurrent quality assurance of radiotherapy involving bony regions generally utilises homogeneous phantoms and dose calculations, ignoring the challenges of heterogeneities with dosimetry problems likely occurring around bone. Anthropomorphic phantoms with synthetic bony materials enable realistic end-to-end testing in clinical scenarios. This work reports on measurements and calculated corrections required to directly report dose in bony materials in the context of comprehensive end-to-end dosimetry audit measurements (63 plans, 6 planning systems).Materials and methodsRadiochromic film and microDiamond measurements were performed in an anthropomorphic spine phantom containing bone equivalent materials. Medium dependent correction factors, k_med, were established using 6 MV and 10 MV Linear Accelerator Monte Carlo simulations to account for the detectors being calibrated in water, but measuring in regions of bony material. Both cortical and trabecular bony material were investigated for verification of dose calculations in dose-to-medium (D_m,m) and dose-to-water (D_w,w) scenarios.ResultsFor D_m,m calculations, modelled correction factors for cortical and trabecular bone in film measurements, and for trabecular bone in microDiamond measurements were 0.875(±0.1%), 0.953(±0.3%) and 0.962(±0.4%), respectively. For D_w,w calculations, the corrections were 0.920(±0.1%), 0.982(±0.3%) and 0.993(±0.4%), respectively. In the audit, application of the correction factors improves the mean agreement between treatment plans and measured microDiamond dose from −2.4%(±3.9%) to 0.4%(±3.7%).ConclusionMonte Carlo simulations provide a method for correcting the dose measured in bony materials allowing more accurate comparison with treatment planning system doses. In verification measurements, algorithm specific correction factors should be applied to account for variations in bony material for calculations based on D_m,m and D_w,w.     

Optimized matching of film dosimetry with calculated doses for IMRT quality assurance

We develop a new method with a global optimization for registering films to calculate doses for intensity-modulated radiation therapy (IMRT) and intensity-modulated radiosurgery (IMRS) quality assurance (QA). Both absolute point dosimetry and two-dimensional (2D) film dosimetry are performed through the IMRT and IMRS using Clinac 21EX's 120 millenium MLC and BrainLab's micro-MLC, respectively. The measured and calculated dose distributions are superimposed by coincidence of their origins, followed by comparison of the point doses at all matched positions. Then, with the optimization algorithm the setup error of the dosimeter is corrected. An example of IMRT cases shows that the average percentage showing 3% of dose difference for 10 patients has been reduced from 19% to 9%, before and after optimization and weight, respectively. Similar results are obtained for IMRS. This method dramatically reduces the difference between measured and calculated dose distributions in all cases investigated.     

QA of dynamic MLC based on EPID portal dosimetry

PurposeDynamic delivery of intensity modulated beams (dIMRT) requires not only accurate verification of leaf positioning but also a control on the speed of motion. The latter is a parameter that has a major impact on the dose delivered to the patient. Time consumed in quality assurance (QA) procedures is an issue of relevance in any radiotherapy department. Electronic portal imaging dosimetry (EPID) can be very efficient for routine tests. The purpose of this work is to investigate the ability of our EPID for detecting small errors in leaf positioning, and to present our daily QA procedures for dIMRT based on EPID.Methods and materialsA Varian 2100 CD Clinac equipped with an 80 leaf Millennium MLC and with amorphous silicon based EPID (aS500, Varian) is used. The daily QA program consists in performing: Stability check of the EPID signal, Garden fence test, Sweeping slit test, and Leaf speed test.Results and discussionThe EPID system exhibits good long term reproducibility. The mean portal dose at the centre of a 10 × 10 cm² static field was 1.002 ± 0.004 (range 1.013–0.995) for the period evaluated of 47 weeks. Garden fence test shows that leaf position errors of up to 0.2 mm can be detected. With the Sweeping slit test we are able to detect small deviations on the gap width and errors of individual leaves of 0.5 and 0.2 mm. With the Leaf speed test problems due to motor fatigue or friction between leaves can be detected.ConclusionsThis set of tests takes no longer than 5 min in the linac treatment room. With EPID dosimetry, a consistent daily QA program can be applied, giving complete information about positioning/speed MLC.     

Comparison of patient-specific intensity modulated radiation therapy quality assurance for the prostate across multiple institutions

AimTo evaluate the success of a patient-specific intensity modulated radiation therapy (IMRT) quality assurance (QA) practice for prostate cancer patients across multiple institutions using a questionnaire survey.BackgroundThe IMRT QA practice involves different methods of dose distribution verification and analysis at different institutions.Materials and MethodsTwo full-arc volumetric modulated arc therapy (VMAT) plan and 7 fixed-gantry IMRT plan with DMLC were used for patient specific QA across 22 institutions. The same computed tomography image and structure set were used for all plans. Each institution recalculated the dose distribution with fixed monitor units and without any modification. Single-point dose measurement with a cylindrical ionization chamber and dose distribution verification with a multi-detector or radiochromic film were performed, according to the QA process at each institution.ResultsTwenty-two institutions performed the patient-specific IMRT QA verifications. With a single-point dose measurement at the isocenter, the average difference between the calculated and measured doses was 0.5 ± 1.9%. For the comparison of dose distributions, 18 institutions used a two or three-dimensional array detector, while the others used Gafchromic film. In the γ test with dose difference/distance-to-agreement criteria of 3%?3 mm and 2%?2 mm with a 30% dose threshold, the median gamma pass rates were 99.3% (range: 41.7%–100.0%) and 96.4% (range: 29.4%–100.0%), respectively.ConclusionThis survey was an informative trial to understand the verification status of patient-specific IMRT QA measurements for prostate cancer. In most institutions, the point dose measurement and dose distribution differences met the desired criteria.     

GafChromic® EBT3 films for patient specific IMRT QA using a multichannel approach

PurposeTo evaluate EBT3 for pre-treatment patient specific quality assurance (QA). The method we propose combines the experience gained in our center with the guidelines of the protocol proposed by Lewis et al. in 2012. To compare the multichannel approach with the single channel dosimetry.MethodsGafchromic® EBT3 films were irradiated both at linac and TomoTherapy and calibration curves were obtained. A series of irradiations with simple fields (uniform dose distributions on regular shaped targets) was performed. In a second stage, films were exposed to full clinical plans at linac (step and shoot IMRT and VMAT). At TomoTherapy dose maps were obtained for a clinical plan in three different coronal planes. Films were digitized using an Epson 10000XL scanner and FilmQA™ Pro software was employed for the analysis.ResultsThe measured calibration curves suggest that, at least for the two beams taken into account (6 MV linac and TomoTherapy), a single calibration can be successfully adopted for each film lot. The application of the multichannel optimization method strongly improves the results in terms of gamma passing rates of the comparison between measured and calculated maps.ConclusionsUp to now EBT films, although attractive, were not preferred for routine patient specific QA due to their complex and time consuming processing and to the challenging work of characterization. The application of the mentioned protocol, together with some additional precautions, and the adoption of the multichannel optimization dosimetry, make this detector a handy and reliable tool for patient specific QA.     

Comprehensive quality assurance phantom for the small animal radiation research platform (SARRP)

PurposeTo develop and test the suitability and performance of a comprehensive quality assurance (QA) phantom for the Small Animal Radiation Research Platform (SARRP).Methods and materialsA QA phantom was developed for carrying out daily, monthly and annual QA tasks including: imaging, dosimetry and treatment planning system (TPS) performance evaluation of the SARRP. The QA phantom consists of 15 (60 × 60 × 5 mm³) kV-energy tissue equivalent solid water slabs. The phantom can incorporate optically stimulated luminescence dosimeters (OSLD), Mosfet or film. One slab, with inserts and another slab with hole patterns are particularly designed for image QA.ResultsOutput constancy measurement results showed daily variations within 3%. Using the Mosfet in phantom as target, results showed that the difference between TPS calculations and measurements was within 5%. Annual QA results for the Percentage depth dose (PDD) curves, lateral beam profiles, beam flatness and beam profile symmetry were found consistent with results obtained at commissioning. PDD curves obtained using film and OSLDs showed good agreement. Image QA was performed monthly, with image-quality parameters assessed in terms of CBCT image geometric accuracy, CT number accuracy, image spatial resolution, noise and image uniformity.ConclusionsThe results show that the developed QA phantom can be employed as a tool for comprehensive performance evaluation of the SARRP. The study provides a useful reference for development of a comprehensive quality assurance program for the SARRP and other similar small animal irradiators, with proposed tolerances and frequency of required tests.     

Dosimetric accuracy of dual isocenter irradiation in low magnetic field resonance guided radiotherapy system for extended abdominal tumours

PurposeDue to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some treatments could require a dual-isocenter planning approach to achieve a complete target coverage and thus exploit the benefits of the online adaptation. This study evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation therapy (IMRT) delivery technique for MR-Linac.Material and MethodsDual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed to evaluate the delivery accuracy of the system. A mono-isocenter plan delivered in clinical practice has then been retrospectively re-planned with dual-isocenter technique. The dual-isocenter plan has been re-calculated and delivered on a 3-dimensional (3D) ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in terms of gamma analysis. Clinical and planning target volume (CTV and PTV respectively) coverage robustness was then investigated after the introduction of ± 2 mm and ± 5 mm positioning errors by shifting the couch.ResultsMLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter irradiation.2D/3D gamma analysis results occurred always to be above 95% if considered a gamma criteria 1%/2 mm and 1%/1 mm respectively for the 2D and 3D analysis.The mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when positioning error was introduced separately in each direction, while the maximum observed variations were 0.9% (CTV) and 3.7% (PTV).ConclusionThe dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac, achieving accurate and robust treatment strategy and improving dose conformality also in presence of targets whose extension exceeds the nominal maximum field size.     

An external dosimetry audit programme to credential static and rotational IMRT delivery for clinical trials quality assurance

PurposeExternal dosimetry audits give confidence in the safe and accurate delivery of radiotherapy. The RTTQA group have performed an on-site audit programme for trial recruiting centres, who have recently implemented static or rotational IMRT, and those with major changes to planning or delivery systems.MethodsMeasurements of reference beam output were performed by the host centre, and by the auditor using independent equipment. Verification of clinical plans was performed using the ArcCheck helical diode array.ResultsA total of 54 measurement sessions were performed between May 2014 and June 2016 at 28 UK institutions, reflecting the different combinations of planning and delivery systems used at each institution. Average ratio of measured output between auditor and host was 1.002 ± 0.006. Average point dose agreement for clinical plans was −0.3 ± 1.8%. Average (and 95% lower confidence intervals) of gamma pass rates at 2%/2 mm, 3%/2 mm and 3%/3 mm respectively were: 92% (80%), 96% (90%) and 98% (94%). Moderately significant differences were seen between fixed gantry angle and rotational IMRT, and between combination of planning systems and linac manufacturer, but not between anatomical treatment site or beam energy.ConclusionAn external audit programme has been implemented for universal and efficient credentialing of IMRT treatments in clinical trials. Good agreement was found between measured and expected doses, with few outliers, leading to a simple table of optimal and mandatory tolerances for approval of dosimetry audit results. Feedback was given to some centres leading to improved clinical practice.     

Optimized method for in vivo dosimetry with small films in pelvic IOERT for rectal cancer

PurposeIntra-Operative Electron Radiation Therapy (IOERT) is used to treat rectal cancer at our institution, and in vivo measurements with Gafchromic EBT3® films were introduced as quality assurance. The purpose of this work was to quantify the uncertainties associated with digitization of very small EBT3 films irradiated simultaneously, in order to optimize in vivo dosimetry for IOERT.MethodsFilm samples of different sizes - M1 (5×5cm²), M2 (1.5×1.5 cm²), M3 (1.0×1.5 cm²) and M4 (0.75×1.5 cm²) – were used to quantify typical variations (uncertainties) due to scanner fluctuations, misalignment, film inhomogeneity, long-term effect of film cutting, small rotations, film curling, edge effects and the influence of opaque templates. Fitting functions and temporal validity of sensitometric curves were also assessed.ResultsFilm curling, intra-film variability and scanner fluctuations are important effects that need to be minimized or considered in the uncertainty budget. Small rotations, misalignments and film cutting have little or no influence on the readings. Most fitting functions perform well, but the quantity used for dose quantification determines over- or under-valuation of dose in the long term. Edge effects and the influence of opaque templates need to be well understood, to allow optimization of methodology to the intended purpose.ConclusionThe proposed method allows practical and simultaneous digitization of up to ten small irradiated film samples, with an experimental uncertainty of 1%.     

Simplification of head and neck volumetric modulated arc therapy patient-specific quality assurance,using a Delta4 PT

Background/AimIn many facilities, intensity-modulated radiation therapy (IMRT), and volumetric modulated arc therapy (VMAT) use intensity-modulated beams, formed by a multi-leaf collimator (MLC). In IMRT and VMAT, MLC and linear accelerator errors (both geometric and dose), can significantly affect the doses administered to patients. Therefore, IMRT and VMAT treatment plans must include the use of patient-specific quality assurance (QA) before treatment to confirm dose accuracy.Materials and methodsIn this study, we compared and analyzed the results of dose verification using a multi-dimensional dose verification system Delta4 PT, an ionization chamber dosimeter, and gafchromic film, using data from 52 patients undergoing head and neck VMAT as the test material.ResultBased on the results of the absolute dose verification for the ionization chamber dosimeter and Delta4 PT, taking an axial view, the upper limit of the 95% confidence interval was 3.13%, and the lower limit was −3.67%, indicating good agreement. These results mean that as long as absolute dose verification for the axial view does not deviate from this range, Delta4 PT can be used as an alternative to an ionization chamber dosimeter for absolute dose verification. When we then reviewed dose distribution verification, the pass rate for Delta4 PT was acceptable, and was less varied than that of gafchromic film.ConclusionThis results in that provided the pass rate result for Delta4 PT does not fall below 96%, it can be used as a substitute for gafchromic film in dose distribution verification. These results indicate that patient-specific QA could be simplified.     

High throughput film dosimetry in homogeneous and heterogeneous media for a small animal irradiator

PurposeWe have established a high-throughput Gafchromic film dosimetry protocol for narrow kilovoltage beams in homogeneous and heterogeneous media for small-animal radiotherapy applications. The kV beam characterization is based on extensive Gafchromic film dosimetry data acquired in homogeneous and heterogeneous media. An empirical model is used for parameterization of depth and off-axis dependence of measured data.MethodsWe have modified previously published methods of film dosimetry to suit the specific tasks of the study. Unlike film protocols used in previous studies, our protocol employs simultaneous multi-channel scanning and analysis of up to nine Gafchromic films per scan. A scanner and background correction were implemented to improve accuracy of the measurements. Measurements were taken in homogeneous and inhomogeneous phantoms at 220 kVp and a field size of 5 × 5 mm². The results were compared against Monte Carlo simulations.ResultsDose differences caused by variations in background signal were effectively removed by the corrections applied. Measurements in homogeneous phantoms were used to empirically characterize beam data in homogeneous and heterogeneous media. Film measurements in inhomogeneous phantoms and their empirical parameterization differed by about 2%–3%. The model differed from MC by about 1% (water, lung) to 7% (bone). Good agreement was found for measured and modelled off-axis ratios.ConclusionsEBT2 films are a valuable tool for characterization of narrow kV beams, though care must be taken to eliminate disturbances caused by varying background signals. The usefulness of the empirical beam model in interpretation and parameterization of film data was demonstrated.     

Evaluation of the ArcCHECK QA system for IMRT and VMAT verification

The purposes of this study were to perform tests for the ArcCHECK QA system, and to evaluate the suitability of this system for IMRT and VMAT verification. The device was tested for short term reproducibility, dose linearity, dose rate dependence, dose per pulse dependence, field size dependence, out of field dependence and directional dependence. Eight simple plans that each used four beams of different field sizes as well as IMRT and VMAT plans for various organs of 10 patients were measured by ArcCHECK. The phantom data was then compared with ion chamber measurements and planned results. The ArcCHECK diodes performed well for all tests except directional dependence, which varies from a minimum of ?4.9% (seen only when the beam is incident on the diode at 180°) to a maximum of 9.1% (approximately at 105°). For simple plan verification, the absolute dose pass rates of γ index (3%/3 mm) were almost identical. They had an average pass rate of 94.6% ± 1.3% when the field size was ≤20 cm in the X direction (right to left direction), but the pass rate fell rapidly when the field size was >20 cm in the X direction. For all patient-specific IMRT and VMAT QA, the pass rates exceeded 95% and 93%, respectively, and high reproducibility of these results has been observed from week to week. The comparative measurements show that the ArcCHECK QA system is completely suitable for clinical IMRT and VMAT verification.