Volumetric Modulated Arc Therapy QA Research Articles

Impact of the Acuros XB spatial discretization error on ArcCHECK VMAT QA for small-field SBRT.

To evaluate the impact of the Acuros XB spatial discretization errors on ArcCHECK volumetric modulated arc therapy (VMAT) QA for small-field SBRT plans. Eighteen SBRT VMAT arcs that failed the ArcCHECK VMAT QA were retrospectively analyzed. Plan verification doses were calculated using Eclipse Acuros XB, and absolute 3%/2mm gamma passing rates were calculated to compare ArcCHECK and MapCHECK2 with MapPHAN. Verification doses were recalculated using AAA in Eclipse and with the EGSnrc Monte Carlo package. In addition, error-reduced Acuros XB doses were calculated by subdividing the entire arc into several sub-arcs ("split-arc" method), with the angular ranges of the sub-arcs optimized to balance accuracy and efficiency. Relative gamma passing rates were calculated and compared for the four methods: (1) Acuros XB; (2) AAA; (3) EGSnrc Monte Carlo; and (4) the split-arc method. The absolute gamma passing rates were below 90% for ArcCHECK and above 95% for MapCHECK2. The averaged relative gamma passing rates were (1) 84.7% for clinical Acuros XB; (2) 96.8% for AAA; (3) 98.8% for EGSnrc Monte Carlo; and (4) 96.8% for the split-arc method with 60° sub-arc angle. Compared to the clinical Acuros XB, the split-arc method improved the relative gamma passing rate by 12.1% on average. No significant difference was found between AAA and the split-arc method (p>0.05). The Acuros XB spatial discretization errors can significantly impact the ArcCHECK VMAT QA results for small-field SBRT plans. The split-arc method may be used to improve the VMAT QA results.

AAPM Task Group Report 307: Use of EPIDs for Patient-Specific IMRT and VMAT QA.

Electronic portal imaging devices (EPIDs) have been widely utilized for patient-specific quality assurance (PSQA) and their use for transit dosimetry applications is emerging. Yet there are no specific guidelines on the potential uses, limitations, and correct utilization of EPIDs for these purposes. The American Association of Physicists in Medicine (AAPM) Task Group 307 (TG-307) provides a comprehensive review of the physics, modeling, algorithms and clinical experience with EPID-based pre-treatment and transit dosimetry techniques. This review also includes the limitations and challenges in the clinical implementation of EPIDs, including recommendations for commissioning, calibration and validation, routine QA, tolerance levels for gamma analysis and risk-based analysis. Characteristics of the currently available EPID systems and EPID-based PSQA techniques are reviewed. The details of the physics, modeling, and algorithms for both pre-treatment and transit dosimetry methods are discussed, including clinical experience with different EPID dosimetry systems. Commissioning, calibration, and validation, tolerance levels and recommended tests, are reviewed, and analyzed. Risk-based analysis for EPID dosimetry is also addressed. Clinical experience, commissioning methods and tolerances for EPID-based PSQA system are described for pre-treatment and transit dosimetry applications. The sensitivity, specificity, and clinical results for EPID dosimetry techniques are presented as well as examples of patient-related and machine-related error detection by these dosimetry solutions. Limitations and challenges in clinical implementation of EPIDs for dosimetric purposes are discussed and acceptance and rejection criteria are outlined. Potential causes of and evaluations of pre-treatment and transit dosimetry failures are discussed. Guidelines and recommendations developed in this report are based on the extensive published data on EPID QA along with the clinical experience of the TG-307 members. TG-307 focused on the commercially available EPID-based dosimetric tools and provides guidance for medical physicists in the clinical implementation of EPID-based patient-specific pre-treatment and transit dosimetry QA solutions including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) treatments.

Commissioning of a Versa HDTM linear accelerator for three commercial treatment planning systems.

In a mixed‐vendor radiation oncology environment, it is advantageous if the department's treatment planning system (TPS) supports the linear accelerators of different vendors. In this publication beam data collection and modeling for the Versa HD linear accelerator in Monaco, Pinnacle, and Eclipse are discussed. In each TPS static field, Intensity‐Modulated Radiation Therapy (IMRT) step and shoot, and Volumetric‐Modulated Arc Therapy (VMAT) plans for flattened and flattening‐filter free photon beams of all available energies were evaluated for field sizes >3 × 3. To compare passing rates, identical beam model validation plans were calculated in each TPS. Eclipse, Monaco, and Pinnacle beam models passed validation measurements in homogeneous materials for a variety of treatment fields, including static, IMRT, and VMAT. In the case of Eclipse, the “dosimetric leaf gap” parameter was found to be critical for passing rates of VMAT plans. The source size parameter plays an important role as well for small fields. In the case of Pinnacle the multileaf collimator offset table needed to be optimized for better VMAT QA results. Each of the investigated treatment planning systems met the criteria to be used clinically in conjunction with Elekta Versa HD linear accelerators. It can be of great advantage to have the option to operate a TPS and linear accelerator from different vendors, as decisions surrounding linear accelerator or TPS purchases are very complicated and not just limited to technical considerations.

Multi-task autoencoder based classification-regression model for patient-specific VMAT QA

Patient-specific quality assurance (PSQA) of volumetric modulated arc therapy (VMAT) to assure accurate treatment delivery is resource-intensive and time-consuming. Recently, machine learning has been increasingly investigated in PSQA results prediction. However, the classification performance of models at different criteria needs further improvement and clinical validation (CV), especially for predicting plans with low gamma passing rates (GPRs). In this study, we developed and validated a novel multi-task model called autoencoder based classification-regression (ACLR) for VMAT PSQA. The classification and regression were integrated into one model, both parts were trained alternatively while minimizing a defined loss function. The classification was used as an intermediate result to improve the regression accuracy. Different tasks of GPRs prediction and classification based on different criteria were trained simultaneously. Balanced sampling techniques were used to improve the prediction accuracy and classification sensitivity for the unbalanced VMAT plans. Fifty-four metrics were selected as inputs to describe the plan modulation-complexity and delivery-characteristics, while the outputs were PSQA GPRs. A total of 426 clinically delivered VMAT plans were used for technical validation (TV), and another 150 VMAT plans were used for CV to evaluate the generalization performance of the model. The ACLR performance was compared with the Poisson Lasso (PL) model and found significant improvement in prediction accuracy. In TV, the absolute prediction error (APE) of ACLR was 1.76%, 2.60%, and 4.66% at 3%/3 mm, 3%/2 mm, and 2%/2 mm, respectively; whereas the APE of PL was 2.10%, 3.04%, and 5.29% at 3%/3 mm, 3%/2 mm, and 2%/2 mm, respectively. No significant difference was found between CV and TV in prediction accuracy. ACLR model set with 3%/3 mm can achieve 100% sensitivity and 83% specificity. The ACLR model could classify the unbalanced VMAT QA results accurately, and it can be readily applied in clinical practice for virtual VMAT QA.

Clinical experience with Mobius FX software for 3D dose verification for prostate VMAT plans and comparison with physical measurements

MobiusFX, an add-on software module from Mobius Medical Systems for IMRT and VMAT QA, uses measurements in linac treatment logs to calculate and verify the 3D dose delivered to patients. In this study, 10 volumetric-modulated arc therapy (VMAT) prostate plans were planned and delivered in a Varian TrueBeam linac. The plans consisted of beams with 6 and 10 MV energy and 2 to 3 arcs per plan. The average gamma value with criterion of 3% and 3mm MobiusFX and TPS: 99.96%, 2% and 2mm MobiusFX and TPS: 98.70 %. Further comparison with ArcCheck measurements was conducted.

Individual volume-based 3D gamma indices for pretreatment VMAT QA.

Although gamma analysis is still a widely accepted quantitative tool to analyze and report patient‐specific QA for intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT), the correlation between the 2D percentage gamma passing rate (%GP), and the clinical dosimetric difference for IMRT and VMAT has been questioned. The purpose of this study was to investigate the feasibility of individual volume‐based 3D gamma indices for pretreatment VMAT QA. Percentage dosimetric errors (%DE) of dose‐volume histogram metrics (includes target volumes and organ at risks) between the treatment planning system and QA‐reconstructed dose distribution, %GPs for individual volume and global gamma indices, as well their correlations and sensitivities were investigated for one‐ and two‐arc VMAT plans. The %GPs of individual volumes had a higher percent of correlation with individual 15 %DE metrics compared with global %GPs. For two‐arc VMAT at 2%/2 mm, 3%/3 mm, and 4%/4 mm criteria, individual volume %GPs were correlated with 9, 12, and 9 out of 15 %DE metrics, while global %GPs were correlated with only 2 out of 15 %DE metrics, respectively. For one‐arc VMAT at 2%/2 mm, 3%/3 mm, and 4%/4 mm criteria, individual volume %GPs were correlated with 18, 16, and 13 out of 23 %DE metrics, and global %GPs were correlated with 19, 12, and 1 out 23 %DE metrics, respectively. The area under curves (AUC) of individual volume %GPs were higher than those of global %GPs for two‐arc VMAT plans, but with mixed results for one‐arc VMAT plans. In a conclusion, the idea of individual volume %GP was created and investigated to better serve for VMAT QA and individual volume‐based %GP had a higher percent of correlation with DVH 15 %DE metrics compared with global %GP for both one‐ and two‐arc VMAT plans.

The Clinical Applications of Polymer Gel Dosimetry Using MRI.

Polymer gel dosimeters are devices that utilize the radiation-induced polymerization reactions of vinyl monomers in a gel to store information of radiation dose. They have some advantages over other dosimeters as the visual conformation and the direct read-out of three-dimensional (3D) radiation dose information for the dosimetric verification of intensity modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), and stereotactic radiotherapy (SRT) with steep dose gradients. In this report, the dosimetric uncertainties and potential for clinical applications of polymer gel dosimetry by the in-house developed 3D dose verification system for IMRT and VMAT QA is outlined.

SU-F-P-63: Individual Volume Based 3D Gamma Indices for Pretreatment VMAT QA

Purpose: To investigate the feasibility of individual volume based 3D gamma indices for pretreatment volumetric modulated arc therapy (VMAT) QA. Methods: Both model- and measurement-based VMAT QA for one- and two-arc VMAT plans were performed with COMPASS system. Percentage dosimetric errors (%DE) of dose-volume histogram (DVH) metrics between TPS and QA reconstructed dose distribution, percentage gamma passing rate (%GP) for individual volume and global gamma indices, as well as the statistical correlations between %GP and %DE were investigated. The sensitivities of different %GP were also investigated. Results: The %DE of model-based QA for most DVH metrics of VMAT plans were within 3%. There were 9, 12, and 9 out of 15 metrics %DE, and only 2 out of 15 %DE metrics were correlated with individual volume %GP and global %GP of nasopharyngeal cancer (NPC) with 2%/2 mm, 3%/3 mm and 4%/4 mm criteria, respectively. For prostate cancer, there were 18, 16 and 13 out of 23 %DE metrics, and 19, 12 and 1 out 23 of %DE metrics were correlated with individual volume %GP and global %GP with 2%/2mm, 3%/3mm and 4%/4mm criteria, respectively. The area under curves (AUC) of individual volume %GP were higher than those of global %GP for NPC patients, but with a mixed results for prostate cancer patients. Conclusion: Individual volume based %GP were strongly correlated with DVH metrics %DE compared with global %GP for both one- and two-arc VMAT plans. Sensitivity analysis indicated that individual volume gamma indices were feasible for VMAT QA.

SU‐F‐T‐294: The Analysis of Gamma Criteria for Delta4 Dosimetry Using Statistical Process Control

Purpose:To evaluate the sensitivity of gamma criteria for patient‐specific volumetric modulated arc therapy(VMAT) quality assurance of the Delta4 dosimetry program using the statistical process control(SPC) methodology.Methods:The authors selected 20 patient‐specific VMAT QA cases which were undertaken MapCHECK and ArcCHECK with gamma pass rate better than 97%. The QAs data were collected Delta4 Phantom+ and Elekta Agility six megavolts without using an angle incrementer. The gamma index(GI) were calculated in 2D planes with normalizing deviation to local dose(local gamma). The sensitivity of the GI methodology using criterion of 3%/3mm, 3%/2mm and 2%/3mm was analyzed with using process acceptability indices. We used local confidence(LC) level, the upper control limit(UCL) and lower control limit(LCL) of I‐MR chart for process capability index(Cp) and a process acceptability index (Cpk).Results:The lower local confidence levels of 3%/3mm, 3%/2mm and 2%/3mm were 92.0%, 83.6% and 78.8% respectively. All of the calculated Cp and Cpk values that used LC level were under 1.0 in this study. The calculated LCLs of I‐MR charts were 89.5%, 79.0% and 70.5% respectively. These values were higher than 1.0 which means good quality of QA. For the generally used lower limit of 90%, we acquired over 1.3 of Cp value for the gamma index of 3%/3mm and lower than 1.0 in the rest of GI.Conclusion:We applied SPC methodology to evaluate the sensitivity of gamma criteria and could see the lower control limits of VMAT QA for the Delta 4 dosimetry and could see that Delta 4 phantom+ dosimetry more affected by the position error and the I‐MR chart derived values are more suitable for establishing lower limits.Acknowledgement: This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (No. 2015R1D1A1A01060463)

SU‐F‐T‐463: Light‐Field Based Dynalog Verification

Purpose:To independently verify leaf positions in so‐called dynalog files for a Varian iX linac with a Millennium 120 MLC. This verification provides a measure of confidence that the files can be used directly as part of a more extensive intensity modulated radiation therapy / volumetric modulated arc therapy QA program.Methods:Initial testing used white paper placed at the collimator plane and a standard hand‐held digital camera to image the light and shadow of a static MLC field through the paper. Known markings on the paper allow for image calibration. Noise reduction was attempted with removal of ‘inherent noise’ from an open‐field light image through the paper, but the method was found to be inconsequential. This is likely because the environment could not be controlled to the precision required for the sort of reproducible characterization of the quantum noise needed in order to meaningfully characterize and account for it. A multi‐scale iterative edge detection algorithm was used for localizing the leaf ends. These were compared with the planned locations from the treatment console.Results:With a very basic setup, the image of the central bank A leaves 15–45, which are arguably the most important for beam modulation, differed from the planned location by [0.38±0.28] mm. Similarly, for bank B leaves 15–45 had a difference of [0.42±0.28] mmConclusion:It should be possible to determine leaf position accurately with not much more than a modern hand‐held camera and some software. This means we can have a periodic and independent verification of the dynalog file information. This is indicated by the precision already achieved using a basic setup and analysis methodology. Currently, work is being done to reduce imaging and setup errors, which will bring the leaf position error down further, and allow meaningful analysis over the full range of leaves.

SU‐E‐T‐115: Analysis of Patient Specific QA for VMAT by Disease Site and Planning‐Delivery System Using the ScandiDos Delta4 Phantom

Purpose:To evaluate patient specific quality assurance (PSQA) for the delivery of volumetric modulated arc therapy (VMAT) by disease site. To compare planning‐delivery system (PDS) PSQA pass rates in a dual vendor institution.Methods:PSQA is performed for VMAT plans using a ScandiDos Delta4 phantom. Verification plans are calculated using Varian Eclipse and Elekta Monaco treatment planning systems (TPS) for patients treated using Varian Truebeam and Elekta linear accelerators respectively. Individual arcs are delivered to the Delta4 phantoms and assessed using the gamma index pass criterion(3% Dose Deviation(DD%), 3mm Distance to Agreement(DTA),10% dose threshold and 90% gamma index).Results:A total of 287 VMAT plans and 680 arcs were analyzed. The passing rates for VMAT QA plans were 95% and 98% for head/neck and pelvis/prostate plans respectively, and 100% for chest/abdomen, spine, lung Stereotactic Body Radiotherapy (SBRT) and Stereotactic Radiosurgery(SRS) plans. Average gamma indices were: (99 ± 2) % for pelvis/prostate, chest/abdomen and lung SBRT plans, (97 ± 4) % for head and neck plans and (98 ± 3) % for spine plans. The average DD% and DTA pass rates ranged from 82% to 90% and 98% to 99% respectively for plans in different disease sites. Paired t‐test analysis (two tails) indicated no significant differences in the gamma indices between plans delivered using different PDS; the P values were: 0.08, 0.45, and 0.94 for lung SBRT, head/neck and pelvis/prostate plans respectively. The statistical power for comparing PDS in different disease sites with an alpha of 0.05 is 1.Conclusion:The Gamma indices based on 3% DD%, 3 mm DTA and 10% dose threshold for the VMAT QA plans in all disease sites were well above 90%, suggesting the possibility of using a more stringent PSQA criterion. No significant differences were observed in the QA of VMAT plans delivered using different PDS.

SU-E-T-105: Development of 3D Dose Verification System for Volumetric Modulated Arc Therapy Using Improved Polyacrylamide-Based Gel Dosimeter

Purpose: The aim of this dosimetric study was to develop 3D dose verification system for volumetric modulated arc therapy (VMAT) using polyacrylamide-based gel (PAGAT) dosimeter improved the sensitivity by magnesium chloride (MgCl2). Methods: PAGAT gel containing MgCl2 as a sensitizer was prepared in this study. Methacrylic-acid-based gel (MAGAT) was also prepared to compare the dosimetric characteristics with PAGAT gel. The cylindrical glass vials (4 cm diameter, 12 cm length) filled with each polymer gel were irradiated with 6 MV photon beam using Novalis Tx linear accelerator (Varian/BrainLAB). The irradiated polymer gel dosimeters were scanned with Signa 1.5 T MRI system (GE), and dose calibration curves were obtained using T2 relaxation rate (R2 = 1/T2). Dose rate (100-600 MU min−1) and fractionation (1-8 fractions) were varied. In addition, a cubic acrylic phantom (10 × 10 × 10 cm3) filled with improved PAGAT gel inserted into the IMRT phantom (IBA) was irradiated with VMAT (RapidArc). C-shape structure was used for the VMAT planning by the Varian Eclipse treatment planning system (TPS). The dose comparison of TPS and measurements with the polymer gel dosimeter was accomplished by the gamma index analysis, overlaying the dose profiles for a set of data on selected planes using in-house developed software. Results: Dose rate and fractionation dependence of improved PAGAT gel were smaller than MAGAT gel. A high similarity was found by overlaying the dose profiles measured with improved PAGAT gel dosimeter and the TPS dose, and the mean pass rate of the gamma index analysis using 3%/3 mm criteria was achieved 90% on orthogonal planes for VMAT using improved PAGAT gel dosimeter. Conclusion: In-house developed 3D dose verification system using improved polyacrylamide-based gel dosimeter had a potential as an effective tool for VMAT QA.

A Comparison Study of Volumetric Modulated Arc Therapy Quality Assurances Using Portal Dosimetry and MapCHECK 2

A Varian Portal Dosimetry system was compared to an isocentrica lly mounted MapCHECK 2 diode array for volumetric modulated arc therapy (VMAT) QA. A Varian TrueBeam STx with an aS-1000 digital imaging panel was used to acquire VMAT QA images for 13 plans using four photon energies (6, 8, 10 and 15 MV). The EPID-based QA images were compared to the Portal Dose Image Prediction calculated in the Varian Eclipse treatment planning system (TPS). An isocentrically mounted Sun Nuclear MapCHECK 2 diode array with 5 cm water-equivalent buildup was also used for the VMAT QAs and the measurements were compared to a composite dose plane from the Eclipse TPS. A γtest was implemented in the Sun Nuclear Patient software with 10% threshold and absolute comparison at 1%/1 mm (dose difference/distance-to-agreement), 2%/2 mm, and 3%/3 mm criteria for both QA methods. The two-tailed paired Student’s t-test was employed to analyze the statistical significance at 95% confidence level. The average γpassing rates were greater than 95% at 3%/3 mm using both methods for all four energies. The differences in the average passing rates between the two methods were within 1.7% and 1.6% of each other when analyzed at 2%/2 mm and 3%/3 mm, respectively. The EPID passing rates were somewhat better than the MapCHECK 2 when analyzed at 1%/1 mm; the difference was lower for 8 MV and 10 MV. However, the differences were not statistically significant for all criteria and energies (p-values ＞0.05). The EPID-based QA showed large off-axis over-response and dependence of γpassing rate on energy, while the MapCHECK 2 was susceptible to the MLC tongue-and-groove effect. The two fluence-based QA techniques can be an alternative tool of VMAT QA to each other, if the limitations of each QA method (mechanical sag, detector response, and detector alignment) are carefully considered.

SU-E-T-155: RapidArc Delivery Performance Using EDR2 Film and EPID Measurement

Purpose: The Varian RapidArc is a system for volumetric modulated arc therapy (VMAT) technique. To obtain the accuracy,efficiency and reliability of the optimal conformal dose during treatment, RapidArc delivery depends on the controlling capabilities and synchronicity such as variable dose rate, variable gantry speed and dynamic multileaf collimator(DMLC). This study describes the development and implementation EDR2 film and electronic portal imaging device(EPID) during commissioning and quality assurance procedures for RapidArc radiotherapy delivery system. Methods: The tests were designed to evaluate RapidArc performance using EDR2 film and EPID tools. The accuracy of DMLC position during gantry rotation, the ability to vary and control the dose‐rate and gantry speed, the synchronization of variable DMLC speed and dose‐rate were examined. Results: The picket fence test of MLC in stationary gantry and RapidArc modes were implemented. EDR2 film and EPID showed good consistency for obtaining the patterns, and the minimal analysis of leaf accuracy test measured with film was better than 0.4mm with EPID. The synchronicity test, a fully dynamic field, was designed to uniformly irradiate seven parts of a square field using different combinations of gantry speed and dose rate. Comparing with a corresponding open field for the influence of beam flatness and asymmetry, both EDR2 film and EPID were found to yield good results, with a mean deviation of 0.24% vs 0.19%. During evaluating accurate correlated motion of DMLC speed, gantry speed and dose‐rates, the dose of four parts in a field showed good agreement, with a mean deviation of 0% vs 0.01%. The analysis of dose patterns was less than 2%. Conclusion: This study demonstrated that EDR2 film and EPID system can be used as reliable and efficient quality assurance tools for RapidArc delivery performance. Of course, the use of VMAT QA measurement with EPID increases the efficiency of routine QA.

A real time dose monitoring and dose reconstruction tool for patient specific VMAT QA and delivery

To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific volumetric modulated arc therapy (VMAT) delivery and quality assurance. The authors develop a VMAT delivery monitor tool called linac data monitor that connects to the linac in clinical mode and records, displays, and compares real time machine parameters with the planned parameters. A new measure, called integral error, keeps a running total of leaf overshoot and undershoot errors in each leaf pair, multiplied by leaf width, and the amount of time during which the error exists in monitor unit delivery. Another tool reconstructs Pinnacle(3)™ format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy. Delivery characteristics of various standard fractionation and stereotactic body radiation therapy (SBRT) VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified. The MLC and gantry errors for all the treatment sites were 0.00 ± 0.59 mm and 0.05 ± 0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than SBRT plans due to frequent dose rate changes. On average, the MLC errors were negligible but larger errors of up to 6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (r = 0.858, p = 0.0004). PTV mean, minimum, and maximum dose discrepancies were 0.87 ± 0.21%, 0.99 ± 0.59%, and 1.18 ± 0.52%, respectively. The organs at risk (OAR) doses were within 2.5%, except some OARs that showed up to 5.6% discrepancy in maximum dose. Real time displayed normalized total positive integral error (normalized to the total monitor units) correlated linearly with MLC (r = 0.9279, p < 0.001) and gantry errors (r = 0.742, p = 0.005). There is a strong correlation between total integral error and PTV mean (r = 0.683, p = 0.015), minimum (r = 0.6147, p = 0.033), and maximum dose (r = 0.6038, p = 0.0376). Errors may exist during complex VMAT planning and delivery. Linac data monitor is capable of detecting and quantifying mechanical and dosimetric errors at various stages of planning and delivery.

Comparison of action levels for patient-specific quality assurance of intensity modulated radiation therapy and volumetric modulated arc therapy treatments

To perform a comprehensive and systematic comparison of fixed-beam IMRT and volumetric modulated arc therapy (VMAT) patient-specific QA measurements for a common set of geometries using typical measurement methods. Fixed-beam IMRT and VMAT plans were constructed for structure set geometries provided by AAPM Task Group 119. The plans were repeatedly delivered across multiple measurement sessions, and the resulting dose distributions were measured with (1) radiochromic film and ionization chamber and (2) a commercial two-dimensional diode array. The resulting QA measurements from each delivery technique were then analyzed, compared, and tested for statistically significant differences. Although differences were noted between QA results for some plans, neither modality showed consistently better agreement of measured and planned doses: of the 22 comparisons, IMRT showed better QA results in 11 cases, and VMAT showed better QA results in 11 cases. No statistically significant differences (p < 0.05) between IMRT and VMAT QA results were found for point doses measured with an ionization chamber, planar doses measured with radiochromic film, or planar doses measured with a two-dimensional diode array. These results suggest that it is appropriate to apply patient-specific QA action levels derived from fixed-beam IMRT to VMAT.

SU‐E‐T‐349: Variation in VMAT QA Pass Rate Caused by Detector Setup Errors

Purpose: To test the sensitivity of pass rates of Gamma Index to the detector setup errors in six degrees of freedom. Methods: VMAT (Volumetric Modulated Arc Therapy) treatment technique is used in our clinic on an Elekta Beam Modulator with a HexaPOD couch. The HexaPOD system is a robotic patient positioning platform with six degrees of freedom. ScandiDos Delta4 system is used for our VMAT patient QAs. Delta4 is a pseudo 3D diode detector array which is capable of calculating 3D Gamma Index. Seven prostate and four H&amp;N VMAT treatment plans were delivered. The measurements were compared with the Pinnacle TPS calculations and the pass rates of Gamma Index were calculated. Then the HexPod couch was shifted or rotated either by 1 mm or 1 degree in any of the six degrees of freedom to simulate the setup errors, and the pass rates were re‐measured and re‐calculated. The criterion used for Gamma Index calculation was 3% and 3mm. A detector point is considered pass if the calculated Gamma Index is smaller than 1. Pass rates of Gamma Index with different detector setup errors in the six degrees of freedom were compared. Results: Regarding the rotational setup error of different directions, the changes of pass rates of Gamma Index are significantly different (p &lt; 0.001). While one degree change in Roll angle causes the least change of the pass rates of Gamma Index, one degree change in Pitch angle causes the biggest change. Regarding the translational setup error of different directions, the changes of pass rates of Gamma Index are insignificant (p &gt; 0.1). Conclusions: Although 1 mm or 1 degree machine‐type tolerance was recommended by AAPM TG reports, they don't translate to the same amount of changes in pass rates of Gamma Index for VMAT QA.

Dosimetric validation of a redundant independent calculation software for VMAT fields

A redundant independent dosimetric calculation (RIDC) prior to treatment has become a basic part of the QA process for 3D conventional radiotherapy, and is strongly recommended in several international publications. On the other hand, the rapid growth in the number of intensity modulated treatments has led to a significant increase in the workflow associated with QA treatments. Diamond (“K&S Associates”) is RIDC software which is capable of calculating VMAT (Volumetric Modulated Arc Therapy) fields. Modeling, validation and commissioning are necessary steps thereby making it a useful tool for VMAT QA. In this paper, a procedure for the validation of the calculation algorithm is demonstrated. A set 3D conventional field was verified in two ways: firstly, a comparison was made between Diamond calculations and experimental measures obtaining an average deviation of −0.1 ± 0.7%(1SD), and secondly, a comparison made between Diamond and the treatment planning system (TPS) Eclipse, obtaining an average deviation of 0.4 ± 0.8%(1SD). For both steps, a plastic slab phantom was used. VMAT validation was carried out by analyzing 59 VMAT plans in two ways: first, Diamond calculation versus experimental measurement with an average deviation of −0.2 ± 1.7%(1SD), and second, Diamond calculation versus TPS calculation with an average deviation of 0.0 ± 1.6%(1SD). In this phase a homogeneous cylindrical phantom was used. These results led us to consider this calculation algorithm validated for use in VMAT verifications.

Evaluation of a 2D detector array for patient-specific VMAT QA with different setups

For pre-treatment plan verification of advanced treatment techniques such as intensity-modulated arc therapy, a fast and reliable dosimetric device is required. In this study, we investigated the suitability of MatriXX in different setups for verification of volumetric modulated arc therapy (VMAT) plans. If MatriXX is used in a stationary phantom (MULTICube), the measured dose is dependent on the beam angle. For the first setup (MatriXX/MULTICube), we developed correction factors (CFs) for each detector element (1020 CFs). We investigated the accuracy of these CFs by verifying 12 VMAT plans. In the second setup, we also assessed the suitability of MatriXX in a dedicated holder. Using this setup (MatriXX/Holder), 30 additional VMAT plans were verified. Deviations of up to ∼17% and ∼11% were noted for one of the ion chambers at 90° and 180° gantry positions. The influence of the beam angle dependence (MULTICube) can explicitly be seen when a gamma criterion of 2%/2 mm was chosen. An overall improvement of 4.3% of passing pixels (pp) was noted after applying beam angular-dependent CFs. When the gamma criterion was 3%/3 mm, the %pp was ⩾95% without and ∼100% with correction. With the second setup, MatriXX/holder, we showed excellent agreement between measurements and calculations. The %pp averaged over all plans (30 VMAT treatment plans) was nearly ∼100%. The combination of MatriXX with MULTICube or with holder proved to be a fast and reliable method for pretreatment verification of arc therapy with sufficient accuracy.