Patient Specific QA Measurements Research Articles

The effect of dose gradients on gamma comparison insensitivity in patient specific QA comparisons.

While many have speculated on the reasons for gamma comparison insensitivity for patient-specific quality assurance analysis, the true reasons for insensitivity have not yet been elucidated. Failing to understand the reasons for this technique's insensitivity limits our ability to either improve the gamma metric to increase sensitivity of the comparison or the capacity to develop new comparison techniques that circumvent the limitations of the gamma comparison. To understand the underlying cause(s) for gamma comparison insensitivity and determine if simple plan characteristics can quantitatively predict for gamma comparison sensitivity. Known MLC and MU errors of varying magnitudes were induced on simple test fields to preliminarily investigate where gamma failures first begin to appear as error magnitude is increased. Gamma value maps between error-induced plan calculations and error-free plan calculations were created for 20 IMRT and 20 VMAT cases, each on three different detector geometries-ArcCHECK, MapCHECK, and Delta4. Gamma value maps were qualitatively compared to dose-gradient maps, and quantitative comparisons were performed between various plan descriptors and the computed gamma sensitivity for five different classes of induced errors were utilized to determine if any plan descriptor could predict the gamma sensitivity on a case-by-case basis. All comparisons were performed in a calculation-only scenario to remove uncertainties introduced by comparisons made with real patient specific QA measurements. Gamma value maps with increasing induced error magnitude illustrated that gamma comparisons fail first in high-dose, low-gradient regions of the field. Conversely, in areas of high gradient, gamma values typically remain low, even in the presence of large errors, regardless of detector geometry and gamma normalization setting. Thus, the complex, and often overlapping, high dose gradients in plans appear to be a limiting factor in gamma comparison sensitivity as the number of points along these gradients may often outnumber the points available for failing the comparison in lower gradient regions of the field. None of the simple plan descriptors studied were able to quantitively predict gamma comparison sensitivity, suggesting that quantitatively predicting the sensitivity of gamma comparisons on a case-by-case basis may require a combination of multiple factors or metrics not studied here. Simple plan descriptors and the number of points in high-dose, low-gradient regions of the field did not quantitively predict for gamma comparison sensitivity. However, it is clear from gradient and gamma value maps that gamma comparisons fail first in high-dose, low-gradient regions of the field in the presence of known induced errors, which we have shown to be independent of detector geometry and gamma comparison normalization setting. Gamma comparison sensitivity is thus limited by the ever-increasing complexity of plans and is particularly important to consider as treatment volumes become smaller and the complexity of overlapping plan gradients increases. This suggests that new methods for patient-specific QA comparisons are required to circumvent this limitation.

The use of a mini-ridge filter with cyclotron-based pencil beam scanning proton therapy.

Pencil beam scanning (PBS) proton therapy allows for far superior dose conformality compared with passive scattering techniques. However, one drawback of PBS is that the beam delivery time can be long, particularly when treating superficial disease. Minimizing beam delivery time is important for patient comfort and precision of treatment delivery. Mini-ridge filters (MRF) have been shown to reduce beam delivery time for synchrotron-based PBS. Given that cyclotron systems are widely used in proton therapy it is necessary to investigate the potential clinical benefit of mini-ridge filters in such systems. To demonstrate the clinical benefit of using a MRF to reduce beam delivery time for patients with large target volumes and superficial disease in cyclotron-based PBS proton therapy. A MRF beam model was generated by simulating the effect of a MRF on our clinical beam data assuming a fixed snout position relative to the isocenter. The beam model was validated with a series of measurements. The model was used to optimize treatment plans in a water phantom and on six patient DICOM datasets to further study the effect of the MRF and for comparison with physician-approved clinical treatment plans. Beam delivery time was measured for six plans with and without the MRF to demonstrate the reduction achievable. Plans with and without MRF were reviewed to confirm clinical acceptability by a radiation oncologist. Patient-specific QA measurements were carried out with a two-dimensional ionization chamber array detector for one representative patient's plan optimized with the MRF beam model. Results show good agreement between the simulated beam model and measurements with mean and maximum deviations of 0.06mm (0.45%) and 0.61mm (4.9%). The increase in Bragg peak width (FWHM) ranged from 2.7mm at 226MeV to 6.1mm at 70 MeV. The mean and maximum reduction in beam delivery time observed per field was 29.1s (32.2%) and 79.7s (55.3%). MRFs can be used to reduce treatment time in cyclotron-based PBS proton therapy without sacrificing plan quality. This is particularly beneficial for patients with large targets and superficial disease such as in breast cancer where treatment times are generally long, as well as patients treated with deep inspiration breath hold (DIBH).

Evaluation of Patient-Specific Quality Assurance for Carbon Ion Radiotherapy Using Full Energy Scanning Method at QST Hospital

Purpose: Patient-specific QA (PSQA) measurements for carbon ion radiotherapy (CIRT) are critical components of processes designed to identify discrepancies between calculated and delivered doses. We report the results of PSQA conducted at the QST Hospital during the period from September 2017 to March 2018. Methods: We analyzed PSQA results for 1448 fields for 10 disease sites with various target volumes, target depths and number of energy layers. For the PSQA, all the planned beams were recalculated on a water phantom with treatment planning software. The recalculated dose distributions were compared with the measured distributions using a 2D ionization chamber array at three depths, including 95% of the area of the prescription dose. These recalculated dose distributions were evaluated using the 3%/3mm gamma index with a passing threshold of 90%. Results: The passing rates for prostate, head and neck, and bone and soft tissue were 96.8%, 99.3%, and 91.7%, respectively. Additionally, 94.7% of lung plans with low energy beams passed. Overall, the CIRT in the QST Hospital reached a high passing rate of more than 95%. Although the remaining 5% failed to pass, there was no dependence between measurement depth and disease sites in these failures. Conclusion: Using PSQA measurements, we confirmed consistency between the planned and delivered doses for CIRT using the full energy scanning method.

PO-1598 Field size dependence of in iPlan patient specific QA measurements

PO-1598 Field size dependence of in iPlan patient specific QA measurements

Patient‐specific quality assurance using machine log files analysis for stereotactic body radiation therapy (SBRT)

An in‐house trajectory log analysis program (LOGQA) was developed to evaluate the delivery accuracy of volumetric‐modulated arc therapy (VMAT) for stereotactic body radiation therapy (SBRT). Methods have been established in LOGQA to provide analysis on dose indices, gantry angles, and multi‐leaf collimator (MLC) positions. Between March 2019 and May 2020, 120 VMAT SBRT plans of various treatment sites using flattening filter‐free (FFF) mode were evaluated using both LOGQA and phantom measurements. Gantry angles, dose indices, and MLC positions were extracted from log and compared with each plan. Integrated transient fluence map (ITFM) was reconstructed from log to examine the deviation of delivered fluence against the planned one. Average correlation coefficient of dose index versus gantry angle and ITFM for all patients were 1.0000, indicating that the delivered beam parameters were in good agreement with planned values. Maximum deviation of gantry angles and monitor units (MU) of all patients were less than 0.2 degree and 0.03 % respectively. Regarding MLC positions, maximum and root‐mean‐square (RMS) deviations from planned values were less than 0.6 mm and 0.3 mm respectively, indicating that MLC positions during delivery followed planned values in precise manner. Results of LOGQA were consistent with measurement, where all gamma‐index passing rates were larger than 95 %, with 2 %/2 mm criteria. Three types of intentional errors were introduced to patient plan for software validation. LOGQA was found to recognize the introduced errors of MLC positions, gantry angles, and dose indices with magnitudes of 1 mm, 1 degree, and 5 %, respectively, which were masked in phantom measurement. LOGQA was demonstrated to have the potential to reduce or even replace patient‐specific QA measurements for SBRT plan delivery provided that the frequency and amount of measurement‐based machine‐specific QA can be increased to ensure the log files record real values of machine parameters.

Automation of Monte Carlo-based treatment plan verification for proton therapy.

PurposeIndependent calculations of proton therapy plans are an important quality control procedure in treatment planning. When using custom Monte Carlo (MC) models of the beamline, deploying the calculations can be laborious, time consuming, and require in‐depth knowledge of the computational environment. We developed an automated framework to remove these barriers and integrate our MC model into the clinical workflow.Materials and MethodsThe Eclipse Scripting Application Programming Interface was used to initiate the automation process. A series of MATLAB scripts were then used for preprocessing of input data and postprocessing of results. Additional scripts were used to monitor the calculation process and appropriately deploy calculations to an institutional high‐performance computing facility. The automated framework and beamline models were validated against 160 patient specific QA measurements from an ionization chamber array and using a ±3%/3 mm gamma criteria.ResultsThe automation reduced the human‐hours required to initiate and run a calculation to 1–2 min without leaving the treatment planning system environment. Validation comparisons had an average passing rate of 99.4% and were performed at depths ranging from 1 to 15 cm.ConclusionAn automated framework for running MC calculations was developed which enables the calculation of dose and linear energy transfer within a clinically relevant workflow and timeline. The models and framework were validated against patient specific QA measurements and exhibited excellent agreement. Before this implementation, execution was prohibitively complex for an untrained individual and its use restricted to a research environment.

EP-2072 Automatic analyzation of patient specific QA measurements made with the Octavius verification device

EP-2072 Automatic analyzation of patient specific QA measurements made with the Octavius verification device

Design and clinical use of a rotational phantom for dosimetric verification of IMRT/VMAT treatments

PurposeTo describe the design and clinical use of a rotational phantom for dosimetric verification of IMRT/VMAT treatment plans using radiochromic film. MethodsA solid water cylindrical phantom was designed with separable upper and lower halves and rests on plastic bearings allowing for 360° rotation about its central axis. The phantom accommodates a half sheet of radiochromic film, and by rotating the cylinder, the film can be placed in any plane between coronal and sagittal. Calculated dose planes coinciding with rotated film measurements are exported by rotating the CT image and dose distribution within the treatment planning system. The process is illustrated with 2 rotated film measurements of an SRS treatment plan involving 4 separate targets. Additionally, 276 patient specific QA measurements were obtained with the phantom and analyzed with a 2%/2 mm gamma criterion. ResultsThe average 2%/2 mm gamma passing rate for all 276 plans was 99.3%. Seventy-two of the 276 plans were measured with the plane of the film rotated between the coronal and sagittal planes and had an average passing rate of 99.4%. ConclusionsThe rotational phantom allows for accurate film measurements in any plane. With this technique, regions of a dose distribution which might otherwise require multiple sagittal or coronal measurements can be verified with as few as a single measurement. This increases efficiency and, in combination with the high spatial resolution inherent to film dosimetry, makes the rotational technique an attractive option for patient-specific QA.

Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218.

Patient-specific IMRT QA measurements are important components of processes designed to identify discrepancies between calculated and delivered radiation doses. Discrepancy tolerance limits are neither well defined nor consistently applied across centers. The AAPM TG-218 report provides a comprehensive review aimed at improving the understanding and consistency of these processes as well as recommendations for methodologies and tolerance limits in patient-specific IMRT QA. The performance of the dose difference/distance-to-agreement (DTA) and γ dose distribution comparison metrics are investigated. Measurement methods are reviewed and followed by a discussion of the pros and cons of each. Methodologies for absolute dose verification are discussed and new IMRT QA verification tools are presented. Literature on the expected or achievable agreement between measurements and calculations for different types of planning and delivery systems are reviewed and analyzed. Tests of vendor implementations of the γ verification algorithm employing benchmark cases are presented. Operational shortcomings that can reduce the γ tool accuracy and subsequent effectiveness for IMRT QA are described. Practical considerations including spatial resolution, normalization, dose threshold, and data interpretation are discussed. Published data on IMRT QA and the clinical experience of the group members are used to develop guidelines and recommendations on tolerance and action limits for IMRT QA. Steps to check failed IMRT QA plans are outlined. Recommendations on delivery methods, data interpretation, dose normalization, the use of γ analysis routines and choice of tolerance limits for IMRT QA are made with focus on detecting differences between calculated and measured doses via the use of robust analysis methods and an in-depth understanding of IMRT verification metrics. The recommendations are intended to improve the IMRT QA process and establish consistent, and comparable IMRT QA criteria among institutions.

Performance of a PTW 60019 microDiamond detector in a 1.5 T MRI-linac

Accurate small-field dosimetry is critical for a magnetic resonance linac (MRI-linac). The PTW 60019 microDiamond is close to an ideal detector for small field dosimetry due to its small physical size, high signal-to-noise ratio and approximate water equivalence. It is important to fully characterise the performance of the detector in a 1.5 T magnetic field prior to its use for MRI-linac commissioning and quality assurance.Standard techniques of detector testing have been implemented, or adapted where necessary to suit the capabilities of the MRI-linac. Detector warmup, constancy, dose linearity, dose rate linearity, field size dependence and leakage were within tolerance. Measurements with the detector were consistent with ion chamber measurements for medium sized fields. The effective point of measurement of the detector when used within a 1.5 T magnetic field was determined to be 0.80 ± 0.23 mm below the top surface of the device, consistent with the existing vendor recommendation and alignment mark at 1.0 mm. The angular dependence was assessed. Variations of up to 9.7% were observed, which are significantly greater than in a 0 T environment. Within the expected range of use, the maximum effect is approximately 0.6% which is within tolerance. However for large beams within a magnetic field, the divergence and consequent variation in angle of photon incidence means that the microDiamond would not be ideal for characterising the profiles and it would not be suitable for determining large-field beam parameters such as symmetry. It would also require a correction factor prior to use for patient-specific QA measurements where radiation is delivered from different gantry angles.The results of this study demonstrate that the PTW 60019 microDiamond detector is suitable for measuring small radiation fields within a 1.5 T magnetic field and thus is suitable for use in MRI-linac commissioning and quality assurance.

Demystifying failed VMAT PSQA measurements with ArcCHECK

A means of reducing Patient Specific QA (PSQA) measurements for VMAT is currently a popular topic due to the resource burden it generates. The reluctance to reduce PSQA may be due to the difficulty in identifying the cause/s of plans that fail. Plans may fail due to a number of potential factors caused by the TPS, linac or measurement device. The goal of this study was to uncover the reason/s why a selection of VMAT plans have failed. PSQA was performed with SNC ArcCHECK. Five failing plans and two passing plans were analysed. The control points for each plan were broken up into separate static fields. The fields were delivered to ArcCHECK and compared to the TPS using a Python program. Dose-differences were tracked field-by-field for each diode and categorised according to the location of the diode to the beam: In-field, penumbra and out-of-field. A composite measurement of individual control point fields compared with the conventional PSQA measurement showed minimal difference indicating that the reason for PSQA fail was not due to dynamic delivery. Dose differences due to the out-of-field component had the greatest impact on the overall pass-rates. The latest version of the SNC Patient software provides corrections for diode over-response out-of-field and was investigated as a potential solution. The impact of applying the correction resulted in all previously failing plans passing. Deconstructing PSQA measurements identified out-of-field dose differences as the main cause of plans failing. The results of this study indicate the new out-of-beam correction should be applied.

Clinical Validation of Proton Pencil Beam Scanning on an Anthropomorphic Lung Phantom

To validate the use of proton pencil beam scanning for the treatment of moving lung tumors. Pencil beam scanning (PBS) proton plans are generated for the standard thorax phantom designed by Imaging and Radiation Oncology Core (IROC). Two planning techniques, namely robust optimization against range and setup uncertainties, as well as layer and volumetric repainting, are used for these PBS plans. Patient specific QA measurements are performed using both a water tank and a custom heterogeneous phantom. Tumor motion in this anthropomorphic thorax phantom is 2 cm in magnitude. A custom movable insert is used with the moving IROC thorax phantom to find the optimal number of volumetric repainting. Both pencil beam convolution (PBC) and Monte Carlo (MC) algorithms are used for dose calculation and their accuracy are compared with actual measurements. Single ionization chamber, two-dimensional ionization chamber array, thermoluminescent dosimeter (TLD) and films are utilized to check the point dose and dose distribution accuracy. The optimal number of volumetric repainting is found to be 4 times in our system. The mean dose overestimation on moving target by PBC and MC algorithms are found to be 5.1% and 2.0%, respectively. The mean gamma passing rate for PBC and MC algorithms are 91% and 96% respectively. MC dose algorithm calculation is found to have a good agreement with measurements. Moving lung tumors can be treated using proton pencil beam scanning with robust optimization, volumetric repainting, and Monte Carlo dose calculation.

SU-F-T-569: Implementation of a Patient Specific QA Method Using EBT-XD for CyberKnife SRS/SBRT Plans

Purpose: To implement the new EBT-XD Gafchromic films for accurate dosimetric and geometric validation of stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) CyberKnife (CK) patient specific QA. Methods: Film calibration was performed using a triplechannel film analysis on an Epson 10000XL scanner. Calibration films were irradiated using a Varian Clinac 21EX flattened beam (0 to 20 Gy), to ensure sufficient dose homogeneity. Films were scanned to a resolution of 0.3 mm, 24 hours post irradiation following a well-defined protocol. A set of 12 QA was performed for several types of CK plans: trigeminal neuralgia, brain metastasis, prostate and lung tumors. A custom made insert for the CK head phantom has been manufactured to yield an accurate measured to calculated dose registration. When the high dose region was large enough, absolute dose was also measured with an ionization chamber. Dose calculation is performed using MultiPlan Ray-tracing algorithm for all cases since the phantom is mostly made from near water-equivalent plastic. Results: Good agreement (<2%) was found between the dose to the chamber and the film, when a chamber measurement was possible The average dose difference and standard deviations between film measurements and TPS calculations were respectively 1.75% and 3%. The geometric accuracy has been estimated to be <1 mm, combining robot positioning uncertainty and film registration to calculated dose. Conclusion: Patient specific QA measurements using EBT-XD films yielded a full 2D dose plane with high spatial resolution and acceptable dose accuracy. This method is particularly promising for trigeminal neuralgia plan QA, where the positioning of the spatial dose distribution is equally or more important than the absolute delivered dose to achieve clinical goals.

SU‐F‐T‐290: Modeling MLC Penumbra Within the Eclipse TPS ‐ A Major Mode of IMRT QA Failure

Purpose:To illustrate how inaccurate modeling of the MLC penumbra within the Eclipse TPS can lead to dosimetric differences and decreased IMRT QA passing rates.Methods:Gafchromic film was used to compare 6 MV measured profiles from 2×2 cm2 MLC‐defined fields to profiles calculated by the Eclipse TPS (v13.6) under two different scenarios. The first scenario involved calculations using the original MLC opening (original Eclipse). In the second, we selectively broadened the penumbra by summing two appropriately weighted fields: One with 95% of MUs delivered using the original MLC opening, and a second with the remaining MUs delivered with all MLCs opened an additional 5 mm (modified Eclipse). MATLAB code was written to generate MLC patterns and MU weighting in this manner and used to calculate modified Eclipse doses for 25 different clinical IMRT and VMAT plans. Patient specific QA measurements were made with film and compared to both original and modified Eclipse calculations.Results:For the 2×2 cm2 fields, agreement was observed between measured and original Eclipse profiles everywhere except in the penumbral region 5–10 mm from the end of the MLC where doses 10–30% higher than expected were measured. Near perfect agreement throughout the entire profile was observed when comparing measurement to the modified Eclipse calculation with the broadened penumbra. For the 25 clinical treatment plans, the average 2%/0 mm gamma passing rate using the original Eclipse calculations was 82.3%, while for the modified Eclipse calculations it improved to 90.1%.Conclusion:The approximation used by the Eclipse TPS to model the rounded end of the MLC is a failure mode that could lead to poor quality and/or failing QA for IMRT and VMAT plans. Improved modeling of the penumbra in the region 5–10 mm from the end of the MLC is required to completely correct this discrepancy.

SU-F-T-344: Commissioning Constant Dose Rate VMAT in the Raystation Treatment Planning System for a Varian Clinac IX

Purpose: To demonstrate the commissioning of constant dose rate volumetric modulated arc therapy (VMAT) in the Raystation treatment planning system for a Varian Clinac iX with Exact couch. Methods: Constant dose rate (CDR) VMAT is an option in the Raystation treatment planning system, enabling VMAT delivery on Varian linacs without a RapidArc upgrade. Raystation 4.7 was used to commission CDR-VMAT for a Varian Clinac iX. Raystation arc model parameters were selected to match machine deliverability characteristics. A Varian Exact couch model was added to Raystation 4.7 and commissioned for use in VMAT optimization. CDR-VMAT commissioning checks were performed on the linac, including patient-specific QA measurements for 10 test patients using both the ArcCHECK from Sun Nuclear Corporation and COMPASS from IBA Dosimetry. Multi-criteria optimization (MCO) in Raystation was used for CDR-VMAT planning. Results: Raystation 4.7 generated clinically acceptable and deliverable CDR-VMAT plans for the Varian Clinac. VMAT plans were optimized including a model of the Exact couch with both rails in the out positions. CDR-VMAT plans generated with MCO in Raystation were dosimetrically comparable to Raystation MCO-generated IMRT plans. Patient-specific QA measurements with the ArcCHECK on the couch showed good agreement with the treatment planning system prediction. Patient-specific, structure-specific, multi-statistical parameter 3D QA measurements with gantry-mounted COMPASS also showed good agreement. Conclusion: Constant dose rate VMAT was successfully modeled in Raystation 4.7 for a Varian Clinac iX, and Raystation's multicriteria optimization generated constant dose rate VMAT plans which were deliverable and dosimetrically comparable to IMRT plans.

Commissioning and verification of the collapsed cone convolution superposition algorithm for SBRT delivery using flattening filter‐free beams

Linacs equipped with flattening filter‐free (FFF) megavoltage photon beams are now commercially available. However, the commissioning of FFF beams poses challenges that are not shared with traditional flattened megavoltage X‐ray beams. The planning system must model a beam that is peaked in the center and has an energy spectrum that is softer than the flattened beam. Removing the flattening filter also increases the maximum possible dose rates from 600 MU/min up to 2400 MU/min in some cases; this increase in dose rate affects the recombination correction factor, Pion, used during absolute dose calibration with ionization chambers. We present the first‐reported experience of commissioning, verification, and clinical use of the collapsed cone convolution superposition (CCCS) dose calculation algorithm for commercially available flattening filter‐free beams. Our commissioning data are compared to previously reported measurements and Monte Carlo studies of FFF beams. Commissioning was verified by making point‐dose measurement of test plans, irradiating the RPC lung phantom, and performing patient‐specific QA. The average point‐dose difference between calculations and measurements of all test plans and all patient specific QA measurements is 0.80%, and the RPC phantom absolute dose differences for the two thermoluminescent dosimeters (TLDs) in the phantom planning target volume (PTV) were 1% and 2%, respectively. One hundred percent (100%) of points in the RPC phantom films passed the RPC gamma criteria of 5% and 5 mm. Our results show that the CCCS algorithm can accurately model FFF beams and calculate SBRT dose distributions using those beams.PACS number: 87.55.kh

SU‐E‐T‐199: A Software Tool for the Efficient Correction and Analysis of IMRT QA Experimental Data Using MatriXX

Purpose: The rationale for developing this software was to address shortcomings found while using OmniPro‐I' mRT software to evaluated patient specific IMRT QA measurements taken with MatriXXEvolution 2D ion chamber array. Our software addresses three major issues. It calculates average doses within each ion chamber of the array using 3D TPS DICOM field‐by‐field doses exported as input, corrects the resulting doses to account for gantry angle dependence of Matrixx, and automates the analysis of the measured data resulting in a concise report document ideal for patient electronic medical records (EMR).Materials/Methods: The software was developed in MATLAB to improve and streamline analysis of patient‐specific IMRT QA. It compares measured Matrixx doses to average doses within each ion chamber of the array, which are computed from 3D DICOM doses exported from TPS for each field. A correction factor is applied to the TPS doses to account for angular dependence. The Data is automatically evaluated using the 2D gamma analysis, evaluating a range of passing criteria (3%,4%,5%). This software allows for user‐defined passing criteria and automated reporting of quantitative results, eliminating potential user errors. A single‐sheet report is generated displaying patient information, planar dose distributions, x‐y‐profiles, and gamma plot noting the pixel based passing criteria. The effect of misalignment in direction perpendicular to detector plane is simulated by allowing a non‐zero shift along that direction. Results: This home‐built IMRT QA analysis tool gives a clear quantitative answer based on the user defined criteria. The implementation of an angular dependence correction and the averaging over the volume of the ion chambers improves the accuracy of the analysis accounting for ion chamber size (∼5mm x 5mm x5mm) and spacing (∼7mm × 7mm) Conclusion: Though this tool does not replace the commercial software completely, it improves the accuracy of the analysis and streamlines the process.

SU‐E‐T‐221: Evaluation of Quality Assurance for Rapid Arc Treatments Using EPIQA Software

Purpose: To evaluate the patient specific quality assurance of Rapid Arc treatments using EPID and EpiQA software. Methods: Eclipse TPS is used for the creation and calculation of patient and verification plans. The treatments were delivered on aS1000 EPID attached to the Varian Clinac iX Linear Accelerator. EpiQA software supplied by the EPIdos is used for the analysis of calculated and measured dose maps. In this retrospective study 75 Rapidarc treatment plans consisting of 150 arc fields were considered from three different sites of 25 plans each namely head &amp; neck, pelvis and brain. All plans were treated with 6 MV photons with two arcs and QA measurements acquired with Portal Vision aS1000 and analyzed by EpiQA software. To evaluate the QA results, we used Gamma agreement Index (GAI). GAI represents the percentage of the field area with gamma value greater than 1.0 to the gamma criteria of 3mm DTA and 3% Dose Difference. Results: Calculated mean values and standard deviations are 1.95 ± 1.25, 2.44 ± 1.35 and 2.69 ± 1.13 for Brain, Pelvis and Head &amp; Neck respectively. It is noted that all the results were well within 95% of pass criteria of gamma agreement index. We found that the mean GAI values for head &amp; neck and pelvis plans are greater than that of brain plans. This deviation may be attributed due to the variations in the geometry of the field, volume of irradiation, depth of penetration and complexity of planning objectives (more critical organs) etc. However further data of measurements is needed for further evaluation. Conclusion: Since all the plans were passed the GAI and the results are comparable with phantom studies, we conclude that the portal dosimetry can be used conveniently to carryout patient specific QA measurements of Rapidarc plans.

SU‐E‐T‐424: Improved Dosimetric Accuracy for Cyberknife Patient Plans Using a Dual‐Detector Measurement Method for Relative Output Factors

Purpose: The measurement of output factors (OFs) for small fields can lead to large dosimetric errors if detector effects are not accounted for. With its high spatial resolution and tissue equivalence, GAFCHROMIC film provides a correction free measure of OFs. We recently changed the OFs used in our Cyberknife planning system from uncorrected diode values to a dual detector method employing a diode with Monte‐Carlo corrections for the smallest collimators and a micro ion chamber for collimators &gt;10 mm in diameter. Methods: We measured OFs for the CyberKnife G4 fixed collimators (5 to 60 mm) using an A16 microchamber and an Edge diode detector. The diode measured OFs for collimator sizes &lt;10 mm were corrected using Monte‐Carlo correction factors. OFs were also measured using GAFCHROMIC film. We evaluated how this change in OFs influenced the dosimetric accuracy of patient specific QA measurements for 13 patient plans (9 before and 4 after the OF change) using film and the A16 chamber. Results: The OFs measured using the dual‐detector method agree with film to within two standard deviations for the full range of collimator sizes. When the dual detector method OFs are used, we achieve better dosimetric agreement (&lt;2 sigma for pixels within the 80% isodose) than with uncorrected diode OFs for all patient specific QA plans measured using film. For patient specific QA using the microchamber, we get good agreement (&lt;3%) for collimator sizes &gt;5 mm, with differences observed for the 5 mm collimator consistent with volume averaging and a 1 mm setup uncertainty. Conclusions: OFs can be determined consistently using the dual‐detector method and verified using film. For patient specific QA, we achieve good agreement with microchambers for collimators &gt;5 mm in diameter but film is the most appropriate detector for patient specific QA using the 5 mm collimator.

SU‐E‐T‐433: Evaluation of the ArcCheck Device for Patient‐Specific Quality Assurance in Step‐And‐Shoot IMRT for Single‐Fraction Spinal SBRT

Purpose: Current standard for patient specific spine SBRT IMRT QA in our institution is a combination of a point dose measurement in solid water under delivery of the beams at actual treatment angles, and 2D planar verification using an ion chamber (IC) array with all beams delivered at standard gantry angle of zero. The very small active area of ArcChecks diodes (∼0.6mm2) has the potential to improve measurement accuracy in high dose gradient regions present in spinal SBRT. Given high clinical workload we seek to improve ease of workflow and decrease time necessary for patient‐specific QA, by obtaining 2D verification and point dose measurement at actual treatment angles in one single measurement. Methods: The results of 10 patient specific QA measurements, as performed with single‐point and 2D IC array were compared to results obtained with ArcCheck. ArcCheck and 2D IC array pass rates are evaluated for 3%/3mm gamma criteria, single‐point measurement based on percent deviation from QA plan dose. The constancy of the device was assessed by re‐measuring the same treatment plan over the course of three weeks, and evaluating pass rate variationResults: The gamma pass rates are generally higher for ArcCheck (average 99.3%) than for Matrixx (average 93.1%). Average absolute deviation for the single‐point IC measurement was 3.4%. Maximum variation in pass rate between any two ArcCheck measurements within the three‐week evaluation period was 0.5%. Since ArcCheck allows for the use of an insert with an ion chamber in place, simultaneous redundant point measurement is possible with no time addedConclusion: Based on our results patient‐specific QA for spine SBRT treatments will be performed using ArcCheck with pass/fail criteria 98%. High sensitivity towards geometrical shifts necessitates careful initial alignment of the device.