QA Device Research Articles

Implementation of an ionization chamber array for linear accelerator monthly dosimetry QA.

The IC Profiler (ICP) manufactured by Sun Nuclear Corporation (SNC) is an ionization chamber (IC) array used for linear accelerator dosimetry measurements. Previous work characterized response of the ICP under various conditions, but there is limited work of its implementation into monthly QA measurement procedures. This work quantifies ICP accuracy and variables that affect accuracy for beam output measurements, and demonstrates feasibility of using the ICP for all recommended monthly dosimetry measurements. A total of 1985 output measurements on six Varian TrueBeam and Edge linear accelerators were performed using three ICP with quad wedges (QWs) and were compared with conventional IC measurements. The accuracy of the ICP for beam output was characterized as the difference between the ICP and IC. Variables that affect ICP accuracy, including gain settings, calibrations, and template baselining as well as machine or energy-specific bias were investigated. Measurements of profile constancy, energy, dose rate constancy, wedge factors, and gating were performed. The initially observed mean output difference between the ICP and IC was 0.16% (0.61%). When gain settings were optimized, the output difference accuracy improved to -0.02% (0.38%). The output accuracy of the ICP was not dependent on array, dose, temperature and pressure calibrations, or template baselining. Statistically, ICP output accuracy was dependent on machine and beam energy, but clinically, all measurements fell within 0.5% of unity. ICP measurements of energy, dose rate constancy, and wedge factors matched passing results with conventional IC in water measurements. Gating and beam profile constancy measurements demonstrated good stability using the ICP. Finally, monthly dosimetry QA using ICP was completed in an average of 33 min compared to 66 min using the IC. This work demonstrated the feasibility and efficiency of using the ICP, with specific considerations, as a measurement device for dosimetric linear accelerator monthly QA.

Wobbling nature of gamma passing rate as a function of calibration field sizes in patient-specific quality assurance

Abstract Purpose: This study aimed to investigate the influence of calibration field size on the gamma passing rate (GPR) in patient-specific quality assurance (PSQA). Methods: Two independent detectors, PTW OCTAVIUS 4D (4DOCT) and Arc Check, were utilised in volumetric modulated arc therapy plans for 26 patients (14 with Arc Check and 12 with 4DOCT). Plans were administered using Varian Unique machine (with 4DOCT) and Varian TrueBeam (with Arc Check), each employing different calibration factors (CFs): 4 × 4, 6 × 6, 8 × 8, 10 × 10, 12 × 12 and 15 × 15 cm2 field sizes. Gamma analysis was conducted with 2%2mm, 2%3mm and 3%3mm gamma criteria. Results: GPR exhibited variations across different CFs. GPR demonstrated an increasing trend below 10 × 10 cm² CFs, while it displayed a decreasing trend above 10 × 10 cm². Both detectors exhibited similar GPR patterns. The correlation between 4DOCT and Arc Check was strong in tighter criteria (2%2mm) with an R² value of 0·9957, moderate criteria (2%3mm) with an R² value of 0·9868, but reduced in liberal criteria (3%3mm) with an R² value of 0·4226. Conclusion: This study demonstrates that calibration field sizes significantly influence GPR in PSQA. This study recommends the plan specific calibration field must obtain to calibrate the QA devices for modulated plans.

Clinical implementation of a log file-based machine and patient QA system for IMRT and VMAT treatment plans

PurposeTo determine the error detection sensitivity of a commercial log file-based system (LINACWatch®, LW) for integration into clinical routine and to compare it with a measurement device (OCTAVIUS 4D, Oct4D) for IMRT and VMAT delivery QA. Materials and methods76 VMAT/IMRT plans (H&N, prostate, rectum and breast) preliminarily classified according to their Modulation Complexity Score (MCS) calculated by LW, were considered. Receiver Operating Characteristic (ROC) Curves were used to establish gamma criteria for LW. 12 plans (3 for each site) were intentionally modified in order to introduce delivery errors regarding MLC, jaws, collimator, gantry and MU (for a total set of 168 incorrect plans) and irradiated on Oct4D; the corresponding log files were analysed by LW. Each incorrect plan was compared to the error-free plan using γ-index analysis for MLC, jaws and MU errors investigation and Root-Mean-Square (RMS) values for gantry and collimator errors investigation. ResultsMCS ranges values were: 0.10–0.20 for H&N, 0.21–0.40 for prostate and rectum, 0.41–1.00 for breast. From ROC curves, the Gamma Passing Rate (GPR) thresholds were: 87%, 92%, 99% for H&N, prostate and rectum, and breast, respectively. The 1.5%/1.5 mm/local criteria were adopted for the γ-analysis. LW sensitivity in detecting the introduced errors was higher when compared to Oct4D: 48.5% vs 30.4% respectively. ConclusionsLW can be considered useful complement to phantom-based delivery QA of IMRT/VMAT plans. The MCS tool is effective in detecting over or under modulated plans prior to pre-treatment QA. However, rigorous and routinely machine QCs are recommended.

PO15: Systematic Dwell Position Deviation of Elekta MR Compatible HDR Ring Applicators

<h3>Purpose</h3> (1) To identify a systematic dwell position deviation of Elekta MR compatible HDR ring applicators due to its design and (2) to demonstrate the dosimetric impact of the deviation. <h3>Materials and Methods</h3> In these ring applicators, the source is designed to travel precisely along the central axis of the ring tube. However, the actual source positions deviate from the central axis due to the semi-rigid design of the wire carrying the source. To quantify the deviation, in-house QA devices were built using decommissioned ring applicators of three different diameters (26mm, 30mm, and 34mm). The ring applicators were modified by replacing their top caps with a 1 mm thick transparent plastic plate. This allowed proper visualization of both the vendor-provided wire QA markers and the actual Ir-192 source. The high-resolution camera of the Elekta Flexitron system was used to acquire images of the applicator and the source. The source was driven to dwell positions with a 5 mm increment to cover the full visible range of the ring. An image was acquired at each dwell position. This procedure was repeated 5 times for each ring size. Two observers independently measured the arc length traveled by the source from the first dwell position to other dwell positions. The deviation was calculated as the difference between the measured arc length and the intended dwell position (converted to arc length). <h3>Results</h3> The deviation was found to be a function of the arc distance from the first dwell position. It increases almost linearly with arc length, experiences a sharp increase before settling in a range between 5 mm and 6 mm. Up to 6 mm deviation was found for ring applicators of all three different diameters. Although there were small interobserver differences (mean = 0.1 mm, SD = 0.13 mm), the results were highly reproducible (SD = 0.09 mm). The dwell position deviations result in remarkable differences of the isodose curves surrounding the ring applicator. The measured deviations were applied to our library plan for cervical cancer using 30 mm ring and 60 mm tandem. The isodose lines in the plane of the ring before and after the offsets were applied are shown in figure 1(a) and (b), respectively. <h3>Conclusions</h3> Large systematic dwell positions deviations of the Elekta MR compatible HDR ring applicators has been quantified. These deviations may have significant dosimetric impact on HDR treatment. Therefore, they should be considered by the treatment planning system for more accurate dose computations.

PHSOR11 Presentation Time: 1:20 PM: Verify the Transit Time Correction Accuracy of the Bravos Afterloader System Using a Novel Scintillation-Based Brachytherapy Quality Assurance Device

<h3>Purpose</h3> Compared to the older generations, the new Bravos high dose rate (HDR) afterloader offers the function to automatically correct dwell time to compensate the transit dose effect. To our best knowledge, there is no QA attempt to verify the accuracy of this correction. To address this issue, a new transit time measurement module has been developed and included in the OriQA HDR QA device, an autonomous phantom for HDR brachytherapy quality assurance. In this work, we employ the OriQA system to verify the transit time correction accuracy of the Bravos afterloader system. <h3>Method</h3> The OriQA device utilizes radioluminescence imaging technique to accurately measure HDR source strength, dwell time, and dwell position. It consists of a 20 cm needle applicator insert, a radioluminescence phosphor sheet placed directly atop the applicator, a digital camera, and software for signal processing and displaying results. Radiation-induced luminescence photons are emitted from the phosphor sheet and captured by the camera. The signal intensity and location are processed to yield relative source strength and dwell information. For the verification test, one of the Bravos channels was connected to the OriQA applicator using a one-meter transfer tube. After specific plan delivery, the raw data of OriQA measurement including source position and time was exported to Matlab for further analysis. The real dwell time at each dwell position and the transit time between dwells during cable extension and retraction were extracted. The transit time correction method provided by the vendor was applied to the measurement, yielding the delivered dwell time. The percentage difference between the delivered and planned dwell time was calculated and used as an indicator of the transit time correction accuracy. Several verification plans were created including seven with fixed dwell steps, one with variable dwell steps, and one with variable dwell time. Plans with fixed dwell steps were delivered three times. The rest were delivered once. Plan details are listed in table 1. For each plan, the mean and standard deviation of the difference between planned and delivered dwell time were calculated from all dwell positions and deliveries. <h3>Results</h3> The deviations between the planned and delivered dwell time are listed in table 1. For plans with less than 4 cm dwell step, the accuracy of transit time correction is, on average, less than 1.2% of the dwell time. For plans with 4 cm and 9 cm dwell step, the accuracy is 1.5% and 2.85%, on average, respectively. Dwell time variation does not affect the correction accuracy, as shown in plan 9. Standard deviations of plan 3, 4, and 9 are greater than 1% due to the relatively large difference at the proximal dwell. <h3>Conclusion</h3> This is the first study that successfully verifies the transit time correction accuracy of the Bravos system. Less than 2% accuracy is observed for clinically relevant dwell steps (0.3 cm to 4 cm). Variations in dwell step, dwell position, and dwell time do not worsen the accuracy.

Needs and Strengths Assessment for Radiotherapy Centers in Africa Transitioning to IMRT

Multiple centers globally are interested in implementing intensity-modulated radiotherapy (IMRT) programs, however little data exists comparing granular center-specific needs and strengths of radiotherapy (RT) centers interested in establishing an IMRT program, in low-resource countries in this position. In conjunction with up-scaling training, methods are needed to assess multiple centers efficiently. Using a novel approach, we assess centers' needs and strengths to tailor a multi-institutional remote training program.To assess strengths and needs of centers interested in establishing an IMRT program. Core RT leaders from 16 RT centers in 7 African countries (Ghana, Ivory Coast, Kenya, Nigeria, Senegal, Morocco, Zambia and one sister center in Pakistan) were individually invited to complete a 30-minute, 55-question electronic survey followed by 60-75-minute video conference with 2-4 lead members of a non-profit global health radiation oncology organization prior to participating in a remote IMRT training program.14 centers responded and were surveyed, and interviewed, including 11 public, 3 private and 1 hybrid institutions. Among centers 4 (28.6%) and 1 (7.1%) are currently treating patients with IMRT and both, IMRT and VMAT, respectively. In terms of strengths all 14 (100%) centers emphasized high energy and commitment to learning potential research collaboration, 1 (7.1%) center is a designated International Atomic Energy Agency training center and 1 (7.1%) is the only center in East Africa with Joint Commission International Accreditation. All 14 (100%) centers emphasized the need for education and training in advanced treatment planning procedures and techniques, 4 (28.6%) centers reported major gaps (including CT simulators, linear accelerator) in equipment, 5 (35.5%) minor gaps (including in-room image guidance, multi-leaf collimator) in equipment, and 5 (35.7%) gaps in physics QA devices (including film/array detectors, dose calculation software, phantoms for machine output checks). Using benchmark recommended staffing levels from Health Economics in Radiation Oncology data (HERO), 14 (100%) centers were well-staffed with physicists, 8 (57%) well-staffed with radiation oncologists and 8 (57%) well-staffed with radiation therapists.Among a motivated group of African RT centers with demonstrated interest in commissioning an IMRT program, we noted gaps in equipment for safe IMRT treatment delivery. Staffing met recommendation for physicists but not radiation oncologists and therapists. Combined survey and video interview assessment can help guide curriculum design for training programs and other areas for improvement.

Commissioning and clinical implementation of an Autoencoder based Classification-Regression model for VMAT patient-specific QA in a multi-institution scenario

Background and purposeTo commission and implement an Autoencoder based Classification-Regression (ACLR) model for VMAT patient-specific quality assurance (PSQA) in a multi-institution scenario. Materials and methods1835 VMAT plans from seven institutions were collected for the ACLR model commissioning and multi-institutional validation. We established three scenarios to validate the gamma passing rates (GPRs) prediction and classification accuracy with the ACLR model for different delivery equipment, QA devices, and treatment planning systems (TPS). The prediction performance of the ACLR model was evaluated using mean absolute error (MAE) and root mean square error (RMSE). The classification performance was evaluated using sensitivity and specificity. An independent end-to-end test (E2E) and routine QA of the ACLR model were performed to validate the clinical use of the model. ResultsFor multi-institution validations, the MAEs were 1.30–2.80% and 2.42–4.60% at 3%/3 mm and 3%/2 mm, respectively, and RMSEs were 1.55–2.98% and 2.83–4.95% at 3%/3 mm and 3%/2 mm, respectively, with different delivery equipment, QA devices, and TPS, while the sensitivity was 90% and specificity was 70.1% at 3%/2 mm. For the E2E, the deviations between the predicted and measured results were within 3%, and the model passed the consistency check for clinical implementation. The predicted results of the model were the same in daily QA, while the deviations between the repeated monthly measured GPRs were all within 2%. ConclusionsThe performance of the ACLR model in multi-institution scenarios was validated on a large scale. Routine QA of the ACLR model was established and the model could be used for VMAT PSQA clinically.

Intrinsic detector sensitivity analysis as a tool to characterize ArcCHECK and EPID sensitivity to variations in delivery for lung SBRT VMAT plans.

PurposeTo investigate intrinsic sensitivity of an electronic portal imaging device (EPID) and the ArcCHECK detector and to use this in assessing their performance in detecting delivery variations for lung SBRT VMAT. The effect of detector spatial resolution and dose matrix interpolation on the gamma pass rate was also considered.Materials and methodsFifteen patients’ lung SBRT VMAT plans were used. Delivery variations (errors) were introduced by modifying collimator angles, multi‐leaf collimator (MLC) field sizes and MLC field shifts by ±5, ±2, and ±1 degrees or mm (investigating 103 plans in total). EPID and ArcCHECK measured signals with introduced variations were compared to measured signals without variations (baseline), using OmniPro‐I'mRT software and gamma criteria of 3%/3 mm, 2%/2 mm, 2%/1 mm, and 1%/1 mm, to test each system's basic performance. The measurement sampling resolution for each was also changed to 1 mm and results compared to those with the default detector system resolution.ResultsIntrinsic detector sensitivity analysis, that is, comparing measurement to baseline measurement, rather than measurement to plan, demonstrated the intrinsic constraints of each detector and indicated the limiting performance that users might expect. Changes in the gamma pass rates for ArcCHECK, for a given introduced error, were affected only by dose difference (DD %) criteria. However, the EPID showed only slight changes when changing DD%, but greater effects when changing distance‐to‐agreement criteria. This is pertinent for lung SBRT where the minimum dose to the target will drop dramatically with geometric errors. Detector resolution and dose matrix interpolation have an impact on the gamma results for these SBRT plans and can lead to false positives or negatives in error detection if not understood.ConclusionThe intrinsic sensitivity approach may help in the selection of more meaningful gamma criteria and the choice of optimal QA device for site‐specific dose verification.

Evaluations of a flat-panel based compact daily quality assurance device for proton pencil beam scanning (PBS) system.

To evaluate the flat-panel detector quenching effect and clinical usability of a flat-panel based compact QA device for PBS daily constancy measurements. The QA device, named Sphinx Compact, is composed of a 20x20 cm2 flat-panel imager mounted on a portable frame with removable plastic modules for constancy checks of proton energy (100MeV, 150MeV, 200MeV), Spread-Out-Bragg-Peak (SOBP) profile, and machine output. The potential quenching effect of the flat-panel detector was evaluated. Daily PBS QA tests of X-ray/proton isocenter coincidence, the constancy of proton spot position and sigma as well as the energy of pristine proton beam, and the flatness of SOBP proton beam through the 'transformed' profile were performed and analyzed. Furthermore, the sensitivity of detecting energy changes of pristine proton beam was also evaluated. The quenching effect was observed at depths near the pristine peak regions. The flat-panel measured range of the distal 80% is within 0.9mm to the defined ranges of the delivered proton beams. X-ray/proton isocenter coincidence tests demonstrated maximum mismatch of 0.3mm between the two isocenters. The device can detect 0.1mm change of spot position and 0.1MeV energy changes of pristine proton beams. The measured transformed SOBP beam profile through the wedge module rendered as flat. Even though the flat-panel detector exhibited quenching effect at the Bragg peak region, the proton range can still be accurately measured. The device can fulfill the requirements of the daily QA tests recommended by the AAPM TG224 Report.

Investigation of Error Detection Capabilities of Various Patient-Specific Intensity Modulated Radiotherapy Quality Assurance Devices

The capability of error detection of patient-specific QA tools plays an important role in verifying MLC motion accuracy. The goal of this study was to investigate the capability in error detection of portal dosimetry, MapCHECK2 and MatriXX QA tools in IMRT plans. The 9 fields IMRT for 4 head and neck plans and 7 fields IMRT for 4 prostate plans were selected for the error detection of QA devices. The measurements were undertaken for the original plan and the modified plans, where the known errors were introduced for increasing and decreasing of prescribed dose (±2%, ±4% and ±6%) and position shifted in X-axis and Y-axis (±1, ±2, ±3 and ±5 mm). After measurement, the results were compared between calculated and measured values using gamma analysis at 3%/3 mm criteria. The average gamma pass for no errors introduced in head and neck plans was 96.9%, 98.6%, and 98.8%, while prostate plans presented 99.4%, 99.0%, and 99.7%, for portal dosimetry, MapCHECK2 and MatriXX system, respectively. In head and neck plan, the shifted error detections were 1 mm for portal dosimetry, 2 mm for MapCHECK2, and 3 mm for MatriXX system. In prostate plan, the shifted error detections were 2 mm for portal dosimetry, 3 mm for MapCHECK2, and 5 mm for MatriXX system. For the dose error detection, the portal dosimetry system could detect at 2% dose deviation in head and neck and 4% in prostate plans, while other two devices could detect at 4% dose deviation in both head and neck and prostate plans. Portal dosimetry shows slightly more capability to detect the error compared with MapCHECK2 and MatriXX system, especially in the complicated plan. It may be due to higher resolution of the detector; however, all three-detector types can detect various errors and can be used for patient-specific IMRT QA.

A single-optical kernel for a phosphor-screen-based geometric QA system (RavenQA™) as a tool for patient-specific IMRT/VMAT QA

It has been proven that portal dosimetry can be derived from a mirror-based fluorescent EPID system by applying multiple kernels that are position dependent. The purpose of this study is to show that patient-specific IMRT/VMAT verification with a single kernel which is acquired from a series of output measurements of a few field sizes is feasible using a commercially available phosphor-screen–based geometric QA system.The optical scatter component in the RavenQA™ (LAP GmbH Laser Applications; Lüneberg, Germany) is corrected by deconvolution with a two-dimensional (2D) spatially invariant single optical scatter kernel (OSK). We assume that the OSK is a 2D isotropic point spread function that decreases as a function of distance from the scatter center. The OSK is determined by comparing output factors of various field sizes. We report on performance testing of the system using 12 intensity-modulated radiation therapy and three volumetric-modulated arc therapy cases.A single spatially invariant OSK can be employed, because the shapes of the OSK across the image plate are almost identical. The average 3%/3 mm gamma passing rate for 15 patients was 97.6% ± 1.1%. The passing rate was >95% for all patients.It is feasible to perform the patient-specific IMRT/VMAT verification with a single kernel using a commercially available phosphor-screen-based mechanical QA device in accordance with AAPM TG-142. It is also practical to implement since it only requires to measure the optical intensities of the field centers of several square fields, in order to obtain the OSK.

OA198] Evaluation of the needs for PPV measurement from the legal point of view

Purpose The main aim of the study is to evaluate whether it is mandatory to have QA device (D&IR Diagnostic & Interventional Radiology application) with possibility of PPV (Practical Peak Voltage) measurement. The secondary aim of the study is to find weather all available measuring devices on the European market provide all necessary readings. The Purpose of the study do not cover investigation whether PPV measurements should be used instead of kVp measurements. Methods European regulation has been verified. Evaluation and comparison of existing devices on the marked has been performed. Results On the European market all device uses in medical sector should be CE marked and should meet requirements of Medical Device Directive 93/42/EEC. There is a list of international standards harmonized with MDD directive, which means that if any device is covered by any of those standards, the specified medical device must meet additional requirements. One of such a standard is EN 61676:2002 Medical electrical equipment - Dosimetric instruments used for non-invasive measurement of X-ray tube voltage in diagnostic radiology IEC 61676:2002, which provide need of PPV (practical peak voltage) measurements, however it is common in practice to choose kVp measurements rather than PPV. 5 main leading European manufacturers has been evaluated. 4 manufacturers offer device with possibility of PPV measurement, while one vendor do not give user possibility to choose PPV instead of kVp if needed. This vendor provide only devices with kVp measurements, nevertheless the devices are CE marked. Conclusions It has been found that from European regulation point of view the devices uses for QA in D&IR purposes should has possibility of PPV measurement to fulfill requirements of MDD Directive due to harmonized standard EN-IEC 61676:2002. Some of the devices existing on the market, provided by leading vendor, do not meet MDD requirements, therefore their legal use for the QA purposes in D&IR might be questionable. It is also very interesting that the device without PPV possibility is CE marked, probably international organizations like COCIR could check the quality of external audits during CE marking process.

Activation of QA devices and phantom materials under clinical scanning proton beams—a gamma spectrometry study

Activation of detectors and phantoms used for commissioning and quality assurance of clinical proton beams may lead to radiation protection issues. Good understanding of the activation nuclide vectors involved is necessary to assess radiation risk for the personnel working with these devices on a daily basis or to fulfill legal requirements regarding transport of radioactive material and its release to the public. 11 devices and material samples were irradiated with a 220 MeV proton pencil beam (PBS, Proton Therapy Center, Prague). This study focuses on devices manufactured by IBA Dosimetry GmbH: MatriXX PT, PPC05, Stingray, Zebra, Lynx, a Blue Phantom rail and samples of RW3, PMMA, titanium, copper and carbon fibre plastic. Monitor units (MU) were monitored during delivery. Gamma spectrometry was then performed for each item using a HPGe detector, with a focus on longer lived gamma emitting radionuclides. Activities were quantified for all found isotopes and compared to relevant legal limits for exemption and clearance of radioactive objects. Activation was found to be significant after long irradiation sessions, as done during commissioning of a proton therapy room. Some of the investigated devices may also cumulate activity in time, depending on the scenario of periodic irradiation in routine clinical practice. However, the levels of activity and resulting beta/gamma doses are more comparable to internationally recommended concentration limits for exemption than to dose limits for radiation workers. Results of this study will help to determine nuclide inventories required by some legal authorities for radiation protection purposes.

EP-1792: A non-measuring way to compare pre-treatment QA devices and set gamma analysis parameters

EP-1792: A non-measuring way to compare pre-treatment QA devices and set gamma analysis parameters

Verification of high-dose-rate brachytherapy treatment planning dose distribution using liquid-filled ionization chamber array.

PurposeThis study aims to investigate the dosimetric performance of a liquid-filled ionization chamber array in high-dose-rate (HDR) brachytherapy dosimetry. A comparative study was carried out with air-filled ionization chamber array and EBT3 Gafchromic films to demonstrate its suitability in brachytherapy.Material and methodsThe PTW OCTAVIUS detector 1000 SRS (IA 2.5-5 mm) is a liquid-filled ionization chamber array of area 11 x 11 cm2 and chamber spacing of 2.5-5 mm, whereas the PTW OCTAVIUS detector 729 (IA 10 mm) is an air vented ionization chamber array of area 27 x 27 cm2 and chamber spacing of 10 mm. EBT3 films were exposed to doses up to a maximum of 6 Gy and evaluated using multi-channel analysis. The detectors were evaluated using test plans to mimic a HDR intracavitary gynecological treatment. The plan was calculated and delivered with the applicator plane placed 20 mm from the detector plane. The acquired measurements were compared to the treatment plan. In addition to point dose measurement, profile/isodose, gamma analysis, and uncertainty analysis were performed. Detector sensitivity was evaluated by introducing simulated errors to the test plans.ResultsThe mean point dose differences between measured and calculated plans were 0.2% ± 1.6%, 1.8% ± 1.0%, and 1.5% ± 0.81% for film, IA 10 mm, and IA 2.5-5 mm, respectively. The average percentage of passed gamma (global/local) values using 3%/3 mm criteria was above 99.8% for all three detectors on the original plan. For IA 2.5-5 mm, local gamma criteria of 2%/1 mm with a passing rate of at least 95% was found to be sensitive when simulated positional errors of 1 mm was introduced.ConclusionThe dosimetric properties of IA 2.5-5 mm showed the applicability of liquid-filled ionization chamber array as a potential QA device for HDR brachytherapy treatment planning systems.

MO-FG-202-09: Virtual IMRT QA Using Machine Learning: A Multi-Institutional Validation

Purpose: To validate a machine learning approach to Virtual IMRT QA for accurately predicting gamma passing rates using different QA devices at different institutions. Methods: A Virtual IMRT QA was constructed using a machine learning algorithm based on 416 IMRT plans, in which QA measurements were performed using diode-array detectors and a 3%local/3mm with 10% threshold. An independent set of 139 IMRT measurements from a different institution, with QA data based on portal dosimetry using the same gamma index and 10% threshold, was used to further test the algorithm. Plans were characterized by 90 different complexity metrics. A weighted poison regression with Lasso regularization was trained to predict passing rates using the complexity metrics as input. Results: In addition to predicting passing rates with 3% accuracy for all composite plans using diode-array detectors, passing rates for portal dosimetry on per-beam basis were predicted with an error <3.5% for 120 IMRT measurements. The remaining measurements (19) had large areas of low CU, where portal dosimetry has larger disagreement with the calculated dose and, as such, large errors were expected. These beams need to be further modeled to correct the under-response in low dose regions. Important features selected by Lasso to predict gamma passing rates were: complete irradiated area outline (CIAO) area, jaw position, fraction of MLC leafs with gaps smaller than 20 mm or 5mm, fraction of area receiving less than 50% of the total CU, fraction of the area receiving dose from penumbra, weighted Average Irregularity Factor, duty cycle among others. Conclusion: We have demonstrated that the Virtual IMRT QA can predict passing rates using different QA devices and across multiple institutions. Prediction of QA passing rates could have profound implications on the current IMRT process.

SU-F-T-567: Sensitivity and Reproducibility of the Portal Imaging Panel for Routine FFF QC and Patient Plan Dose Measurements

Purpose: The purpose of this work was to see if the EPID is a viable alternative to other QA devices for routine FFF QA and plan dose measurements. Methods: Sensitivity measurements were made to assess response to small changes in field size and beam steering. QA plans were created where field size was varied from baseline values (5–5.5cm, 20–20.5cm). Beam steering was adjusted by altering values in service mode (Symmetry 0–3%). Plans were measured using the Varian portal imager (aS1200 DMI panel), QA3 (Sun Nuclear), and Starcheck Maxi (PTW). FFF beam parameters as stated in Fogliata et al were calculated. Constancy measurements were taken using all 3 QC devices to measure a MLC defined 20×20cm field. Two clinical SABR patient plans were measured on a Varian Edge linac, using the Portal Dosimetry module in ARIA, and results compared with analysis made using Delta4 (ScandiDos). Results: The EPID and the Starcheck performed better at detecting clinically relevant changes in field size with the QA3 performing better when detecting similar changes in beam symmetry. Consistency measurements with the EPID and Starcheck were equivalent, with comparable standard deviations. Clinical plan measurements on the EPID compared well with Delta4 results at 3%/1mm. Conclusion: Our results show that for FFF QA measurements such as field size and symmetry, using the EPID is a viable alternative to other QA devices. The EPID could potentially be used for QC measurements with a focus on geometric accuracy, such as MLC positional QA, due to its high resolution compared to other QA devices (EPID 0.34mm, Starcheck 3mm, QA3 5mm). Good agreement between Delta4 and portal dosimetry also indicated the EPID may be a suitable alternative for measurement of clinical plans.

SU‐F‐T‐282: Quality Assurance for IMRT/VMAT QA Devices: Issues Affecting the Timing for ArcCHECK Recalibration

Purpose:To discuss several factors surrounding the decision on when to recalibrate the ArcCHECK device as well as present a simple and efficient monthly check to evaluate ArcCHECK calibrations.Methods:ArcCheck (Sun Nuclear) calibrations were evaluated monthly by measuring a 25×25cm2 field with 100 MU. Since ArcCHECK measurements are run on an almost nightly basis, such additional square field measurements are obtained with minimal additional effort. An in‐house MATLAB script compares two radial (y‐direction) profiles from the top/center of the new measurement relative to a baseline measurement acquired at the last device calibration. The program automatically generates PDF profile and percent difference comparisons for inspection. Recalibration is based on inspection of measurement profile shapes and percent differences from the baseline measurement.Results:The method presented here shows the utility of a simple monthly check for evaluating ArcCHECK calibrations, and in addition shows the importance of recalibrating after Linac beam steering. Our device required recalibration approximately every 8–10 months. However, for ease of scheduling, we propose a bi‐annual recalibration interval. Clinics with a lighter/heavier IMRT/VMAT QA case load may require different recalibration intervals, which are easily determined using the single‐field method presented. Analysis of additional square fields is also easily incorporated, if desired. We further illustrate the importance of array recalibration given that diode irradiation is not uniform over the entire device, with central diodes receiving more than 900 Gy over the course of 10 months and peripheral diodes receiving as little as 50 Gy (in our experience). Finally, we show that timely device recalibration decreases spread in clinical IMRT/VMAT QA gamma passing rates.Conclusion:Quality assurance for ArcCHECK array calibrations is important to ensure quality IMRT/VMAT QA comparisons. For many clinics, calibrations should be performed at least annually, if not more frequently, based on use of the proposed monthly check.

SU‐F‐T‐287: A Preliminary Study On Patient Specific VMAT Verification Using a Phosphor‐Screen Based Geometric QA System (Raven QA)

Purpose:The RavenQA system (LAP Laser, Germany) is a QA device with a phosphor screen detector for performing the QA tasks of TG‐142. This study tested if it is feasible to use the system for the patient specific QA of the Volumetric Modulated Arc Therapy (VMAT).Methods:Water equivalent material (5cm) is attached to the front of the detector plate of the RavenQA for dosimetry purpose. Then the plate is attached to the gantry to synchronize the movement between the detector and the gantry. Since the detector moves together with gantry, The 'Reset gantry to 0’ function of the Eclipse planning system (Varian, CA) is used to simulate the measurement situation when calculating dose of the detector plate. The same gantry setup is used when delivering the treatment beam for feasibility test purposes. Cumulative dose is acquired for each arc. The optical scatter component of each captured image from the CCD camera is corrected by deconvolving the 2D spatial invariant optical scatter kernel (OSK). We assume that the OSK is a 2D isotropic point spread function with inverse‐squared decrease as a function of radius from the center.Results:Three cases of VMAT plans including head &amp; neck, whole pelvis and abdomen‐pelvis are tested. Setup time for measurements was less than 5 minutes. Passing rates of absolute gamma were 99.3, 98.2, 95.9 respectively for 3%/3mm criteria and 96.2, 97.1, 86.4 for 2%/2mm criteria. The abdomen‐pelvis field has long treatment fields, 37cm, which are longer than the detector plate (25cm). This plan showed relatively lower passing rate than other plans.Conclusion:An algorithm for IMRT/VMAT verification using the RavenQA has been developed and tested. The model of spatially invariant OSK works well for deconvolution purpose. It is proved that the RavenQA can be used for the patient specific verification of VMAT.This work is funded in part by a Maryland Industrial Partnership Program grant to University of Maryland and to JPLC who owns the Raven technology. John Wong is a co‐founder of JPLC.

SU‐G‐TeP4‐01: A High Resolution Pre‐Treatment VMAT QA Technique Based On EPID for Single Isocenter Multiple Mets Stereotactic Radiosurgery

Purpose:For multiple mets radiosurgery, it is advantageous to use VMAT with a single isocenter for the ease in planning and efficiency in delivery. Insightful pre‐treatment QA for these cases is difficult to obtain because (1) target size is often small and the same order of magnitude as the detector size for commercial array based VMAT pre‐treatment QA devices; and furthermore (2) targets are located away from central axis where the array detectors are located. In this study, we present and validate an in‐house developed VMAT QA technique that is capable of addressing these challenges.Methods:The VMAT plan is delivered on a Varian Truebeam STX with HDMLC. The MV cine images are converted to portal dose (PD) after calibration. The gantry and MU index encoded in each image are used to extract a sub‐arc from the plan to compute corresponding PD. A 3D matrix is constructed with the 3rd axis representing gantry angle for subsequent γ‐analysis. The EPID has a resolution of 0.8×0.8 mm2 and field size up to 40×30 cm2. Measurements were performed for 21 arcs from 4 patients. Average(range) number of mets is 7(4–9), with volume of 0.36(0.01–1.39)cc. Varian's integrated PD was compared.Results:In PD analysis with 3%, 3mm, and 5% in threshold, pass rate of γ&gt;1.0 is (99.6±0.3)%. Our EPID 3D analysis has the pass rate of (98.8±0.9)%, with additional criteria of 3° in angle‐to‐agreement. Projecting the 3D matrix along each axis, our analysis is reduced to 3 2D analyses, with one mimicking PD analysis at (99.0±0.9)%. Results on other planes are (99.5±0.4)% and (99.5±0.4)%, respectively. Additional criteria can be chosen to diagnose potential treatment abnormalities.Conclusion:We have developed an efficient EPID‐based QA technique for VMAT that has sufficient spatial and gantry angular resolution for use with single isocenter VMAT radiosurgery of multiple mets.