(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Diode-based patient specific QA (PSQA) devices such as ArcCHECK should undergo recalibration at regular intervals to account for differences in relative sensitivities of individual detectors. For ArcCHECK, the manufacturer suggests calibration be done annually; however, there is no recommendation on how to evaluate the array calibration. In this work, we present a method for validating a new array calibration and examine the impact of array calibration differences on clinical gamma pass rates (GPR).
Methods: To assess array calibration validity, a verification field was created to deliver uniform dose to all diodes via a conformal arc. A new array calibration was completed and verification field was measured on ArcCHECK (Sun Nuclear Corporation, Melbourne FL) to ensure uniformity was achieved. To examine the impact of array calibration on GPR, simple fields and complex patient plans were measured on the ArcCHECK with the new and old array calibrations using an Elekta Agility 6MV and 6FFF beams (Elekta, AB, Stockholm, Sweden). Gamma analysis was performed in the SNC-Patient software using the AAPM TG-218 recommended criteria of 3%/2mm.
Results: 6MV simple fields had an average GPR of 98.0% with the old calibration and 100.0% with the new calibration. For patient plans, average GPR was 95.4% and 93.5% with old and the new calibrations, respectively. 6MV-FFF results were similar, with slightly lower GPR values seen with the new calibration for complex plans.
Conclusion: A test to evaluate array calibration validity was created, and the impact of array calibration differences on PSQA GPR was examined. Results indicate that the new array calibration improved non-uniform diode response to uniform dose delivery, and also improved the GPR for simple fields; however, it marginally decreased the GPR for complex patient plans. This suggests that older array calibration may be masking failed points, which could impact GPR on borderline plans.
Keywords
Intensity Modulation, Quality Assurance
Taxonomy
TH- External Beam- Photons: Quality Assurance - IMRT/VMAT
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