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Retrospective gamma analysis of patient absorbed dose distributions in a view of new tolerance criteria recommendations

Introduction: In radiation oncology, uncertainties in treatment delivery should be kept as low as possible. Establishment of a comprehensive quality assurance (QA) program is a crucial step of uncertainties reduction. Important part of a QA program is verification of calculated patient absorbed dose distributions, also known as patient specific dosimetry (PSD). At Medical Physics Department, PSD verification is undertaken for each advanced radiation oncology treatment. Calculated absorbed dose distributions are validated using gamma method. Since 2018 international tolerance criteria for gamma passing rate calculation have become more stringent. This was a motivation for a retrospective verification of calculated dose distributions validated before 2018 according to new tolerance criteria. Methods and materials: Two different treatment planning system’s dose calculation algorithms were used for absorbed dose calculation. Standard Superposition algorithm (Elekta XiO ver. 5.10) for “field in field” (FiF) technique and Monte Carlo based algorithm (Elekta Monaco ver. 5.11) for intensity modulated radiation therapy (IMRT) technique. Detector IBA Matrixx, along with MultiCube phantom, was used for measurement of absorbed dose distributions. Linear accelerators Siemens Oncor Impression and Siemens Oncor Expression were used for dose delivery. Calculated and measured absorbed doses were compared using IBA OmniPro I’mRT. Up to 2018, recommended tolerance criteria were 3% dose-difference and 3mm distance-to-agreement (DTA). Gamma passing rate was set to ≥95%. In 2018, these criteria were changed to be 3% dose-difference and 2mm DTA. Retrospectively, absorbed dose distributions calculated using the 3%/3mm criteria were re- validated using more stringent criteria. Over 1800 FiF and over 250 IMRT dose distributions were validated. The results were presented per anatomical sites: breast, rectum, H&N, CNS, prostate, lung (FiF) and H&N, CNS, prostate (IMRT). Results and discussion: Applying the 3mm/2% criteria, for FiF, average gamma passing rates for anatomical sites, along with their standard deviation, were as follows (in %): 96.99 (1.67) for breast, 95.22 (4.12) for rectum, 96.98 (1.98) for H&N, 98.39 (2.01) for CNS, 98.17 (1.13) for prostate and 99.22 (2.32) for lung. Correspondingly, for IMRT, average gamma passing rates for anatomical sites, along with their standard deviation, were as follows (in %): 96.35 (3.95) for H&N, 97.09 (1.87) for CNS and 95.78 (1.89) for prostate. Presented results shows that despite the validation according to more stringent tolerance criteria, average gamma passing rates exceed the ≥95% tolerance limit for all observed treatment sites for both treatment planning modalities. Conclusion: Validated average gamma passing rates exceeded the ≥95% tolerance criteria when new recommendations were followed. This shows that a prerequisite to deliver prescribed absorbed dose to the patient was fulfilled and calculated absorbed dose distributions were optimized in a very good manner.

dose calculation algorithm, pre-treatment dosimetric verification, new tolerance criteria

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