GAMMA ANALYSIS DEPENDENCE ON SPECIFIED LOW DOSE THRESHOLDS FOR MULTIPL VMAT FIELDS QUALITY ASSURANCE (QA)
Journal Title: Kocatepe Medical Journal - Year 2024, Vol 25, Issue 2
Abstract
OBJECTIVE: In its report TG-119, the American Association of Physicists in Medicine (AAPM) recommends the use of a 10% low-dose threshold or a collimator-determined assessment area when evaluating quality assurance (QA) data for planar dose distribution gamma analysis. In this study, it was aimed to examine the effect of various low-dose thresholds ranging from 0% to 15% according to standard and more rigid acceptance criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm) on quality control of treatment plans. MATERIAL AND METHODS: A total of 44 multiple VMAT (Volumetric Modulated Arc Therapy) treatment plans including 11 head and neck, 11 brain, 11 lung and 11 prostate cancer cases were randomly selected from the Varian Eclipse® treatment planning system (TPS). Gamma analysis was then performed using portal dosimetry software. RESULTS: Using global normalization, the gamma pass through rate (%GP) decreased as the value of the low dose threshold increased. For the low dose thresholds used, the %GP values exceeded 95% for the 3%/3mm and 2%/2mm criteria, except for lung cancer. The percentage change in %GP was 1.69%, 5.65% and 20.03% for lung cancer plans for the 3%/3mm, 2%/2mm and 1%/1mm criteria, respectively, for a low dose threshold of 15%. In the local gamma analysis, the %GP increased as the low dose threshold increased, which is the opposite result of the global gamma method. In all cases, it was above 95% for 3%/3mm and 5% low dose threshold and above. For the local gamma method, the percentage change in %GP was 2.98%, 7.88% and 23.86% for lung cancer plans for a low dose threshold of 15%, 3%/3mm, 2%/2mm and 1%/1mm, respectively. When rigid acceptance criteria were applied, the %GP result started to decrease. For both the 2%/2mm and 1%/1mm criteria, the average GP was below 95% at all low dose thresholds. CONCLUSIONS: Global normalization shows that applying gamma analysis using different low-dose threshold levels has no critical impact on the QA results. In local normalization, the low dose threshold level should be chosen carefully, as low dose points can cause the average %GP to increase rapidly.
Authors and Affiliations
Taha ERDOĞAN
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- EP ID EP740631
- DOI https://doi.org/10.18229/kocatepetip.1236007
- Views 31
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