Impact of iso-dose levels on pre-treatment quality assurance in volumetric modulated arc therapy.

The current standard of modern radiotherapy pre-treatment evaluation of dose distributions measured using gamma pass percentage is the predominant metric for Patient-Specific Quality Assurance (PSQA). The aim of the study was to analyze the impact of the different dose levels for three different gamma criteria in various anatomical sites. Retrospectively selected 120 VMAT plans of the brain, head and neck, thorax, and pelvic sites were considered for this study. Quality assurance plans were created and measurements were done using PTW Octavius 4D phantom. Three different gamma criteria were used to evaluate different dose levels' volume gamma passing rates. The maximum gamma passing rate for all dose levels except 100% with a 3%/3 mm criterion was observed for the pelvis site. A significant variation in dose levels was observed with the 3%/2 mm gamma criteria for head and neck sites, particularly above the 85% dose level compared to other anatomical sites. Using 2%/2 mm gamma criteria, there was a rapid fall in the gamma passing rate and all the dose levels showed a significant variation among different sites. This study demonstrated significant variations in gamma passing rates across anatomical sites and dose levels, emphasizing the importance of tailored QA protocols. The findings underscore the need for site-specific action limits and highlight the sensitivity of stricter gamma criteria for detecting errors in complex treatment plans.