Gamma index variation in the verification of step & shoot IMRT

Abstract

Introduction IMRT plan verification is one of the most important parameters in the modern radiotherapy chain in order to achieve high quality and reproducible treatment delivery. Purpose The purpose of this study is to present a detailed analysis of the variation of gamma index (γ-index), which is used as a primary quantitative parameter in IMRT plan verification. Materials and methods We retrospectively considered 60 Prostate and 30 Head and Neck (H&N) step and shoot IMRT cases treated in our department. All plans were created on the Oncentra v4.5 TPS (Elekta, Sweden), using 5–7–9 beams, maximum 10 segments/beam and minimum 4 cm2 segments. Plan verification was implemented using the ArcCheck phantom (SunNuclear, FL, USA). γ-index was calculated at 3%–3 mm and at 2%–2 mm using a 10% threshold value with and without global correction. Results γ-Index (3%/3 mm, global) for Prostate and H&N was 97.1 ± 1.7 and 97 ± 2.1 respectively, γ-index was 9% less when not using global corection factor. 1 mm y-shift variation occurred in 70% of verification plans, while x-shift was negligible. Correcting for y-shift, γ-index was 0.4% and 0.9% higher for prostate and H&N respectively. γ-index variation showed no dependance upon the number of beams used. Conclusion Arccheck is an efficient QA tool for individual patient verification, exhibiting γ-index >97% and high reproducibility of results. Further work is done to correlate γ-index values with ROI specific passing rates, as estimated by the 3DVH software analysis.