Retrospective analysis of portal dosimetry pre-treatment quality assurance of prostate volumetric-modulated arc therapy (VMAT) plans

Background: Electronic portal imaging device (EPID) offers high-resolution digital image that can be compared with a predicted portal dose image. A very common method to quantitatively compare a measured and calculated dose distribution that is routinely used for quality assurance (QA) of volumetric-modulated arc therapy (VMAT) and intensity-modulated radiation therapy treatment plans is the evaluation of the gamma index. The purpose of this work was to evaluate the gamma passing rate (% GP), maximum gamma (gamma(max)), average gamma (gamma(ave)), maximum dose difference (DDmax) and the average dose difference (DDave) for various regions of interest using Varian's implementation of three absolute dose gamma calculation techniques of improved, local, and combined improved and local. Methods and materials: We analyzed 232 portal dose images from 100 prostate cancer patients' VMAT plans obtained using the Varian EPID on TrueBeam Linacs. Results: Our data show that the % GP, gamma(max) and gamma(ave) depend on the gamma calculation method and the acceptance criteria. Higher % GP values were obtained compared with both our current institutional action level and the American Association of Physicists in Medicine Task Group 119 recommendations. Conclusions: The results of this study can be used to establish stricter action levels for pre-treatment QA of prostate VMAT plans. A stricter 3%/3mm improved gamma criterion with a passing rate of 97% or the 2%/2mm improved gamma criterion with a passing rate of 95% can be achieved without additional measurements or configurations.