Retrospective analysis of portal dosimetry pre-treatment quality assurance of prostate volumetric-modulated arc therapy (VMAT) plans. Issue 1 (22nd August 2017)

• Record Type: Journal Article
• Authors: Maraghechi, Borna; Davis, Jack; Badu, Shyam; Fleck, Andre; Darko, Johnson; Osei, Ernest
• Is Part Of: Journal of radiotherapy in practice. Volume 17:Issue 1(2018)
• Page Start: 44
• Page End: 52
• Publication Date: 2017-08-22
• Subjects: EPID, -- gamma index, -- portal dosimetry, -- VMAT quality assurance; Radiotherapy -- Periodicals; 615.842005
• DOI: 10.1017/S1460396917000486
• Languages: English
• ISSNs: 1460-3969

Abstract

Abstract: 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 ( γ max ), average gamma ( γ 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, γ max and γ 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%/3 mm improved gamma criterion with a passing rate of 97% or the 2%/2 mm improved gamma criterion with a passing rate of 95% can be achieved without additional measurements or configurations.