Evaluation of Dosimetric Impact in Volumetric Modulated Arc Therapy Using Rolled-up Gafchromic Film and Gamma Analysis

Abstract

Evaluation for gamma models on dose evaluation of volumetric modulated arc therapy (VMAT) fields was investigated depending on analytic models and variable gamma parameters. Gamma evaluation was applied to compare calculated doses with measured doses using Gafchromic EBT2 films in 45° wedge and VMAT field of prostate cancer. Two gamma models were employed for dose evaluation; typically used gamma model with dose difference and distance-to-agreement and improved gamma model calculating local dose gradient. Effect of each gamma model was evaluated according to the grid size (0.5 mm, 1 mm, and 2 mm) and searching range (1 mm, 2 mm, and 4 mm). Gamma model compensating dose gradient in VMAT with small searching range showed higher pass ratio than other model not compensated. Dose evaluation using grid size of 1 mm closely corresponded to 0.5 mm at each gamma model and each field. Dose evaluation was varied by the selection of applied gamma model and evaluation parameters. Evaluated gamma model reflecting compensated dose gradient could be useful in modulated fields with 1 mm grid size and searching range.