<title>Creation of a reference image with Monte Carlo simulations for online EPID verification of daily patient setup</title>

Abstract

Patient positioning accuracy remains an issue for external beam radiotherapy. Currently, kilovoltage verification images are used as reference by clinicians to compare the actual patient treatment position with the planned position. These images are qualitatively different from treatment-time megavoltage portal images. This study will investigate the feasibility of using PEREGRINE, a 3D Monte Carlo calculation engine, to create reference images for portal image comparisons. Portal images were acquired using an amorphous-silicon flat-panel EPID for 1) the head and pelvic sections of an anthropomorphic phantom with 7 -8 mm displacements applied, and 2) a prostate patient on five treatment days. Planning CT scans were used to generate simulated reference images with PEREGRINE. A correlation algorithm quantified the setup deviations between simulated and portal images. Monte Carlo simulated images exhibit similar qualities to portal images, the phantom slabs appear clearly. Initial positioning differences and applied displacements were detected and quantified. We find that images simulated with Monte Carlo methods can be used as reference images to detect and quantify set-up errors during treatment.