SU‐E‐T‐335: Transit Dosimetry for Verification of Dose Delivery Using Electronic Portal Imaging Device (EPID)

Abstract

Purpose:To evaluate the effectiveness of transit dose, measured with an electronic portal imaging device (EPID), in verifying actual dose delivery to patientsMethods:Plans of 5 patients with lung cancer, who received IMRT treatment, were examined using homogeneous solid water phantom and inhomogeneous anthropomorphic phantom. To simulate error in patient positioning, the anthropomorphic phantom was displaced from 5 mm to 10 mm in the inferior to superior (IS), superior to inferior (SI), left to right (LR), and right to left (RL) directions. The transit dose distribution was measured with EPID and was compared to the planed dose using gamma index.Results:Although the average passing rate based on gamma index (GI) with a 3% dose and a 3 mm distance‐to‐dose agreement tolerance limit was 94.34 % for the transit dose with homogeneous phantom, it was reduced to 84.63 % for the transit dose with inhomogeneous anthropomorphic phantom. The Result also shows that the setup error of 5mm (10mm) in IS, SI, LR and SI direction can Result in the decrease in values of GI passing rates by 1.3% (3.0%), 2.2% (4.3%), 5.9% (10.9%), and 8.9% (16.3%), respectively.Conclusion:Our feasibility study suggests that the transit dose‐based quality assurance may provide information regarding accuracy of dose delivery as well as patient positioning.