SU‐E‐T‐236: Evaluation of Electronic Portal Imaging Device in the Quality Assurance of Volumetric Modulated Arc Therapy

Abstract

Purpose: To evaluate the feasibility of engaging electronic portal imaging device (EPID) in the routine VMAT QA programs. Methods: A small radiation field of 1.6×1.6cm was used to irradiate an 8mm diameter ball bearing positioned precisely at machine isocenter and projected images at different gantry angles were recorded using EPID. A dynamic‐MLC based picket fence pattern was created both in (a) static mode and (b) VMAT mode. In either mode, a total of 40 control points were evenly divided into ten strips of 2cm in width. The ratio of radiation fluence between two neighboring strips was set at 2. Elekta linear accelerators operate with binned dose rates. The dose rate transition is instant and in strict synchronization with the gantry rotation speed and MLC movement. The entire delivery of the picket fence pattern was recorded in accumulation mode with Elekta iView. Image analysis was performed using an in‐house program written in MATLAB. Results: During a full gantry rotation, EPID panel was found to move up to 1.3 mm in the gantry‐target direction but less than 0.5mm in the lateral direction. The MLC speed and positioning were well controlled in both static mode and VMAT mode. At any time point during radiation delivery, the positions of individual leaves in each bank were within 0.5mm of the planned position in static mode. Comparing the picket fence pattern in VMAT mode with that in static mode, we noted minimal discrepancy (<0.5mm) between the corresponding leaf edges. In addition, we observed a negligible impact of gantry rotation speed and MLC travel speed on the accuracy of leaf positioning. Conclusions: Through this study, we demonstrate that EPID is indeed a feasible tool in routine VMAT QA program given that the motion of EPID panel per se is carefully considered.