SU‐E‐J‐49: Reliability Study of ExacTrac‐ System Image Isocenter Using an On‐Demand QA Procedure

Purpose: To develop a pointer based quality assurance (QA) procedure that quickly verifies the congruence of ExacTrac® image isocenter with the radiation isocenter of a linear accelerator (linac), and use it to study the accuracy and reliability of the ExacTrac® image isocenter. Method and Materials: A 6 mm diameter tungsten ball was aligned with the room laserusing a pointer alignment system. By measuring the positions of ExacTrac®image isocenter and linac radiation isocenter relative to the ball, thedeviations between the two isocenters was easily calculated. The ExacTrac®image isocenter relative to the ball was measured using the built‐in Winston‐Lutz Pointer test, in which kV images of the ball are projected to flat panels and the center of the ball to the image isocenter are calculated by ExacTrac® software. The linac radiation isocenter relative to the ball was measured by taking the portal images of the ball at four gantry angles, 0,90, 180 and 270 with a 6 × 6 cm² field. An in‐house developed software was used to identify the center of the ball and the center of the radiation. This procedure was used in weekly QA for a year in this work. Results: A simple QA procedure was developed to ensure the accuracy of ExacTrac® imageisocenter against the linac radiation isocenter. The procedure can be executed in less than 20 minutes. The data show the position error were ‐ 0.27±0.30, 0.52±0.24, 0.18±0.22 mm in the lateral, vertical and longitudinal directions, and the max deviation were 0.86, 0.89 and 0.54 mm respectively. Conclusion: The weekly verification data showed the ExacTrac® image isocenter can be very reliable with sub‐millimeter accuracy. This level of imaging alignment accuracy is necessary for imaging guided SBRT procedures. The quick QA procedure can be applied on‐demand to ensure the accuracy of ExacTrac® isocenter.