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

Abstract

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. 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. 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. 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.