SU-FF-T-390: Study of the Long Term Stability of the Radiation Isocenter of the Cone-Based SRS System On Varian's Trilogy Linac

Abstract

Purpose: The accuracy of the SRS radiation isocenter is of critical importance since it directly affects the dose distribution in patients undergoing SRS procedures. The cone‐based SRS system on Varian's Trilogy® linac consists of a gantry mount and a set of 13 cones with sizes varying from 5mm–35mm. The system has a rotational isocenter accuracy of better than 0.5mm according to manufacturer's specification. To evaluate the long term stability of the SRS radiation isocenter, we analyzed its off‐set based on the data collected since January 2006. Method and Materials: Before each SRS procedure, the radiation isocenter of the SRS system is checked from different gantry angles to make sure it is within specification. A small steel ball is positioned accurately at the ideal isocenter and a 12mm cone is mounted onto the gantry. Gafchromic® films are then exposed at five angles (240°, 270°, 0°, 90°, and 120°) along the most commonly used arcs in our treatment plan. A template of isocentered circles and an Edmund Optics® magnifier are then used to measure the off‐set of circular radiation field from the center of the steel ball. This method can provide an accuracy of 0.1mm and a total of 23 measurements are analyzed in this study. Results: During the period of more than one year, the average off‐sets of the isocenter at the 5 angles are 0.31±0.09mm (240°), 0.27±0.06mm(270°), 0.48±0.08mm(0°), 0.21±0.07mm (90°), and 0.33±0.09mm(120°). We observed that they all fall within the 0.5mm specification of the system. We further observed that the offset at 0 gantry angle (AP) is the largest among the five due to mechanical tilt of the gantry head. Conclusion: The radiation isocenter of Varian Trilogy® cone‐based SRS system is very stable and can provide sub‐millimeter accuracy required for the SRS procedures in the long term.