SU‐E‐T‐54: A New Method for Optimizing Radiation Isocenter for Linac‐Based SRS

Abstract

Purpose:To develop a new method to minimize deviation of linac x‐ray beams from the centroid of the volumetric radiation isocenter for all combinations of gantry and table angle.Methods:A set of ball‐bearing (Winston‐Lutz) images was used to determine the gantry radiation isocenter as the midrange of deviation values. Deviations in the cross‐plane direction were minimized by calibration of MLC leaf position offset, and by adjusting beam position steering for each energy. Special attention was also paid to matching the absolute position of isocenter across all energies by adjusting position steering in the gun‐target axis. Displacement of table axis from the gantry isocenter, and recommended table axis adjustment for contemporary Elekta linacs, was also determined. Eight images were used to characterize the volumetric isocenter for the full range of gantry and table rotations available. Tabulation of deviation for each beam was used to test compliance with isocenter tolerance.Results:Four contemporary Elekta linacs were evaluated and the radius in the gun‐target axis of the radiation isocenter was 0.5 to 0.7 mm. After beam steering adjustment, the radius in the cross‐plane direction was typically 0.2 to 0.4 mm. Position matching between energies can be reduced to 0.28 mm. Maximum total deviation was 0.68 to 1.07 mm, depending primarily on the effect of systematic table axis wobble with rotation.Conclusion:This new method effectively facilitates minimization of deviation between beam center and target position. The test, which requires a few minutes to perform, can be easily incorporated into a routine machine QA program. A tighter radiation isocenter for contemporary Elekta linacs would require reducing the effect of gantry arm flex and/or table axis wobble that are the two main components of deviation from the designated isocenter point.