Sci—Sat AM: Stereo — 05: The Development of Quality Assurance Methods for Trajectory based Cranial SRS Treatments

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Abstract

The goal of this work was to develop and validate non-planar linac beam trajectories defined by the dynamic motion of the gantry, couch, jaws, collimator and MLCs. This was conducted on the Varian TrueBeam linac by taking advantage of the linac's advanced control features in a non-clinical mode (termed developers mode). In this work, we present quality assurance methods that we have developed to test for the positional and temporal accuracy of the linac's moving components. The first QA method focuses on the coordination of couch and gantry. For this test, we developed a cylindrical phantom which has a film insert. Using this phantom we delivered a plan with dynamic motion of the couch and gantry. We found the mean absolute deviation of the entrance position from its expected value to be 0.5mm, with a standard deviation of 0.5mm. This was within the tolerances set by the machine's mechanical accuracy and the setup accuracy of the phantom. We also present an altered picket fence test which has added dynamic and simultaneous rotations of the couch and the collimator. While the test was shown to be sensitive enough to discern errors 1° and greater, we were unable to identify any errors in the coordination of the linacs collimator and couch. When operating under normal conditions, the Varian TrueBeam linac was able to pass both tests and is within tolerances acceptable for complex trajectory based treatments.

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