Evaluation of Varian Machine Performance Checks for Early Fault Detection

Abstract

Varian Machine Performance Check (MPC) is an automated and integrated image-based tool for verification of beam and geometry performance of TrueBeam linacs. The tolerances for MPC are set by Varian. The checks involve placing an Isocal Phantom on the treatment couch and a combination of high resolution kV and MV images are taken at different collimator, gantry and couch angles. The images are analyzed and a pass/fail indication is displayed at the end of the check. Daily Beam, 6 MV, geometry checks, enhanced MLC and couch check have been performed in our clinic since April 15th 2019. The enhanced MLC check from initial baseline was 0.18 mm. This started to move beyond the tolerance of 0.2 mm and was investigated by Varian engineers leading to a decision to move the MLC carriage by 1mm yielding a new baseline of 0.03 mm for the MLC leaf offset. Another test which started to fail was the enhanced couch check. This was initially thought to be related to the couch movement but in fact was related to a flatness and symmetry error within the ion chamber which lead to replacing the ion chamber. This problem would have taken a lot longer to present if daily MPC checks were not performed. Overall MPC has been straight forward to implement. This helped us to safely schedule repairs and allowed for earlier detection of failing components.