Evaluation of the truebeam machine performance check (MPC): mechanical and collimation checks

Machine performance check (MPC) is an automated and integrated image-based tool for verification of beam and geometric performance of the TrueBeam linac. The aims of the study were to evaluate the performance of the MPC geometric tests relevant to beam collimation (MLC and jaws) and mechanical systems (gantry and collimator). Evaluation was performed by comparing MPC to QA tests performed routinely in the department over a 4-month period. The MPC MLC tests were compared to an in-house analysis of the Picket Fence test. The jaw positions were compared against an in-house EPID-based method, against the traditional light field and graph paper technique and against the Daily QA3 device. The MPC collimator and gantry were compared against spirit level and the collimator further compared to Picket Fence analysis. In all cases, the results from the routine QA procedure were presented in a form directly comparable to MPC to allow a like-to-like comparison. The sensitivity of MPC was also tested by deliberately miscalibrating the appropriate linac parameter. The MPC MLC was found to agree with Picket Fence to within 0.3 mm and the MPC jaw check agreed with in-house EPID measurements within 0.2 mm. All MPC parameters were found to be accurately sensitive to deliberately introduced calibration errors. For the tests evaluated, MPC appears to be suitable as a daily QA check device.