Purpose: To determine suitability of the Varian Machine Performance Check (MPC) enhanced couch utility for couch isocentricity testing. This tool uses a specific phantom that contains fiducials for imaging at various gantry, collimator, and couch angles. The intent was to validate the use of MPC as a tool to address isocenter deviation issues.
Methods: A phantom with a single BB was placed at the isocenter of the gantry, collimator, and couch by means of the Winston Lutz technique. The appropriate translations to reach to the isocenter were determined using an open-source python package. The BB phantom was then analyzed at couch 0, 90, 270 with gantry pointing down. Once at the actual machine isocenter, the actual isocenter size was determined by a Winston-Lutz measurement and this data was compared back to the MPC enhanced couch results. With early tests, the final shift to actual isocenter was not done, and larger discrepancies between measurement and MPC were found.
Results: It was found that applying table isocenter offsets to the BB phantom was necessary to replicate the MPC values. The gantry isocenter measurement using the Winston Lutz test was not sensitive to minor offsets from actual isocenter. When the magnitude of the shift vector was less than 0.3 mm, the MPC analysis of the couch isocenter was within an average of 0.13±0.12 mm of the Winston Lutz measurement.
Conclusion: The accuracy of the MPC enhanced couch results as it pertains to the max table offset parameter was confirmed to be within acceptable limits. The MPC utility is an accurate tool for isocenter measurement.