Beam modeling and validation for a 1.5 T MR-linac in an alternative treatment planning system

Objective.MR-guided adaptive radiotherapy (MRgART) is ideally suited to adjust the treatment plan for anatomical changes. The online nature of MRgART involves multiple process steps during each treatment fraction, which may greatly benefit from automation. TheRayStationtreatment planning system incorporates automation features by design, but requires dose calculations in the presence of an external magnetic field and the modeling of the MRgART system. This study developed and commissioned a comprehensive model for the 1.5 T MR-guided linear accelerators (MR-linacs) inRayStation.Approach.MR-linac dose profiles and output factors were used to model the Unity MR-linac inRayStation. The MR cryostat was implemented in the Monte Carlo simulation as a multi-layer barrel, adjusted using simulations and in-air output measurements. The cryostat transmission variation with gantry angle was added as a fluence correction. Dose distributions of on-axis square, off-axis square, and 90 IMRT fields of ten prostate MR-linac treatment plans were evaluated with the PTW Octavius 1500MR detector array. The radiofrequency coils and couch were characterized with megavoltage imager transmission measurements. CT scans of the coils were used to model its geometry inRayStationand mass densities were assigned to match the measured attenuation.Results.Differences between measured and calculated depth dose showed aγ< 0.5 beyondd_max. For both the inline and crossline profiles, theγevaluation exhibited aγ< 1.0 for nearly all evaluated points. The maximum difference between the measured and calculated cryostat scatter was 0.6%. The averageγpass rate for the on-axis, off-axis, and IMRT fields was >98.3% (range: 91.2%-100%). The average coil transmissions were 0.6 (anterior) and 2.2% (posterior).Significance.We successfully modeled and commissioned the 1.5 T MR-linac inRayStationwell within tolerance limits as specified by the American Association of Physicists in Medicine TG-157 report.