A study of multi-parameter impact on beam position accuracy for scanning proton therapy

Pencil beam scanning (PBS) has become the dominant beam delivery method in proton therapy facilities, owing to its high dose conformality and high beam utilization. While prescribed dose distribution necessitates accurate control of both irradiation position and dose, critical factors including magnetic field fluctuation, mechanical deformation, and beam orbit distortion may impact irradiation position accuracy. This study quantitatively evaluates the impacts of these three factors on irradiation position accuracy for the SC200 proton therapy system. Firstly, three-dimensional magnetic field data of nozzle were acquired through COMSOL simulation and subsequently validated against experimental measurements. Secondly, the simulated field data were utilized to Monte Carlo assessment of individual parameter effects: (1) power supply ripple and (2) beam orbit distortion on irradiation position accuracy, along with (3) analysis of gantry deformation-induced isocenter shifts. Thirdly, the combined effects of these three critical parameters on irradiation accuracy were systematically analyzed. Results reveal that irradiation position deviation patterns under multi-parameter interactions exhibit complex behavior, with beam orbit fluctuations showing the most significant impact in three-dimensional magnetic field, particularly through quadrant-dependent deviation inconsistencies. The study further proposes orienting beam orbit distortion toward the fourth quadrant, demonstrating that this approach can largely maintain irradiation position deviations within ±0.5 mm in both X and Y directions under combined parameter effects. These findings establish a foundation for future high-precision dose delivery system design.