Proton minibeam (pMBRT) radiation therapy: experimental validation of Monte Carlo dose calculation in the RayStation TPS.

Proton minibeam radiation therapy (pMBRT) dose profile is characterized by highly heterogeneous dose in the plane perpendicular to the beam direction and rapidly changing depth dose profiles. Typically, dose measurements are benchmarked against in-house Monte Carlo simulation tools. It is essential to have a treatment planning system (TPS) that can accurately predict pMBRT doses in tissue and be available via commercial platform for preclinical and clinical use. Methods: The pMBRT beam model was implemented in RayStation for the IBA Proteus®ONE single-room compact proton machine. The RayStation pMBRT beam model is an add-on to the clinically used beam model. The adjustable parameters include air gap, slit thickness, slit pitch, number of slits, slits direction and slit thickness. The pMBRT TPS is validated experimentally against measurements. Six different collimators with various slit widths and center-to-center slit distance are used. The slit width varies from 0.4 mm to 1.4 mm, and the center to center (c-t-c) distance varies from 2.8 mm to 4.0 mm. The slits are non-divergent with a total of 5 slits. Results: When comparing the average depth dose measurements against the RayStation dose MC calculation, the agreement is better than a 95% gamma passing rate using 3mm/3% criteria except the 0.4 mm slit width. However, after we adjusted the slit width by 40 - 60 μm to account for machining uncertainty, the agreement again exceeds a 95% gamma passing rate using 3mm/3% criteria. When comparing the PDDs of the peaks and valleys between RayStation and film measurements, the agreement is above 90% using 2mm/5% criteria. When comparing later profiles at various depths, the agreement is above 90% for all curves using 0.2mm/5%. Conclusions: The pMBRT beam modeling has been successfully established for our Proteus®ONE-based pMBRT system using the RayStation TPS, with demonstrated accuracy through experimental validation.