Dosimetric Evaluation of Large-area Proton Minibeam Radiation Therapy System for Clinical Applications

Proton minibeam radiation therapy (pMBRT) has emerged as a promising radiation treatment modality, offering enhanced tissue-sparing effects compared to conventional proton therapy. For clinical application, we developed a pMBRT system with the largest field size (30 x 40 cm²) in the world, and conducted a comprehensive dosimetric evaluation of its characteristics. Our system consists of a large-area multi-slit collimator (MSC), a depth-dose modulator, a neutron absorber, a range shifter, and a custom snout compatible with the conventional proton therapy machines. In our experiments, we assessed the dose uniformity of the minibeams generated by the MSC, the depth-dose modulation effect of the lead scatterers using PMMA phantoms and radiochromic films, and the neutron absorption effect through Monte Carlo simulations. Two energy conditions (170 MeV with a range shifter and 200 MeV without a range shifter) were investigated while varying the air gap and scatterer configurations to replicate clinical treatment scenarios. Lateral dose profiles along the phantom depth demonstrated that the standard deviation of peak and valley doses remained below 5.5% of their respective mean values. The characteristics of the scatterers designed for selective treatment of shallow-depth tumors were analyzed using the peak-to-valley dose ratio (PVDR). As scatterer thickness increased, the PVDR approached unity just beneath the phantom surface, whereas under low-scattering conditions, the PVDR at the skin exceeded 15. Additionally, the neutron absorption effect was evaluated via Monte Carlo simulations, confirming a significant reduction in secondary neutrons. In conclusion, our system generated minibeams with very uniform dose envelope, and excellent depth-dose modulation using scatterers facilitated simultaneous skin protection and shallow-depth tumor treatment. Furthermore, by mitigating secondary neutrons, our system can also reduce radiation toxicity, enhancing its clinical viability.