Monte Carlo-based characterization of proton minibeam radiation therapy across clinically relevant beam parameters.

BACKGROUND AND PURPOSE: Proton minibeam radiotherapy (pMBRT) uses a 1D array of narrow beams to widen the therapeutic window of difficult-to-treat tumors. With the aim of identifying tumor locations that could benefit most from pMBRT, we evaluated how irradiation parameters shape 3D dose distributions. MATERIALS AND METHODS: Monte Carlo simulations were used to compute dose distributions in water for different proton energies, beam widths (bws) and center-to-center distances (ctcs). Optimal parameter combinations were selected according to three criteria: (i) minimization of the bw in normal tissue; (ii) maximization of the valley dose in the target; and (iii) minimization of the peak dose in normal tissue. RESULTS: For shallow tumors (≤ 2 cm), 0.5 mm beams with ctc = 3bw kept normal-tissue widths < 1 mm with Bragg-peak-to-entrance dose ratio (BEDR) > 1. For intermediate and deep-seated tumors (8-20 cm), 1.0-1.5 mm beams with ctc = 4-5bw kept normal-tissue widths < 7 mm with peak-to-valley dose ratio (PVDR) > 3 and achieved lateral dose homogeneity in the target. For very deep-seated tumors (> 20 cm), 2 mm beams with ctc = 4bw maintained normal-tissue widths < 10 mm with PVDR > 3 at the cost of BEDR ∼ 0.5. CONCLUSION: pMBRT may offer advantages over conventional proton therapy and GRID therapy for treating shallow and deep-seated tumors. For very deep-seated tumors (> 20 cm), feasibility will depend on tumor size and proximity of organs at risk.