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

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.

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.

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.

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.