Proton Minibeam Delivery at Ultra High Dose Rates-Experimental Demonstration
Abstract
Purpose
Proton minibeam radiation therapy (pMBRT) reduces normal tissue toxicity while preserving tumor control through spatial dose fractionation. However, multi-slit collimators (MSCs) substantially increase monitor unit requirements and delivery times, limiting clinical translation. This study demonstrates the feasibility of delivering pMBRT using an ultra-high-dose-rate (UHDR) proton beam to overcome delivery inefficiencies and enhance clinical applicability.
Methods
Experiments were performed on an IBA Proteus®ONE compact proton therapy system equipped with a securely mounted MSC integrated into the treatment nozzle. The MSC featured a center-to-center slit spacing of 2.8 mm, a slit width of 1.0 mm, and brass thicknesses of 6.5 cm and 10 cm. UHDR delivery was achieved using 228 MeV protons at a beam current of 125 nA, compared with conventional clinical beam currents of approximately 1–5 nA. Dose distributions were measured using Gafchromic film in solid-water phantoms and compared with Monte Carlo simulations performed using TOPAS. Delivery times were evaluated for UHDR, FLASH, and conventional clinical beams. Pencil-beam scanning (PBS) dose rates were calculated from machine log files.
Results
pMBRT dose distributions were successfully delivered under UHDR conditions, reducing treatment times to approximately 2.5 s compared with ~3 min for conventional clinical delivery. The 10 cm-thick MSC produced higher peak-to-valley dose ratios (PVDR = 4.36 at 2 cm depth) than the 6.5 cm MSC (PVDR = 2.57). The PBS peak dose rate exceeded 40 Gy/s, well above commonly cited FLASH thresholds, while preserving spatial dose modulation.
Conclusion
This study demonstrates the feasibility of integrating pMBRT with UHDR delivery using a clinical proton therapy system. By mitigating delivery-time limitations associated with MSC-based pMBRT, this work supports clinical translation and highlights the potential of combining spatial fractionation with ultra-high-dose-rate irradiation for precision cancer therapy.