Experimental validation of time-driven proton PBS FLASH delivery on a clinical proton therapy system

Clinical proton pencil beam scanning systems use dose-driven delivery, limiting operation at ultra-high dose rates required for FLASH radiotherapy. Time-driven spot delivery, controlled by irradiation time rather than monitor chamber feedback, may enable FLASH operation. We implemented a non-clinical time-driven FLASH strategy on a clinical proton PBS system and experimentally validated its performance.

FLASH delivery was achieved by increasing beamline transmission through the energy-selection system and replacing dose-driven spot control with time-driven delivery in the Scanning Controller Diagnostic mode. Beam transmission was evaluated at gantry angle 0°. Depth-dose distributions, beam optics, scanning accuracy and uniform field generation were assessed for 228.4 MeV protons at gantry angles of 0°, 180° and 270°.

Opening the ESS divergence-limiting slits increased beamline transmission from 8% to 57%, resulting in approximately 273 nA at the isocentre (1.7 × 10¹² protons s^-1) and an estimated peak on-axis average dose rate of 2.0 × 10³ Gy s^-1. An 11 × 11 spot field delivered 25 Gy in approximately 0.6 s, corresponding to an average field dose rate of 42 Gy s^-1, with up to 90 Gy s^-1 measured at higher doses. Proton range and Bragg peak width remained unchanged. Spot position deviations were below 0.5 mm, and uniform 4 × 4 cm² fields with <3% flatness were achieved.

Time-driven delivery enables experimental proton PBS operation at UHDR while maintaining beam characteristics suitable for reproducible dosimetry and radiobiological studies, providing a practical and experimentally characterised platform for preclinical proton PBS FLASH research.