Retrofitting a clinical pencil beam scanning proton therapy unit for 2D ultra-high dose rate scanning

Purpose.This work aimed to demonstrate how existing clinical infrastructure can be adapted to enable ultra-high dose rate (UHDR) pencil beam scanning (PBS) for FLASH research. Leveraging available hardware and minimizing modifications, we extended the capabilities of PSI Gantry 1, previously used as a flexible UHDR fixed-beam delivery platform, by enabling true 2D PBS for UHDR small-field irradiations through the addition of a second scanning direction and multiple gantry angles.Methods.A second (vertical) scanning direction was implemented by repurposing an existing steering magnet and integrating it into the control system. Inter-spot dead times were minimized through software optimizations that synchronized control processes and magnet settling times, optimizing the local average dose rate. A precise spot map acquisition process ensured accurate dose delivery across different gantry angles. Additionally, a1kHzlogging system was introduced, enabling the reconstruction of lateral dose and dose-rate distributions from recorded beam parameters.Results.Gantry 1 was successfully commissioned for 2D scanning of small fields (100×24mm2) at gantry angles from 0∘(beam towards floor) to-120∘. Dose-uniform field deliveries were achieved at local average dose rates up to75Gys-1. The reconstructed lateral dose distributions were in good agreement with CCD measurements regarding the90%iso-dose contour, while the dose-rate distributions were validated againstmicro-Diamonddetector measurements, confirming the precision of the recalculated dose rates.Conclusions.By repurposing existing hardware and optimizing beam delivery, Gantry 1 has been upgraded to a true 2D PBS UHDR scanning system, capable of delivering fields at local average dose rates exceeding70Gys-1. The high-frequency logging system enables future reanalysis of data as the mechanisms underlying the FLASH effect become clearer. This transformation makes Gantry 1 a versatile platform for preclinical research and small animal irradiation, advancing the investigation of the FLASH effect.