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

Clinical Snapshot

Key Findings

• P Value: Not applicable for technical validation study

• Effect Size: Local average dose rates up to 75 Gy/s achieved

• Primary Outcome: Successful implementation of 2D ultra-high dose rate pencil beam scanning

• Nnt Or Sensitivity: 90% iso-dose contour agreement between reconstructed and CCD-measured dose distributions

• Confidence Interval: Not reported for primary outcomes

Clinical Application

High feasibility for centers with existing scanning magnets and control systems, requiring primarily software modifications Relevant for Australian proton therapy facilities planning FLASH research programs, though limited by current proton therapy infrastructure in Australia Proton therapy centers with similar pencil beam scanning infrastructure seeking FLASH research capabilities

Abstract

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.

References

1. 1.Dellepiane, G., Colizzi, I., Meer, D., Psoroulas, S., & Schaefer, R. (2026). Retrofitting a clinical pencil beam scanning proton therapy unit for 2D ultra-high dose rate scanning. Physics in Medicine and Biology. https://doi.org/10.1088/1361-6560/ae6d6b