Commissioning and Validation of a Synchrocyclotron - Based Ultra-High-Dose-Rate Beamline for FLASH.

PURPOSE: FLASH radiotherapy delivers ultra-high dose rate radiation (>40 Gy/s) has shown promise in reducing normal tissue toxicity while maintaining tumor control. IBA's single-room proton system, equipped with S2C2 superconducting synchrocyclotron accelerator, has recently demonstrated to achieve UHDR delivery. Integrating the UHDR beam line in the treatment planning system (TPS) is crucial for accurate dose calculation in preclinical study, optimization of the 2D dose profile as well as paving the way for further accessory development for spread-out Bragg peak FLASH. This study aims to commission and validate a synchrocyclotron-based pencil beam scanning UHDR proton beamline on the IBA ProteusONE system in RayStation TPS. The goal is to establish a framework for TPS modeling and validation, facilitating preclinical FLASH radiotherapy studies.

METHODS: The transmission UHDR beamline using scanning proton beam energy of 228 MeV was characterized at gantry 0° using comprehensive point dose and 2D lateral profiles measurements. The beam model was developed in RayStation, incorporating key parameters such as virtual source position, spot size, integrated depth dose (IDD), and absolute dose calibration. Extensive validations were conducted using ionization chambers, film dosimetry, and 2D scintillation detectors, with gamma analysis performed to assess the accuracy of the TPS model with open field and field in the presence of brass apertures.

RESULTS: The UHDR beamline achieved ultra-high dose rates exceeding 40 Gy/s (average dose rate for a 2.5 x 2.5 cm field) with consistent dose output validated across multiple detectors. The nozzle current was measured to be linear with respect to the requested MU in the range of 45 to 126 nA. The RayStation beam model demonstrated excellent agreement with experimental measurements, achieving less than 2.5% deviation for all point dose measurements. For 2D profile measurements, gamma passing rates >95% under 2%/2 mm criteria for all fields. The TPS allowed optimization of the spot pattern for UHDR FLASH beams aligned closely with clinical beam profiles, enabling accurate preclinical study comparisons.

CONCLUSIONS: A synchrocyclotron-based UHDR beamline was successfully commissioned and validated through a reliable TPS model for transmission FLASH application. The results provide a foundation for preclinical FLASH-RT research and future clinical applications, demonstrating the feasibility of integrating FLASH-RT into existing proton therapy platforms. Future work will extend the commissioning to all gantry angles and explore spread-out Bragg peak FLASH delivery for improved dose conformality.