Ultra-High Dose-Rate Particle Therapy: On the Delivery of Active Beam Scanning at a Synchrotron-Based Facility With p, 4He, 12C and 16O

Abstract

<h3>Purpose/Objective(s)</h3> Despite being a principal research interest, feasibility of raster-scanned ultra-high dose-rate particle beams remains largely unknown. Moreover, with the increasing evidence of potential normal tissue sparing with uHDR delivery, there is an urgent need for comprehensive assessments of facility-specific delivery capacity and characteristics to ultimately drive forward radiobiological investigations of the FLASH effect and subsequent modeling. This work aims to establish and assess the range and limits of uHDR delivery for raster-scanned particle therapy at a synchrotron-based facility for "light" (p, He) and "heavy" (C, O) ion beams. <h3>Materials/Methods</h3> For p, He, C and O ions, different settings of the synchrotron are tested to assess the linearity window for clinically ideal spill characteristics at FLASH dose levels and dose-rates. For instance, different parameters require modification and/or monitoring for each delivery scenario, e.g., monitor chamber current, delivery time, field size, the number of scanned spot positions, the number of particles and thus the achievable dose level. The dose can be assessed in two particular positions, either the entrance channel of a pristine peak or within a spread-out Bragg peak using a range modulator. Limitations in dose level and dose-rate were investigated within the entrance channel where, conceptually, normal tissues reside within the irradiation field. In addition, the influences of the field size and number of scanned-spots on spatially dependent dose-time structure and delivery time are investigated. <h3>Results</h3> For the entrance channel, parameters which primarily impact dose delivery characteristics were both the number of particles to be delivered within a single spill by the synchrotron and the requested beam intensities. Parameter settings influence delivery characteristics differently for "light" and "heavy" particles: for "light ions" maximizing a number of particles per spill to FLASH dose levels can be limited while for "heavy" ions, extraction intensity is the main obstacle in achieving FLASH dose-rates. Characterized for the four ions, the dose-time evolution is spatially dependent and varies with the number of spots and scanning patterns. <h3>Conclusion</h3> In parallel with mean delivery time and mean dose-rate measured live via monitoring systems, temporal delivery aspects of in-field dose distributions can be assessed and verified (experimentally or in-silico) for active beam delivery that could complement further bio-modelling. The results of this work make evident the importance of a proper assessment of dosimetry and delivery characteristics prior to biological experimentation, as demonstrated at our synchrotron-based facility where delivery parameters may fluctuate between treatment sessions.