Commissioning of a Proton Conformal Flash Radiotherapy Delivery System for a Head and Neck Patient Clinical Trial
Abstract
Purpose
A proton conformal FLASH radiotherapy system was developed to generate spread-out Bragg peaks at tumor targets utilizing beam-modifying devices, consist of conformal energy modulator(CEM), range shifter(RS) and brass aperture(BA). The system delivers scanned beams at ~500nA current to achieve ultra-high dose rate(UHDR) irradiation. A FLASH treatment planning system(TPS) generates patient plans incorporating customized device designs. Key commissioning challenges included validation of machine characteristics and TPS beam model, integrity of manufactured devices, plan dose and dose-rate verification, and robust end-to-end testing.
Methods
We employed cross-validated dosimetry systems including ionization chambers(ICs) and Faraday cups(FCs) with ultra-fast electrometer, together with machine logs, to characterize the FLASH machine performance. TPS-generated plans for multiple regular-shaped targets in a uniform phantom were verified for multi-planar dose distributions and associated dose rates. Manufactured device quality was assured, including using CT-based evaluations for the 3-D printed CEM quality. Comprehensive mock patient QAs are being conducted using conformal FLASH plans generated for retrospectively identified head-and-neck(HN) patients, to be followed by end-to-end test on an anthropomorphic phantom.
Results
Measured R90 values from integrated depth-dose curves in water, delivered through multiple RS slabs and CEM blocks, were concordant with TPS calculations within 1.5 mm. Absolute dose using a 0.65-mm-gap parallel-plate IC agreed with TPS reference fields’ predictions within 2%, with ion recombination verified to be <1% against FC measurements. Beam spot sizes measured using a scintillator system were consistent with TPS specifications. Manufactured device qualities were assured. Multi-planar IC-array-measured dose distributions satisfied 3%/3 mm gamma criteria of 90% threshold. Spot-by-spot delivery timing and UHDR performance were confirmed using machine log files and FC measurements.
Conclusion
Robust dosimetry and verification methodologies were established to test reliable integrated dose and UHDR performance for a proton conformal FLASH system with corresponding TPS, supporting successful commissioning and readiness for a FLASH HN clinical trial.