Purpose: A challenge of implementing FLASH proton therapy using a cyclotron-based accelerator is the need to deliver treatment plans with the largest possible beam energy to achieve ultra-high dose-rates (UHDR)s. Recent hardware developments have achieved 500nA proton beamlet flux at the nozzle, greatly enhancing the pencil beam scanning (PBS) beamlet dose-rate. Here we present the design of a patient-specific ridge filter (RF) which delivers a proton PBS treatment plan with optimal dose distribution using a single energy layer.
Methods: The RF design consists of a base structure, ensuring dose conformality at the tumor’s distal end, and a ridge structure composed of a series of spikes centered over each spot position for range modulation. The shape of each spike correlates to the beam energies and proton fluence in the original treatment plan. Correction factors are applied to each spike to account for the beamlet fluence’s approximated Gaussian distribution. Optimization fine-tunes the shape of the RF spikes and the proton fluence delivered to each spot to account for lateral scattering within the RF.
Results: RF optimized for a single beam head and neck patient with 40 Gy in five fractions achieved promising dose distributions around the target structure with 95% of the volume receiving >90% of the prescribed dose. While further optimization is ongoing, one current plan is estimated to be delivered at UHDR within 0.239s. For tumor target voxels, on average 50% of the dose is estimated to be delivered with >282 Gy/s spot dose-rate; in Mandible, Larynx and Constrictor voxels receiving >5Gy/fraction on average 80% and 50% of the dose is estimated to be delivered with >100 and >302 Gy/s spot dose-rate.
Conclusion: We have designed and assessed the performance of a patient-specific RF on a head and neck patient, allowing for UHDR beam delivery necessary for proton FLASH radiotherapy.
Funding Support, Disclosures, and Conflict of Interest: This project is supported by NIH grant #1P01CA257904-01A1 and by an IBA industrial grant. Several co-authors for this study are employed by Ion Beam Applications (IBA)
Not Applicable / None Entered.