Purpose: FLASH irradiation implies delivering the treatment dose at high dose-rates (typically > 40 Gy/s) to elicit sparing in irradiated healthy tissue. To fulfill the FLASH time constraints and achieve high dose-rates in proton PBS irradiation, a working approach is scanning the pencil-beam in a monoenergetic delivery, whilst ensuring the target coverage within the generated Spread-Out-Bragg-Peak (SOBP).
Methods: We present a way to generate a proton FLASH treatment plan unraveling the full potential of proton Bragg-peak for high dose-rate FLASH irradiations, alleviating the limitations in the current shoot-through, transmission-FLASH approach that negates the advantage of the Bragg-peak
Results: The solution consists in passing the proton pencil-beam at the highest cyclotron energy through a range shifter setting the beam maximum range and through a spatially non-uniform ridge filter (the “hedgehog”). The ridge filter base height controls the distal modulation while the height of its spikes defines the proximal modulation depth at each position, giving a conformal dose distribution over the non-uniform CTV surfaces. The spike shapes determine the SOBP plateau flatness.The ridge filter and range shifter are patient and field specific, responsible in generating the SOBP covering the target volume. Its design is based on the weight of each beamlet in the field, by optimizing a standard IMPT plan using an analytical algorithm in MIROpt (http://openmiropt.org/), alongside the MCsquare dose engine (http://www.openmcsquare.org/). A modified MIROpt version optimizes the spot scanning trajectory and computes the percentile dose-rate at each CT voxel.The shape of the ridge filter generated by MIROPT and MCsquare is validated by comparing with the dose distribution computed using MCNPX Monte Carlo.
Conclusion: An open-source TPS considering the time constraints of FLASH irradiation was designed, based on a Conformal Energy Ridge Filter approach that converts a monoenergetic 230MeV proton beam delivery into an SOBP.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Walloon Region, grant No. 8341.