Novel Treatment Planning Strategy using Single Switching of Universal Range Shifters in Bragg Peak Proton FLASH Radiotherapy.

[BACKGROUND] Bragg peak (BP) proton FLASH radiotherapy (FLASH-RT) holds promise for achieving conformal dose distributions while maintaining ultra-high dose rates (UHDR). However, achieving dose conformality may necessitate patient or beam specific modulation devices. Moreover, the FLASH effect is substantially diminished during energy layer switching in multi-energy BP FLASH-RT.

[PURPOSE] This study presents a novel optimization framework that enables single-energy BP FLASH-RT using a single switching of universal range shifters (URS). The proposed approach offers a more generalizable method for conformal FLASH-RT and addresses the impact of energy layer switching on treatment efficacy.

[METHODS AND MATERIALS] In single-energy BP FLASH-RT planning with a URS (URS plan), both the dose and URS thickness were simultaneously optimized. To emulate an equivalent lower energy layer without switching the proton beam energy, a switchable URS was implemented, thereby generating a treatment plan (sURS plan) that features a single switching of URS event. Subsequently, gradient-based optimization was applied to adjust spot placement within the beam to obtain the final plan, further improving the conformality index. For 10 brain cancer cases and 10 lung cancer cases, both plan types were designed and compared against conventional multi-energy IMPT plans. Additionally, their delivery parameters, dose metrics, and dose rate indicators were analyzed and compared.

[RESULTS] Both URS and sURS plans met clinical dose criteria and robustness requirements, with significantly fewer energy layers and shorter delivery times compared to IMPT. The treatment times for the URS and sURS plans were 0.8 s and 0.9 s for brain cases and 0.7 s and 0.8 s for lung cases, respectively, compared to 42.3 s for brain and 21.4 s for lung in the IMPT plans. After adding an additional URS thickness, the conformality indexes improved from 0.63 to 0.72 for brain cases and from 0.68 to 0.77 for lung cases, bringing them closer to the high conformity of intensity-modulated proton therapy (IMPT) plans that achieved CI of 0.82 and 0.85. Additionally, the volume of normal tissues exposed to the ADR-based V in the URS plan was 57.8% in brain cases and 49.1% in lung cases. After adding a single switching of URS, these volumes changed to 55.2% and 44.6%, respectively, while IMPT plans showed negligible UHDR coverage.

[CONCLUSION] The optimization framework enables URS-based plans with a single switching of URS to achieve ultra-high dose rate coverage for normal tissues and enhances conformality.