A novel approach for spot boosting selection and treatment plan optimization to enhance prompt gamma monitoring of proton therapy.

BACKGROUND AND PURPOSE: Despite continuous progress, proton therapy remains challenged by delivery uncertainties. In vivo monitoring can help address these limitations. Prompt-gamma (PG) imaging is a monitoring technique suitable for adaptive particle therapy, though it suffers from low signal at typical fraction doses. A treatment planning strategy was evaluated, that boosts selected spots to enhance monitoring. MATERIAL AND METHODS: A prostate and a head-and-neck plans were generated with a research version of RayStation (RaySearch, Sweden) treatment planning system. A detailed workflow was implemented to select candidate spots to artificially increase the number of protons (boosting) delivered according to specific criteria. Two boosting modalities were applied: (1) assigning a fixed number of protons per boosted spot; (2) maximizing the number of protons, given a lower bound. Reoptimized, boosted plans were compared to the initial one in terms of dose, dose-averaged linear-energy-transfer (LET), LET-weighted dose and robustness. RESULTS: Boosted plans preserved target coverage and normal tissue sparing until a boost level of around 7 × 108 protons, although dependent on patient case and number of boosted spots. Tumor coverage was ensured after dose rescaling. Sparing of normal tissue was deemed acceptable compared to the initial plan, until a certain boost level. In silico PG signals after boosting confirmed higher number of counts and noise level reduction. CONCLUSIONS: Spot boosting is a feasible approach to enhance monitoring with secondary radiation, like PG, without compromising the plan quality. Its integration into clinics could enable an improved delivery quality assurance and a more precise proton therapy.