Hydrogel rectum-prostate spacers and anterior-oblique proton beams ”

Introduction: Anterior-oblique (AO) proton beams form an attractive option for prostate patients receiving external beam radiotherapy (EBRT) as they avoid the femoral heads. For a cohort with hydrogel prostate-rectum spacers, we asked whether it was possible to generate AO proton plans robust to end-of-range elevations in linear energy transfer (LET) and modelled relative biological effectiveness (RBE). Additionally we considered how rectal spacers influenced planned dose distributions for AO and standard bi-lateral (SB) proton beams versus intensity-modulated radiotherapy (IMRT). Material and methods: We studied three treatment strategies for ten patients with rectal spacers: (A) AO proton beams, (B) SB proton beams and (C) IMRT. For strategy (A) dose and LET distributions were simulated (using the TOPAS Monte Carlo platform) and the McNamara model was used to calculate proton RBE as a function of LET, dose per fraction, and photon α/β. All calculations were performed on pre-treatment scans: interand intrafractional changes in anatomy/set-up were not considered. Results: For 9/10 patients, rectal spacers enabled generation of AO proton plans robust to modelled RBE elevations: rectal dose constraints were fulfilled even when the variable RBE model was applied with a conservative α/β=2Gy. Amongst a subset of patients the proton rectal doses for the PTV plans were remarkably low: for 2/10 SB plans and 4/10 AO plans, ≤10% of the rectum received ≥20 Gy. AO proton plans delivered integral doses a factor of ~3 lower than IMRT and spared the femoral heads almost entirely. Conclusion: Typically, rectal spacers enabled the generation of anterior beam proton plans that appeared robust to modelled variation in RBE. However, further analysis of day-to-day robustness would be required prior to a clinical implementation of AO proton beams. Such beams offer almost complete femoral head sparing, but their broader value relative to IMRT and SB protons remains unclear.