Robust Four-Field and Arc LET-Optimized Proton Prostate Cancer Treatment Plans With Urethral Sparing

To create LET-optimized pencil beam scanning proton prostate cancer treatment plans with clinically acceptable range robustness and urethral sparing.

Two-beam, four-beam, and arc LET-optimized treatment plans were created for a prostate cancer patient with varying levels of robustness to evaluate LET_d and several performance metrics. Urethral sparing was incorporated by de-escalating the physical dose to the urethra while maintaining high LET_d to prevent underdosing the periurethral prostate by merit of its lower $\alpha /\beta$ . Four-beam and arc plans were created with a targeted robustness level for 12 patients along with reference SFO plans, and statistical comparisons were performed on clinical target volume prostate and seminal vesicles minus urethra (CTV PSV_-U) mean LET_d, target volume and organs at risk (OAR) relative biological effectiveness (RBE)-doses, minimum average RBE-doses, target-to-OAR dose ratios (TODRs), and OAR volumes receiving at least 80% of the CTV PSV D95 dose (V80%).

A tradeoff between robustness and high LET_d was demonstrated. Statistically significant differences in LET_d were found, with 2.5 keV/µm in SFO plans, 3.3 keV/µm in arc plans, and 3.5 keV/µm in four-beam plans. Statistically significant increases in CTV PSV_-U D95 RBE-dose resulted in 6.3%-8.1% increases in four-beam plans over SFO. High LET_d in the periurethral prostate prevented underdosage with urethral sparing. V80%s were statistically decreased in four-beam and most arc comparisons relative to SFO. While increased TODRs were demonstrated in a test patient, lack of statistical significance (or superiority in SFO over arc in the case of the rectum) in the 12 patient cohort occurred due to some patients already having low OAR RBE-doses in SFO plans, which could be addressed with additional optimization objectives at lower doses.

LET-optimized treatment plans were created with high LET_d and clinically acceptable robustness with the potential to increase the therapeutic ratio and implemented urethral sparing without underdosing the periurethral prostate.