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 α / β . 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) 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 potential to increase the therapeutic ratio and implemented urethral sparing without underdosing the periurethral prostate.
Vieceli et al. (Wed,) studied this question.