Hypofractionated Proton Therapy in Early Prostate Cancer: Results of a Phase I/II Trial at Loma Linda University

Purpose: To determine whether a hypofractionated proton therapy regimen will control early-stage disease and maintain low rates of side effects similar to results obtained using standard-fraction proton therapy at our institution. Materials and Methods: A cohort of 146 patients with low-risk prostate cancer according to National Comprehensive Cancer Network guidelines (Gleason score <7, prostate-specific antigen [PSA] <10, tumor stage of T1-T2a) received 60 Gy (cobalt Gy equivalent) of proton therapy (20 fractions of 3.0 Gy per fraction) in 4 weeks, a dose biologically equivalent to standard fractionation (44-45 fractions of 1.8 Gy to a total of 79.2 to 81 Gy in 0 weeks). Patients were evaluated at least weekly during treatment, at which time documentation of treatment tolerance and acute reactions was obtained. Follow-up visits were conducted every 3 months for the first 1 years, every 6 months for the next 3 years, then annually. Follow-up visits consisted of history and physical examination, PSA measurements, and evaluation of toxicity. Results: The median follow-up time was 42 months (range, 3-96 months). Acute grade 2 urinary toxicity occurred in 16% (20/120) of the patients; acute grade 2 or higher gastrointestinal toxicity was seen in 1.7% (2/120). At 9 months, 1 patient had late grade 3 urinary toxicity, which resolved by 12 months; no grade 3 gastrointestinal toxicities occurred. The 3-year biochemical survival rate was 99.3% (144/145). The median time to PSA nadir was 30 months. Conclusion: Hypofractionated proton therapy of 60 Gy in 20 fractions was safe and effective for patients with low-risk prostate cancer.