Image-guided moderately hypofractionated proton beam therapy for localized prostate cancer: 5-year outcomes of a phase II trial

The purpose of the present study is to confirm the efficacy and feasibility of moderately hypofractionated proton beam therapy (PBT) at a total dose of 63 Gy relative biological effectiveness (RBE) in 21 fractions for prostate cancer. This phase II clinical trial (PPS-002: UMIN 000017679) enrolled 125 men with histologically confirmed prostate cancer (cT1c-3aN0M0). The primary endpoint was grade ≥2 late gastrointestinal (GI) toxicities. Secondary endpoints included biochemical recurrence rates, clinical recurrence rates, overall survival (OS) and acute toxicities and late genitourinary (GU) toxicities. With a median follow-up period of 71.0 months (IQR: 60-84), the 5-year cumulative incidences of late grade 2 or higher GI and GU adverse events (AEs) were 3.3% and 8.1%, respectively. Biochemical recurrences were observed in three patients; no clinical recurrences occurred. The 5-year biochemical recurrence-free, clinical recurrence-free and OS rates were 97.6% (95% CI: 94.9-100.0), 100% and 97.5% (95% CI: 94.8-100.0), respectively. The long-term follow-up results of moderately hypofractionated PBT of 63 Gy (RBE) in 21 fractions confirmed low incidences of late AEs, successfully meeting the primary endpoint. Furthermore, the low incidence of tumor recurrence supports the overall safety and feasibility of this fractionation schedule.