Long-term clinical outcomes of moderate hypofractionated proton beam therapy for localized prostate cancer: a single-institution retrospective analysis

To evaluate long-term oncologic outcomes and toxicity of moderate hypofractionated proton beam therapy (PBT) in patients with localized prostate cancer. We retrospectively analyzed 192 patients with localized prostate cancer (cT1–T4N0M0) treated with definitive PBT between 2010 and 2021. All patients received 70 Gy (RBE) in 28 fractions. Androgen deprivation therapy (ADT) was administered in 26.6% of patients. The primary endpoint was biochemical failure-free survival (BFFS), defined by the Phoenix criteria. Secondary endpoints included local progression-free survival (LPFS), prostate cancer-specific survival (PCSS), and treatment-related toxicity. Survival outcomes were estimated using the Kaplan–Meier method. With a median follow-up of 90.6 months, the 10-year BFFS rate was 78.5%. Risk-stratified 10-year BFFS rates were 100.0%, 80.3%, and 66.6% for low-, intermediate-, and high-risk groups, respectively. The overall 10-year LPFS rate was 91.3% (100.0%, 88.9%, and 89.0% for low-, intermediate-, and high-risk groups, respectively), and the 10-year PCSS rate was 100.0% in all risk groups. Among 27 patients with biochemical recurrence, intraprostatic local recurrence ( n = 11) was the most common clinical failure pattern. On multivariable analysis, NCCN risk group and ADT use were significantly associated with BFFS. Grade ≥ 3 gastrointestinal and genitourinary toxicities were uncommon, occurring in 5.2% and 3.1% of patients, respectively, with no treatment-related deaths. Moderate hypofractionated PBT provides favorable long-term disease control with acceptable toxicity in localized prostate cancer, despite a relatively high proportion of high-risk patients and limited ADT use. Intraprostatic recurrence appeared to be the predominant pattern of clinical failure in this cohort. Not applicable.