3371 - Outcomes of Carbon-Ion Radiotherapy for Very High-Risk Prostate Cancer in a Contemporary Consecutive Cohort

Purpose/Objective(s): Very high-risk (VHR) prostate cancer remains associated with poor outcomes despite multimodal therapy. We hypothesized that carbon-ion radiotherapy (CIRT) would provide durable biochemical recurrence-free survival (bRFS) and distant metastasis-free survival (DMFS) in patients with NCCN 2020 VHR prostate cancer.

Materials/Methods: We retrospectively analyzed 850 consecutive patients who initiated CIRT between February 25, 2021 and June 30, 2023. All received 51.6 Gy (RBE) in 12 fractions. Androgen deprivation therapy was risk-adapted: none for low-risk disease, approximately 6 months for intermediate-risk disease, and approximately 24 months for high- and VHR disease. Risk groups were defined per NCCN 2020 criteria. The primary endpoint was bRFS using the Phoenix definition. Secondary endpoints included local, pelvic, and distant metastasis-free survival (DMFS). Survival outcomes were estimated using the Kaplan–Meier method. Adverse events were graded using CTCAE v5.0. Median follow-up was 36 months.

Results:

Risk distribution included 58 low-risk, 409 intermediate-risk, 237 high-risk, and 145 VHR patients; one patient was not evaluable for NCCN risk classification. Biochemical recurrence occurred in 2 low-risk, 15 intermediate-risk, 4 high-risk, and 11 VHR patients. In the overall cohort, 3-year bRFS was 97.2% (95% CI, 95.6–98.2). In the VHR subset, 3-year bRFS was 93.3% (95% CI, 87.1–96.6), and 3-year DMFS was 99.3% (95% CI, 95.0–99.9). Two isolated local recurrences and two distant metastases were observed in the VHR subset. No isolated pelvic recurrences occurred. One grade 3 genitourinary adverse event (hematuria) was reported; no grade =3 gastrointestinal adverse events were observed.

Conclusion: In this contemporary consecutive cohort treated with uniform CIRT and risk-adapted ADT, VHR patients demonstrated encouraging biochemical and distant disease control with minimal severe adverse events at mid-term follow-up. These findings support CIRT as a promising therapeutic option for very high-risk prostate cancer, while further treatment optimization remains under investigation.