Background: The salvage treatments for biochemical recurrence (BCR) include local external beam radiation therapy (RT) and systemic androgen-deprivation therapy (ADT). Methods: We reviewed patients who underwent radical prostatectomy (RP) and developed BCR at three institutions. After excluding patients whose nadir prostate-specific antigen (PSA) was higher than 0.2 ng/mL, those who received neoadjuvant/adjuvant therapy, and those whose BCR was not treated until their PSA exceeded 4.0 ng/mL, the remaining 335 patients comprised the cohort of this study. Salvage RT and ADT were performed for 154 and 181 patients, respectively. After the failure of salvage RT, all patients received subsequent ADT. The starting point of this study was the timing of BCR and the endpoint was the development of castration-resistant prostate cancer (CRPC). Results: During the mean follow-up period of 8.5 years after BCR, CRPC was observed in 13 patients administered RT and 24 patients administered ADT. Kaplan–Meier curves demonstrated no significant difference in CRPC-free survival between the RT and ADT groups (10-year CRPC-free survival 89.9 vs. 86.3%, p = 0.199). On the other hand, we found a significant difference in CRPC-free survival between the RT and ADT groups in 50 high-risk patients with two risk factors of Grade Group ≥ 4 and PSA-doubling time < 6 months (10-year CRPC-free survival 73.4 vs. 40.3%, p = 0.040). Conclusion: This study revealed that salvage RT increases the CRPC-free survival rate compared with salvage ADT in high-risk patients with Grade Group ≥ 4 and PSA-doubling time < 6 months.
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