Predictive Value of the Prostate-Specific Antigen Doubling Time for the Effectiveness of Metastasis-Directed Radiotherapy in Patients with Oligometastases after Radical Treatment for Non-metastatic Prostate Cancer: A Single Institution Retrospective Study

Background: Data on metastasis-directed radiotherapy (MDRT) are limited, particularly regarding its association with the prostate-specific antigen (PSA) doubling time (PSADT). The present study aims to evaluate the oncological outcomes of MDRT on the basis of the PSADT in patients who developed oligometastases after radical treatment for non-metastatic prostate cancer. Methods: : We retrospectively reviewed the clinical data of 35 MDRTs for 29 patients targeting oligometastatic prostate cancer developed after radical prostatectomy or radiotherapy for non-metastatic prostate cancer. Thirty-five MDRTs were classified into the PSADT > 3 months (n = 25) or PSADT ≤ 3 months group (n = 10). Statistical analyses were performed to compare associations between the two PSADT groups and the oncological outcomes such as progression-free survival (PFS) and PSA response after MDRT. Results: : There were no significant differences between the groups in terms of age, PSA level, Gleason score, or TNM stage. Kaplan–Meier analysis showed that PFS was significantly better in the PSADT > 3 months group than in the PSADT ≤ 3 months group (median: 13.3 versus [vs.] 2.6 months, P = 0.046). Regarding castration sensitivity, the predictive role of PSADT > 3 months was maintained in 21 patients who received MDRT without prior salvage hormone therapy (median PFS: 12.7 vs. 2.6 months, P = 0.024). In the castration-resistant setting (n = 14), the frequency of a decrease in serum PSA levels after MDRT by 90% was 54.5% (median PFS: 23.1 months). Conclusions: : MDRT can provide benefit especially for patients with PSADT ≥ 3 months who had oligo-recurrence after the radical treatment for non-metastatic prostate cancer.