Evaluating Local Doses for Prostate Cancer Treatment Using Low-Dose-Rate Brachytherapy Considering Oncological Control and Toxicity.

Background: This study evaluated factors associated with clinical failure and toxicity in patients with low-, intermediate-, and high-risk prostate cancer treated with low-dose-rate brachytherapy (LDR-BT) alone or combined with external beam radiation therapy (EBRT).

Methods: Seven hundred thirteen patients (low risk: n = 323; intermediate-risk: n = 390) underwent LDR-BT alone, whereas 534 patients (intermediate-risk: n = 225; high risk: n = 309) underwent LDR-BT combined with EBRT. The Fine-Gray hazard model was used to identify factors associated with clinical failure, and the Youden index was employed to determine BED cut-off values for Grade 3 or higher toxicity.

Results: In patients treated with LDR-BT alone, BED thresholds of ≥ 180 Gy2 (low-risk; hazard ratio [HR]: 0.38; 95% confidence interval [95% CI], 0.15-0.98, intermediate-risk; HR: 0.29; 95% CI, 0.12-0.70) was significantly associated with better clinical outcomes. Patients with Grade 3 or higher toxicity had significantly higher BEDs than those without toxicity (p = 0.008). A BED threshold of 200 Gy2 was identified as a cut-off value for toxicity risk. In patients treated with LDR-BT combined with EBRT, BED was not significantly associated with improved clinical failure-free rates. Patients experiencing Grade 3 or higher toxicity exhibited significantly higher BEDs than those without toxicity (p = 0.04). A BED cut-off of 220 Gy2 was determined for toxicity.

Conclusion: For patients undergoing LDR-BT alone, a BED of 180-200 Gy2 is optimal. For patients undergoing LDR-BT combined with EBRT, a BED of 200-220 Gy2 may be more appropriate.