3194 - Association between Intraoperative Dosimetric Parameters and Urinary Acute and Late Toxicity after Low-Dose-Rate Brachytherapy

Purpose/Objective(s): Urinary toxicity is a common adverse event following low-dose-rate (LDR) brachytherapy for prostate cancer. Identifying predictive dosimetric parameters could help optimize treatment planning and minimize toxicity. This study investigates the association between intraoperative dosimetric parameters and both early and late urinary toxicity.

Materials/Methods: The data from 100 patients treated with LDR brachytherapy for prostate cancer were retrospectively analyzed. The dosimetric parameters of interest D10%_pre and D30%_pre, which correspond to the minimum dose received by the 10% and 30% most irradiated volumes of the prostate, were measured immediately after implantation. Early urinary toxicity (=G2 at 3 months) was analyzed using group comparisons (Wilcoxon rank-sum test) and ROC analysis to determine discriminative ability and optimal thresholds. For late urinary toxicity (=G2 beyond 6 months), we performed a time-to-event analysis using Cox proportional hazards regression to evaluate the association between intraoperative dosimetric parameters and the risk of late toxicity.

Results: The median age at diagnosis was 66 years (IQR: 61–69), and the median PSA before treatment was 6.39 ng/mL (IQR: 5–8.22). Diabetes was present in 7% of patients. The majority had a performance status of 0 (92%). Based on the AMICO classification, 67% of patients were classified as low-risk and 33% as intermediate-risk (favorable or unfavorable). Early urinary toxicity (=G2) was observed in 16 patients (16%). D10%_pre was not significantly different between groups (p = 0.2172), whereas D30%_pre was significantly higher in the high-toxicity group (median = 192 Gy, IQR: 176–205 Gy) compared to the low-toxicity group (median = 178 Gy, IQR: 163–189 Gy, p = 0.0385). ROC analysis indicated a moderate discriminative ability (AUC = 0.6658, 95% CI: 0.5083–0.8233), with an optimal threshold of 189.99 Gy based on Youden’s index. After a median follow-up of 41.5 months (IQR: 24–60), high late urinary toxicity occurred in 18 patients (18%). D10%_pre was not associated with late urinary toxicity (HR = 1.004, 95% CI: 0.9881–1.02, p = 0.623), confirming that this parameter is not a relevant predictor for either early or late toxicity. However, D30%_pre was a significant predictor of late urinary toxicity when analyzed as a continuous variable (HR = 1.027, 95% CI: 1.009–1.046, p = 0.004). When applying the 189.99 Gy threshold, a non-significant trend toward increased late toxicity in the high-dose group was observed (HR = 0.476, 95% CI: 0.18–1.21, p = 0.118).

Conclusion: D30%_pre was significantly associated with both early and late urinary toxicity after LDR brachytherapy. The 189.99 Gy threshold was predictive of early urinary toxicity and showed a non-significant trend toward late toxicity. These findings highlight the importance of D30%_pre in treatment planning, while further studies are needed to refine dose constraints and confirm its role in late toxicity risk.