Influence of dosimetry on outcomes in HDR prostate brachytherapy monotherapy.

PURPOSE: To investigate the association between brachytherapy dose and oncological outcomes in patients with localized prostate cancer treated with high-dose-rate (HDR) brachytherapy monotherapy. METHODS: We retrospectively analyzed 286 consecutive patients treated with HDR brachytherapy monotherapy (19-20 Gy × 1, 13 Gy × 2, or 10.5 Gy × 3) between 2005 and 2013. Dosimetric parameters included the equivalent dose in 2-Gy fractions covering 90% of the planning target volume (EQD2 D90), V100, V150, and dose homogeneity index (DHI). Biochemical relapse-free interval (bRFI) and overall survival (OS) were estimated using the Kaplan-Meier method. Cox regression evaluated associations between EQD2 D90 and outcomes. RESULTS: At median follow-up times of 108 months for bRFI and 120 months for OS, the 10-year bRFI and OS rates were 72.8% and 78.3%, respectively. In the overall cohort, EQD2 D90 was not associated with bRFI or OS. Similarly, V100, V150, and DHI were not associated with clinical outcomes. In subgroup analyses, EQD2 D90 was not associated with bRFI in any fractionation regimen. Higher EQD2 D90 was associated with improved OS in the 2-fraction subgroup but not in the 1- or 3-fraction subgroups. CONCLUSIONS: In patients treated with HDR brachytherapy monotherapy for localized prostate cancer, increasing EQD2 D90 was not associated with improved bRFI or OS. Within the relatively high and narrow dose range achieved, further increases in D90 may provide limited additional benefit once adequate target coverage is achieved, without implying equivalence between fractionation schedules. Further studies are needed to validate these findings and optimize dose delivery in HDR brachytherapy.