Purpose

To evaluate how the number of implanted needles per fraction influences target coverage, dose heterogeneity, and organ at risk (OAR) exposure in high-dose-rate (HDR) prostate brachytherapy.

Material and methods

Retrospective planning system data from 2016 to 2025 were analyzed the level of individual treatment fractions (1,850 fractions from 1,008 patients). Fractions were stratified by needle number into three groups: ≤ 14 needles (n = 202, 10.9%), 15-18 needles (n = 761, 41.1%), and 19-24 needles (n = 887, 47.9%). DVH (dose-volume histogram) doses were reported as percentage of prescription dose (%Rx). Kruskal-Wallis test was used between-group comparisons, with Dunn post-hoc correction (Bonferroni), while associations were assessed with Spearman’s ρ. Multivariable mixed-effects models with patient ID as random effect accounted for within-patient clustering.

Results

Prostate volume differed significantly across needle groups (H = 619.66, p < 0.001), with more needles used in larger glands. Target coverage remained stable across all groups (V_100% ≈ 95.3-95.4%, D_90% ≈ 107.6-107.9% Rx). High-dose indicators decreased progressively: Prostate V_200% from 14.33% to 12.41%, and prostate D_1% from 616% to 497% Rx. Urethra D_1% decreased from 129.5% to 124.6% Rx (ρ = –0.22, p < 0.001), whereas rectum D_30% increased modestly from 60.3% to 63.0% Rx (ρ = 0.18, p < 0.001). All pairwise comparisons were significant (all p < 0.001), except rectum D_30%, between the ≤ 14 and 15-18 needle groups (p = 0.52). The needle number remained a significant independent predictor of all key dosimetric outcomes after adjustment for prostate volume.

Conclusions

A higher needle number was associated with reduced prostate hot spots and lower urethral peak dose while preserving target coverage. These findings support volume-adapted implant catheter density as a practical strategy to improve dose homogeneity in routine HDR prostate brachytherapy. Prostate volume is a major confounder, and should be considered in outcome-linked analyses and institutional benchmarking.