Purpose

To investigate the relation between radiation dose to organs at risk (OARs), including internal and external urethral sphincters (IUS, EUS), and the occurrence of urinary toxicity in patients treated with high-dose-rate (HDR) and pulsed-dose-rate (PDR) prostate brachytherapy.

Material and methods

Two-hundred eighty patients treated with HDR or PDR brachytherapy as monotherapy or boost after external beam radiation therapy (EBRT) between 2012 and 2020 were analyzed retrospectively. Prostate, urethra, bladder, rectum, IUS, and EUS were contoured on transrectal ultrasound (TRUS). Physical dose-volume histograms (DVHs) were converted to EQD₂ (α/β = 3 Gy) and for PDR, corrected for incomplete repair. Urinary toxicity (dysuria, incontinence) was graded using LENT/SOMA scale. Dose-response relationships were assessed with Barnard’s test, while survival and toxicity outcomes were analyzed using Kaplan-Meier and log-rank tests.

Results

Of the 280 patients considered (median follow-up, 92 months; range, 1-146 months), 29 received brachytherapy as monotherapy and 251 as boost after EBRT. Five-year overall survival (OS), disease-free estimate (DFE), local recurrence rate, and biochemical disease-free estimate (bDFE) were 89%, 86%, 3.6%, and 81% for brachytherapy alone, and 85%, 86%, 2.9%, and 85% for EBRT plus brachytherapy, respectively. The cumulative incidence of grade 3 dysuria and incontinence was 3% and 7% at two-years, respectively. Higher EUS dose (D_0.5cm3 > 102-106 Gy) was associated with increased dysuria and incontinence (p ≤ 0.03), while IUS dose (D_0.1cm3 > 83 Gy) was correlated with increased dysuria (p = 0.04).

Conclusions

Higher sphincter doses, particularly EUS exposure, were significantly associated with urinary toxicity. The IUS primarily influenced dysuria. Our results highlight the importance of considering both sphincters as individual organs at risk, and support the implementation of specific dose constraints to minimize urinary side effects.