Exploring the Impact of External Urethral Length on Acute Urinary Toxicity in Patients Undergoing High Dose Rate Brachytherapy for Prostate Cancer

Abstract

The pre-operative and post-operative length of the external urethra sphincter (EUS) in patients undergoing radical prostatectomy has previously been reported to correlate with urinary incontinence. We aimed to determine if external urethral length impacts rates of urinary incontinence in patients undergoing high dose rate (HDR) brachytherapy. We performed a retrospective cohort study of patients with localized prostate cancer treated with MRI-based HDR brachytherapy monotherapy on a prospectively maintained database of prostate HDR patients treated from 2017-2018. Patients were treated with 13.5 Gy in 2 fractions delivered 1-2 weeks apart. CT images were used for catheter identification, and a T2 weighted MRI was obtained after the implant for target and normal structure delineation. The planning target volume (PTV) consisted of the prostate with an asymmetric expansion of 0-5 mm, with inclusion of the proximal seminal vesicles. Expanded Prostate Cancer Composite - 26 (EPIC-26) was evaluated at baseline, 1 month, 3 months, 6 months, then every 6 months thereafter. Linear mixed effects models were used to estimate mean EPIC-26 scores, International Prostate Symptom Score (IPSS) scores, and CTCAE v.5.0 genitourinary and gastrointestinal toxicity over time based on EUS length which was dichotomized as ≤ (short) or > (long) than the median. During analysis, changes were also classified as minimal clinically important difference (MCID). 46 patients were treated with definitive HDR using MRI-based planning, including 1 NCCN high risk , 32 intermediate risk, and 13 low risk patients. Median follow up was 4.18 months. The median EUS length was 1.7 cm (range 0.5-2.35 cm). There was no difference in baseline mean EPIC-26 scores in any domains. For the Urinary Incontinence domain, baseline mean scores were similar for short and long EUS (87.79 vs. 91.42; p=0.96). Patients with a short EUS had a greater decline in Urinary Incontinence mean scores at 1 month (72.51 vs. 83.59), 3 months (73.16 vs. 92.59), and 6 months (76.47 vs. 93.01), meeting MCID at all time points after treatment, although this was only marginally significantly associated with EPIC-26 urinary incontinence scores (p=0.05). EUS length was not associated with other EPIC-26 scores, changes in IPSS scores, or acute GU or GI toxicity. In concordance with surgical literature, shorter EUS length may be associated with acute urinary incontinence outcomes as assessed by EPIC-26 scores, however our series is limited by short follow-up and small sample size. Further research is required to validate our findings and, if so, may guide discussion with patients in the future regarding risks and benefits of brachytherapy.