Risk factors for rectal bleeding in prostate cancer after radiotherapy with a validation of current rectal dose constraints.

Background: Rectal bleeding is a well-known adverse event following pelvic external beam radiotherapy (EBRT) for prostate cancer. This study investigates risk factors for rectal bleeding and validate our current rectal dose constraints in a real-world setting.

Material and methods: This prospective study includes 248 prostate cancer patients who received EBRT between 2017-2022. EBRT consisted of 56 Gy/39 fractions to the prostate, elective lymph nodes, and seminal vesicles with an integrated boost of 78 Gy to the prostate alone (≤T3a) or to the prostate and seminal vesicles (T3b). Rectal dose constraints were V50 Gy ≤50%, V70 Gy ≤20%, and V74 Gy ≤12%. Rectal bleeding was recorded at baseline and regularly duringfollow-up and included staff reported morbidity and patient reported outcome measures. Risk factors were evaluated in multivariate cox regression analysis.

Results: Median follow-up was 18 months (range 1-61 months).  Sixteen percent (CI:11%;22%) of patients reported rectal bleeding as "rarely", 4%(CI:2%;8%) "about half the time", 0% "usually", and 2%(CI:0%;4%) "always". Five percent reported rectal bleeding as bothersome. It was possible to comply with current rectal dose constraint (V74 Gy ≤12%) in 99.6% of all patients. Body mass index (BMI) (BMI:25-29.9, HR:0.54(CI:0.30;0.98), p=.044 or BMI>29.9, HR:0.40(CI:0.20;0.79), p=0.008)) were predictors for rectal bleeding.

Interpretation: Patient-reported rectal bleeding is common after prostate cancer radiotherapy. High BMI was a protective factor against rectal bleeding. No correlation was observed between rectal dose-volume constraints and the occurrence of rectal bleeding, suggesting that a rectal high-dose constraint of V74 Gy ≤12% is an adequate threshold to minimize patient-reported rectal bleeding.