Baseline Post-Void Residual Urinary Volume Is Associated with Acute Genitourinary Toxicity After Five-Fraction Stereotactic Body Radiation Therapy for Localized Prostate Cancer

Background/Objectives: We investigated the association between baseline post-void residual urine volume (PVR) and acute genitourinary (GU) toxicity after prostate stereotactic body radiation therapy (SBRT). Methods: We retrospectively analyzed 140 consecutive patients treated with 36.25 Gy in five every-other-day fractions. The primary endpoint was the maximum clinically assessed acute Grade 2 GU toxicity during treatment or within 90 days after SBRT. Results: Grade 2 toxicity occurred in 26/140 patients (18.6%). For PVR ≥ 40 mL, Grade 2 toxicity occurred in 12/34 (35.3%) versus 14/106 (13.2%) patients (unadjusted OR 3.58, 95% CI 1.46–8.82). After adjustment for age and baseline IPSS, the OR was 3.464 (95% CI 1.389–8.637; p = 0.0077). Apparent discrimination of the adjusted model was modest (AUC 0.670 (95% CI 0.539–0.801)). Conclusions: Higher baseline PVR was associated with clinically assessed acute GU toxicity; the exploratory association requires prospective validation.