Large Prostate Volume Predicts Acute Urinary Pain after Prostate SBRT: Threshold Effect Mediated By Bladder Low-Dose Exposure
Abstract
Purpose
Acute urinary pain is common after prostate stereotactic body radiotherapy (SBRT). Although treatment breaks and bladder filling protocols are adopted to mitigate toxicity, evidence supporting these interventions is limited. We investigated the effects of prostate volume, genitourinary dosimetry, treatment timing, and bladder filling on acute urinary pain.
Methods
We retrospectively analyzed 118 patients who underwent prostate SBRT. Acute urinary pain was defined as CTCAE v5.0 grade ≥1 within one month. Prostate volume was analyzed continuously (per 10mL) and in quartiles. Bladder, trigone, and urethral dose metrics were evaluated, including the bladder V18 (per 10mL). Treatment timing was compared between consecutive daily treatment and treatment with unplanned weekend breaks between fractions. The multivariable logistic regression models included prostate volume, bladder V18, age, baseline IPSS, NCCN risk, medications, bladder filling status, and treatment timing.
Results
Urinary pain occurred in 49 patients (41.5%). Prostate volume was significantly associated with pain (OR 1.41 per 10mL, 95%CI 1.15–1.78, p=0.002). Quartile analysis revealed a threshold effect: highest-quartile patients had markedly increased odds versus lowest quartile (OR 4.46, 95%CI 1.24–16.70, p=0.022), with no differences among lower quartiles. Bladder V18 was the only dosimetric parameter significantly correlated with prostate volume (Pearson’s r=0.41, Spearman’s ρ=0.40, p<0.0001). Bladder V18 was associated with urinary pain univariably but lost significance after adjustment, suggesting mediation. Neither weekend treatment breaks nor bladder filling status were independently associated with urinary pain in the multivariable analysis.
Conclusion
Large prostate volume strongly predicts acute urinary pain after SBRT, with a non-linear threshold at the upper quartile, mediated by increased bladder low-dose exposure. Despite common clinical practice, neither weekend treatment breaks nor bladder filling protocols independently reduced urinary pain in this cohort. Prostate volume and bladder dosimetry, rather than treatment timing or bladder filling protocols, should guide SBRT planning and patient selection strategies.