Predictors of urinary toxicity with MRI-assisted radiosurgery for low-dose-rate prostate brachytherapy

MRI-assisted radiosurgery (MARS) is a modern technique for prostate brachytherapy that provides superior soft tissue contrast. The purpose of this analysis was to evaluate treatment planning factors associated with urinary toxicity, particularly damage to the membranous urethra (MUL) and external urethral sphincter (EUS), after MARS. We retrospectively reviewed 227 patients treated with MARS. Comparisons were made between several factors including preimplantation length of the MUL and EUS dosimetric characteristics after implantation with longitudinal changes in American Urological Association (AUA) urinary symptom score. Rates of grade 3 urinary incontinence and obstructive urinary symptoms were 4% and 2%. A piecewise mixed univariate model revealed that MUL and V200, V150, V125, and D5 to the EUS were all associated with increased rates of urinary toxicity over time. On univariate logistic regression, MUL >14.2 mm (odds ratio [OR] 2.03 per cm3, 95% confidence interval [CI] 1.10–3.77, p = 0.025), V125 to the EUS (OR 3.21 cm3, 95% CI 1.18–8.71, p = 0.022), and use of the I-125 isotope (OR 3.45, 95% CI 1.55–7.70, p = 0.001) were associated with subacute urinary toxicity (i.e., that occurring at 4–8 months). Optimal dose-constraint limits to the EUS were determined to be V200