Acute Urinary Retention After I-125 Prostate Brachytherapy in Relation to Dose in Different Regions of the Prostate

Purpose: To assess the influence of dose in different prostate regions, and the influence of anatomic variation on the risk of acute urinary retention (AUR) after I-125 prostate brachytherapy. Methods and Materials: In this case-control study, dosimetry and anatomy were compared between 50 patients with AUR (cases) and 50 patients without AUR (controls). Cases and controls were randomly selected from our database. The following structures were delineated on magnetic resonance imaging: prostate, urethra, peripheral zone, transitional zone, apex, base, midprostate, lower sphincter, and bladder neck. The dosimetric parameters analyzed were D10, D50, D90, V100, V150, and V200. The anatomic parameters analyzed were prostate protrusion into the bladder, bladder overlap, urethra angle, and urethra-bladder angle. The delineator was blinded to the patient's AUR status. Logistic regression analysis was used to investigate the association of these factors with AUR. Results: The dose delivered to different regions of the prostate was not significantly associated with the risk of AUR. Only dose to the bladder neck was significantly associated with AUR (odds ratio 1.13 per 10 Gy; 95% CI 1.02;1.26; p = 0.023). Mean bladder neck D90 was 65 Gy in AUR cases vs. 56 Gy in controls (p = 0.016), and mean bladder neck D10 was 128 Gy vs. 107 Gy, respectively (p = 0.018). Furthermore, on univariate analysis, a larger extent of both bladder overlap and of prostate protrusion were associated with a higher risk of AUR (odds ratio 1.16; 95% CI 1.04-1.28; p = 0.005, and odds ratio 1.83; 95% CI 1.37-2.45; p < 0.001, respectively). The mean extent of prostate protrusion was 3.5 mm in AUR cases vs. 1.0 mm in controls (p < 0.001). Odds ratios did not change substantially after adjustment for potential confounders. On multivariate analysis, the extent of prostate protrusion seemed to be a stronger risk factor for AUR than bladder overlap. Conclusion: The risk of AUR is not associated with dose delivered to different regions of the prostate. However, a higher dose to the bladder neck and a larger extent of prostate protrusion into the bladder are risk factors for the development of AUR after I-125 prostate brachytherapy.