Obstructive Uropathy after Prostate SBRT is Rare: Characterizing Clinical and Dosimetric Predictors From a Large Patient Cohort

Abstract

Acute urinary obstruction is a known potential sequela of prostate radiation. Several series have characterized risk factors for patients undergoing brachytherapy. While preliminary data indicate a more favorable incidence of obstruction for patients receiving Stereotactic Body Radiation Therapy (SBRT), there is a paucity of long-term data to define risk factors in this cohort. Herein, we assess these risk factors after prostate SBRT and explore the related potential long-term outcomes in a large hospital experience. 3308 patients with prostate cancer were treated with non-coplanar SBRT at an academic institution from April 10, 2006 to November 25, 2019. Patients had NCCN low (22.3%), favorable intermediate (24.6%), unfavorable intermediate (35.5%) and high risk (17.6%) disease. 2945 (89.0%) treatments consisted of a 5-fraction regimen of 35.0-36.25 Gy while 363 (11.0%) involved pelvic irradiation (median dose 45Gy) followed by 3 fraction SBRT as boost (19.5-21Gy). ADT was prescribed in 774 (23.9%) cases. Acute obstruction was defined as requiring foley catheterization ≤6 months after treatment, with long-term toxicity scored as occurring ≥6 months from the completion of SBRT. Survival curves were calculated by the Kaplan-Meier method and multivariate analysis was assessed by Cox regression. 65 (2.0%) patients experienced acute urinary obstruction. These individuals were more likely to be older than age 70 (54.7% vs. 41.4%, p = .023), with a greater likelihood of taking 5-alpha reductase inhibitor medication (12.1% vs. 5.6%, p = .045). They were more likely to have prior history of TURP procedure (5.2% vs. 1.3%, p = .046) and to have prostate gland size ≥70cc’s (37.5% vs. 13.8%, p = .001). Patients treated with 5-fraction SBRT were more likely to experience acute obstruction if treated with consecutive vs. QOD fractionation (7.0% vs. 0%, p = .013) and if their Bladder V34 was ≥ 12cc’s (56% vs. 28.4%, p = .004). Pelvic irradiation and baseline patient EPIC questionnaires did not predict for acute obstruction. Those experiencing acute obstruction were more likely to undergo subsequent TURP procedure (4.9% vs. 0.5%, p = .023). They were more likely to experience 5-year cumulative grade 2+ (12.5% vs. 3.2%, p = .002) and grade 3+ (12.5% vs. 2.0%, p<.0001) genitourinary toxicity, with no difference in biochemical outcome. On MVA, prostate size ≥70cc’s (HR 9.25, CI 3.01-28.37, p<.001) and Bladder V34 >12cc’s (HR 3.21, CI 1.05-9.80, p = .04) predicted for acute obstruction. Prostate size ≥70cc’s (HR 4.46, CI 1.21-16.43, p = .025) was the lone predictor for grade 3+ genitourinary outcome. Acute obstructive uropathy is measurable, though rare, in a large series of men undergoing prostate SBRT. Prostate size ≥70cc’s and Bladder V34 >12cc’s portend for an increased risk of acute obstruction. Those experiencing acute obstruction do not appear more likely to experience long-term genitourinary toxicity after definitive prostate SBRT.