Stereotactic body radiation therapy (SBRT) is a specialized form of radiation therapy (RT) in which extremely high doses of radiation are delivered with high technical precision, allowing abbreviation of a prostate cancer RT course to just five fractions. While SBRT has become increasingly widespread, with promising safety and efficacy data, there remain concerns about whether it is appropriate for patients with large prostate volumes (≥50 cc). These concerns are derived from the fact that a larger target volume will translate to higher incidental doses to the bladder. The goal of the project was to interrogate patient-reported quality of life outcomes and physician-scored toxicity outcomes in patients with large prostates who received SBRT at a single center.
Physician-scored toxicity was abstracted from the medical record and was scored using the Common Terminology Criteria for Adverse Events (CTCAE) v4.03 scale. Patient-reported quality of life (QOL) was measured using the Expanded Prostate Cancer Index Composite (EPIC) instrument, which measures QOL outcomes across several domains, including urinary incontinence and urinary obstruction. The percentages of patients with domain-specific changes exceeding the minimally clinically detectable threshold (% MCDT) were recorded. Data were compared against two referent cohorts: patients without large prostates receiving SBRT at the same institution, and published data for patients receiving conventionally fractionated radiotherapy, brachytherapy, or SBRT in a multi-institutional study.
Between 2010-2017, 124 patients with prostates ≥50 cc received SBRT (median, 63.8 cc; range, 50-263 cc). Median follow-up was 2.8 years. Rates of acute grade 1, 2, and 3 genitourinary (GU) toxicity were 46.8%, 4.8%, and 0.8%, respectively. Late grade 1, 2, and 3 GU toxicity rates were 41.8%, 15.3%, and 3.2%, respectively. These compared favorably with toxicity outcomes in a parallel cohort of 269 patients with prostates <50 cc who received SBRT over the same time frame, with no significant difference in grade ≥3 (i.e., severe) GU toxicity. There was, however, an increased rate of mild (i.e., grade 1) toxicity (p<0.005). With regards to QOL, the %MCDT at 3, 6, and 12 months was 25.8%, 11.1%, and 20.8% for urinary incontinence and 35.5%, 40.7%, and 50.0% for urinary obstructive symptoms. With respect to published %MCDT values, the rates among patients with large prostates compared favorably for urinary incontinence, but urinary obstructive values were worse when compared with a broader pool of patients receiving SBRT.
Physician-reported severe GU toxicity is not significantly different in men with small versus large prostates receiving SBRT, though mild GU toxicity is significantly higher in men with large prostates. Patient-reported QOL metrics suggest worse urinary obstructive symptomatology with SBRT in men with large prostates, compared with SBRT in a broader population.