Patient and Dosimetric Predictors of Genitourinary and Bowel Quality of Life After Prostate SBRT: Secondary Analysis of a Multi-institutional Trial

Purpose: Stereotactic body radiation therapy (SBRT) is an emerging option for localized prostate cancer. However, there are no standard dosimetric guidelines, and normal tissue tolerances for extreme hypofractionation are not well defined. We analyzed dosimetric correlations with patient-reported urinary and bowel quality of life (QOL) on a prospective trial.

Methods and materials: Patients with low- or intermediate-risk prostate cancer from 18 institutions were enrolled on a phase 2 trial from 2007 to 2012 and treated using robotic SBRT to 38 Gy in 4 fractions on consecutive days. No androgen deprivation was used. Patients received simulation with Foley catheter for urethral delineation. The clinical target volume was prostate (low-risk patients) or prostate plus 1 cm of proximal seminal vesicles (intermediate-risk patients). Multiple dosimetric measures for urethra, bladder, and rectum were prospectively recorded. QOL using the Expanded Prostate Cancer Index Composite was assessed before and after treatment at protocol-specific time points. Linear regression was used to assess factors associated with QOL at 1 month and 2 years.

Results: A total of 259 patients were enrolled. QOL data were available for 98%, 96%, and 84% at baseline, 1 month, and 2 years, respectively. Median age was 69 years. Prior transurethral resection of the prostate and clinical target volume size were associated with 2-year urinary incontinence. There was a trend toward worse 2-year obstruction/irritation in older patients on multivariable analysis. Bladder and urethral doses were not associated with either 1-month or 2-year urinary QOL. In contrast, rectum maximum dose was associated with both 1-month and 2-year bowel QOL. At 2 years, the proportion with moderate or big overall bowel problems (as defined by Expanded Prostate Cancer Index Composite-26) was significantly higher in patients with rectum maximum dose greater than versus less than the median 37.4 Gy (11% vs 2%, Fisher's exact test P = .008).

Conclusions: These results provide novel data that contribute to a better understanding of patient and dosimetric factors associated with adverse QOL effects from prostate SBRT.