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

Abstract

SBRT is an emerging option for localized prostate cancer. However, there are no standard dosimetric guidelines and normal tissue tolerances for extreme hypofractionation are unknown. We analyzed dosimetric correlations with patient-reported urinary and bowel quality of life (QOL). Patients with low or intermediate-risk prostate cancer from 18 institutions were enrolled on a Phase II trial from 2007-2012 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 (CTV) was the prostate (low risk patients) or prostate plus 1cm 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 (EPIC) was assessed prospectively pre-treatment and post-treatment at protocol-specific time points. Linear regression was used to assess factors associated with QOL at 1 month (acute) and 24 months (late). Two hundred fifty-nine patients were enrolled. QOL data were available in 98%, 96%, and 84% at baseline, 1 month, and 2 years, respectively. Median age was 69. Univariate analyses of QOL are shown below (Table). Prior TURP was associated with 2-year urinary incontinence, and CTV volume may be associated with both 2-year urinary incontinence and urinary obstruction/irritation. Rectum Dmax was associated with both 1-month and 2-year bowel QOL. Multivariable analyses adjusting for baseline QOL confirmed these findings. At 2 years, the proportion with “moderate” or “big” bowel problems was significantly higher in patients with rectum Dmax stratified by the median value of all patients: >37.4 Gy vs. ≤37.4 Gy (10.5% vs. 1.8%, chi-square p = 0.008).Abstract 66; TableUnivariate analysis of QOL at 1 month and 2 years. (additional analyzed factors including age, baseline QOL, prescription isodose line, CTV Dmax, Urethra D50%, Rectum D25%, and Rectum D50% did not fit in Table)1 Month2 YearsPBetaPBetaUrinary Incontinence DomainTURP0.600.03<0.001-0.27CTV0.980.0010.028-0.15Urethra Dmax0.250.080.550.04Bladder Dmax0.54-0.040.790.02Urinary Obstructive / Irritative DomainTURP0.0650.120.54-0.04CTV0.370.060.061-0.13Urethra Dmax0.400.050.460.05Bladder Dmax0.11-0.100.65-0.01Bowel DomainRectum Dmax0.002-0.200.019-0.16 Open table in a new tab These results from a prospective trial provide novel data that contribute to a better understanding of patient and dosimetric factors associated with adverse QOL effects from prostate SBRT, a treatment that continues to evolve.