Early Exploratory Analysis for Patient-reported Quality of Life and Dosimetric Correlates in Hypofractionated Stereotactic Body Radiation Therapy (SBRT) for Low-risk and Intermediate-risk Prostate Cancer: Interim Results from a Prospective Phase II Clinical Trial.

Given the relative novelty of stereotactic body radiation therapy as a treatment modality low-risk and intermediate-risk prostate cancer, little data exist evaluating dosimetry and its impact on patient-reported quality of life (PR-QOL) metrics. Herein, we present an interim analysis of a phase II clinical trial of PR-QOL and dosimetric correlates. Patients with biopsy-proven low-risk or intermediate-risk prostate cancer, prostate volume ≤100 cm, and life expectancy ≥10 years were enrolled. Expanded Prostate Cancer Index Composite (EPIC) scores were tabulated by domain and evaluated in relation to dosimetry. Paired t test was performed to compare differences in scores from baseline. Minimally important differences were established using the anchor-based approach and correlations made using the χ test. A total of 95 patients were analyzed with a median follow-up of 18.1 months (range, 3.0 to 76.9 mo). There were no cases of acute or late grade 3+ GI or GU toxicities. Expanded Prostate Cancer Index Composite scores in urinary obstructive/irritative domain at 1 month (-4.8, P=0.03) and bowel domain at 1, 6, and 12 months (-10.8, -6.1, and -5.2) were significantly different from pretreatment, with both returning to nonsignificant differences around 24 months. Higher bladder V37Gy (≥3.35%) was associated with both late urinary incontinence and obstructive/irritative declines. Both higher rectal D5% and rectal V36Gy >0.6 cm were correlated with an enhanced proportion of patients with late minimally important difference declines. Higher dose volumes for the bladder and rectum predicted for poorer PR-QOL. In contrast to prostate brachytherapy data, neither prostate volume nor urethral dosimetry at this dose schedule correlated with urinary symptoms.