Penile Bulb Dose Predicts Sexual Quality of Life after Prostate SBRT: Secondary Analysis of a Multi-Institutional Trial

Abstract

<h3>Purpose/Objective(s)</h3> Prostate radiation is associated with erectile dysfunction. Recently, secondary analyses of the CHIPP (19 vs. 20 vs. 37 fractions) and Hypo-RT trials (7 vs. 39 fractions) demonstrated an association between penile bulb (PB) dose and sexual outcomes. Though stereotactic body radiation (SBRT, 4-5 fractions) is increasingly used, minimal data exist regarding dosimetric predictors of sexual function after SBRT. We herein analyze predictors of erectile dysfunction in patients treated with SBRT on a prospective trial. <h3>Materials/Methods</h3> Patients with low or intermediate risk prostate cancer were enrolled on a multi-institutional phase 2 trial from 2007 to 2012. Patients were treated with robotic SBRT to 38 Gy in 4 fractions on consecutive days, without androgen deprivation therapy. The clinical target volume was prostate alone for low-risk patients or prostate+1 cm of proximal seminal vesicles for intermediate-risk patients. Selected dosimetric parameters for PB and neurovascular bundle were prospectively collected. Erectile function was assessed using the Sexual Health Inventory for Men (SHIM) Questionnaire at baseline and after treatment at protocol-specified time points. The SHIM is a patient reported questionnaire scored from 1-30, with lower scores indicating worse erectile function. Linear regression was used to assess factors associated with SHIM score at 2 years. We chose one PB metric (D10%) for use in multivariable analyses given collinearity of dosimetric variables. <h3>Results</h3> 251 patients with PB data were included and received SBRT at 18 institutions. SHIM score was available in 97% and 79% of patients at baseline and 2 years, respectively. Mean SHIM was 14.1 at baseline, declining to 9.8 at 2 years. Median age was 69 years and median PB D10% was 16 Gy. On univariate analysis, higher age, lower baseline SHIM and each PB metric were all significantly associated with worse erectile function (lower SHIM score) at 2 years (Table). On multivariate analysis accounting for age and baseline SHIM, PB D10% remained significantly associated with lower SHIM score at 2 years (Table). In contrast, dose to the neurovascular bundle was not associated with erectile dysfunction (data not shown). Mean SHIM score at 2 years was 11.2 vs. 8.7 in patients with PB D10% below vs. above the median, respectively (t-test p=0.04). <h3>Conclusion</h3> To the authors' knowledge, this is the largest analysis of dosimetric predictors of erectile dysfunction after SBRT. In addition to known factors such as age and baseline function, penile bulb dose appears to correlate with late erectile dysfunction and should be minimized to preserve sexual QOL in patients receiving SBRT. Table: Linear regression analyses of 2-year SHIM score.