Erectile Function After Stereotactic Body Radiation Therapy for Prostate Cancer: A Validated Model-Based Comparison to Nerve-Sparing Prostatectomy, Conventional Radiation Therapy, and Brachytherapy

Abstract

The use of prostate stereotactic body radiation therapy (SBRT) for localized prostate cancer is increasing. However, concerns exist regarding the impact of SBRT on erectile function given the large dose per fraction. Using validated prospective models of erectile function for brachytherapy (BT), conventional radiotherapy (EBRT), and nerve-sparing radical prostatectomy (RP), erectile function rates are compared to a prospective cohort of SBRT treated men. Between 2008 and 2013, 373 patients with localized prostate cancer were treated with prostate SBRT (35-36.25 Gy in 5 fractions) per an institutional protocol. No men received androgen deprivation therapy. Prospective health-related quality of life (HRQOL) data were collected via the Expanded Prostate Cancer Index Composite (EPIC)-26. Erectile function (EF) was defined as “firm enough for intercourse”. Individual patient data from the SBRT cohort was entered into the three validated models (EBRT, BT, and RP) from the Prostate Cancer Outcomes and Satisfaction with Treatment Quality Assessment (PROSTQA) multicenter study (JAMA 2011), which adjusted as indicated for patient age, HRQOL, BMI, race, and PSA. Observed and model-expected EF rates were compared. The median age was 69, and 33%, 62%, and 4% were low, intermediate and high risk, respectively. Baseline EF was 49%. At 2 years, 84% of patients had follow up with complete HRQOL. Of those, 50% had EF at baseline, and two years post-treatment, 34% had EF (95% CI 29-40%). EF 2-years post-SBRT did not significantly differ from model predicted 2 year rates of erectile function following EBRT (37%, 95% CI 33-40%, p=0.31) or BT (32%, 95% CI 28-35%, p=0.29). EF following SBRT was significantly improved compared to expected rates following RP (16%, 95% CI 15-18%, p<0.01). When restricting analysis to patients with baseline HRQOL 60-100, EF at baseline was 88% and 2-years post-SBRT was 57% (95% CI 49-64%). This did not significantly differ from 2 year expected rates following EBRT (61%, 95% CI 59-64%, p=0.21) or BT (58%, 55-62%, p=0.37), but was improved compared to RP (29%, 95% CI 27-32%, p<0.01). Observed EF rates after SBRT are comparable to model-expected rates of EF from other radiation modalities. Furthermore, nerve-sparing RP appeared to result in half the rate of EF 2 years post-treatment compared to radiotherapeutic modalities. Ongoing randomized trials, such as Prostate Advances in Comparative Evidence (PACE) that is comparing SBRT, EBRT, and RP, will provide additional key information to this important question. Given that erectile dysfunction remains common across modalities, continued efforts to improve EF are warranted.