A comparison of radiation dose to the bulb of the penis in men with and without prostate brachytherapy-induced erectile dysfunction

Abstract

Purpose: To retrospectively evaluate the relationship between the radiation dose to the bulb of the penis and the development of erectile dysfunction (ED) in patients undergoing permanent prostate brachytherapy without external beam radiation therapy. Methods and Materials: Twenty-three men who developed ED after transperineal ultrasound-guided permanent prostate brachytherapy for clinical T1/T2 adenocarcinoma of the prostate gland were paired with 23 similar men who maintained potency after implantation. Potency was defined as an erection sufficient for vaginal penetration. The mean and median follow-up for the entire group was 34.6 ± 13.7 months and 32.8 months, respectively. Patients were implanted with either ¹²⁵I (145 Gy TG-43) or ¹⁰³Pd (115 Gy, pre-NIST-99). No patient received external beam radiation therapy either before or after brachytherapy. The bulb of the penis was outlined at 0.5-cm intervals on the Day 0 postimplant CT scan. The radiation dose distribution to the bulb of the penis was defined in terms of the minimal dose delivered to 25%, 50%, 70%, 75%, 90%, and 95% of the bulb (D₂₅, D₅₀, D₇₀, D₇₅, D₉₀, and D₉₅). Results: The radiation dose delivered to the bulb of the penis in men with postbrachytherapy-induced ED was statistically greater for all evaluated dosimetric parameters (D₂₅, D₅₀, D₇₀, D₇₅, D₉₀, and D₉₅). Multivariate analysis indicated that dose to the bulb of the penis and patient age at the time of implant were predictive of postimplant ED, whereas choice of isotope had no effect. Among potent patients, 19/23 had D₅₀ ≤ 40% of prescribed minimal peripheral dose, whereas for the impotent patients, 19/23 had D₅₀ >40% of the minimal peripheral dose. Of the impotent patients, 17 utilized sildenafil, with 15 experiencing a favorable response (88%). Conclusion: Our data suggest that prostate brachytherapy–induced impotence is highly correlated with the radiation dose delivered to the bulb of the penis. With Day 0 dosimetric evaluation, the radiation dose delivered to 50% of the bulb of the penis should be maintained at 50 Gy or less to maximize post-treatment potency. Fortunately, the majority of the brachytherapy-induced ED population responds favorably to sildenafil.