1001 - Optimizing Radiation Therapy for Prostate Cancer: Can Dose Reduction to the Pudendal Arteries Preserve Sexual Health?

Purpose/Objective(s): In men treated with prostate radiation therapy (RT), dose to the internal pudendal arteries (IPA), which supply blood to the penile bulb and corpora, may contribute to sexual dysfunction via arteriosclerosis. We assessed the relationship between dose to the IPA and sexual health in men treated with non-vessel sparing prostate RT.

Materials/Methods: 170 men were treated with prostate ± pelvic lymph node RT from 2011-2020, with patient reported sexual function assessed with the EPIC-26 survey. Prostate dose was 60 Gy/20 (n=46), 78 Gy/38 (n=80), and 79.2 Gy/44 (n=44); 62 (37%) received concurrent androgen deprivation therapy (ADT). Bilateral IPA were retrospectively contoured per the POTEN-C Trial Atlas with a 5mm brush and craniocaudal borders at the superior edge of the seminal vesicles and penile bulb, respectively. To account for different fractionation schemes, biologically equivalent doses (BED) were calculated for dose (e.g. D1, D10, D20, through D90, D99 and mean) to the bilateral IPA (?/ß=3). Distance between the prostate mid-gland to the IPA was measured (PPD). The primary endpoint was change in global sexual score (GSS); generalized estimated equation (GEE) models and cox regression were used adjusting for baseline GSS, age, and ADT. A change in GSS more than the minimum clinically important difference (MCID, 10), was also assessed, and the two sided t-test was used to compare change in GSS from baseline to last follow-up. A sample size of 170 men is estimated to provide 90% power to detect a significant difference between two groups stratified by IPA dose, assuming an effect size of 0.5 and significance level of 0.05. Median follow-up after RT for the last sexual health assessment was 37 mo (IQR, 21-52 mo).

Results: Median dose to the IPA was 43 Gy BED, and median PDD was 3.6 cm. The median change in GSS was -10 from baseline to last follow up. IPA dose was correlated with PDD (p<0.01). In the overall cohort on GEE analysis, GSS was correlated with age (p=0.03) and ADT (p=0.006), but not mean IPA dose (p=0.21). Assessment of various dose volume relationships with IPA other than mean dose revealed the strongest associations for pudendal D10%BED to GSS (p=0.08). Analysis of the change in GSS from baseline to last follow-up revealed further associations. Larger PDD (>3.6 cm) was correlated with a smaller drop in GSS (-4.8 vs -15, p=0.13). On a subset analysis in men without ADT, men with mean IPA dose > 43 Gy BED had a change in GSS of -16 compared to -1 (p=0.10); 65% had a drop >MCID compared to 43% (p=0.14). In men with a baseline GSS of 50+, men with mean IPA dose > 43 Gy had a change in GSS of -32 compared to -13 (p=0.10); 82% had a drop >MCID compared to 53% (p=0.03).

Conclusion: Sparing dose to the IPA (<43 Gy BED, or <26 Gy EqD2, corresponding roughly to a dose at or below the 50% isodose line) may help preserve sexual health in subsets of men treated with prostate RT. Considering the IPA as a dose constraint in prostate cancer radiation has the potential to improve sexual health outcomes for men with good baseline health.