Impact of Hydrogel Spacer Insertion on Radiation Dose to Erectile Structures and Longitudinal Sexual Function in Prostate Cancer Patients

<b>Background:</b> Erectile dysfunction is a common late effect of prostate radiotherapy. Hydrogel spacers aim to reduce radiation exposure to nearby structures by increasing the distance between the prostate and surrounding tissues, potentially preserving sexual function. <b>Methods:</b> In this retrospective cohort study of 117 prostate cancer patients who received hydrogel spacers, we compared pre- and post-insertion radiation dose and anatomical positioning of erectile structures using paired t-tests. Longitudinal sexual function, assessed via EPIC scores, was modeled using linear mixed-effects regression with natural splines (df = 3), incorporating random intercepts and slopes to account for within-subject variability. <b>Results:</b> Spacer insertion significantly reduced radiation dose to the left and right neurovascular bundles (mean reductions: 1.66 Gy, 95% CI: 1.32-2.00; and 1.64 Gy, 95% CI: 1.28-2.01, respectively; <i>p</i> < 0.01) and the right perineal artery (1.33 Gy, 95% CI: 0.57-2.09; <i>p</i> < 0.01). No significant dose changes were observed for the penile bulb or left perineal artery, nor in anatomical distances. However, spatial displacement was confirmed by significant overlap and integrated volume changes. Longitudinal modeling showed a significant decline in sexual function between 12 and ≥36 months post-treatment (Spline 2: β = -12.72, 95% CI: -18.52--6.92 and Spline 3: β = -6.68, 95% CI: -10.96--2.40; <i>p</i> < 0.01). <b>Conclusions:</b> Hydrogel spacer insertion was associated with significant reductions in radiation dose to erectile structures, most notably the neurovascular bundles and the right perineal artery. However, longitudinal analyses revealed no corresponding preservation of sexual function. These findings suggest that while hydrogel spacers effectively reduce radiation exposure to key anatomical structures, their clinical benefit for maintaining erectile function remains uncertain.