Impact of Hydrogel and Hyaluronic Acid Rectal Spacer on Rectal Dosimetry and Toxicity in Low-Dose-Rate Prostate Brachytherapy: A Multi-Institutional Analysis of Patient Outcomes

PURPOSE/OBJECTIVE(S) To report on rectal dosimetry and toxicity outcomes in men with prostate cancer (PCa) treated with I125 low-dose rate brachytherapy (LDR-BT) with or without hydrogel (HS) or hyaluronic acid (HA) rectal spacer (RS) insertion. The working hypothesis is the increased retroprostatic distance achieved from RS insertion results in reduced rectal dosimetry and subsequent reduction in rectal toxicity outcomes. MATERIALS/METHODS Seventy consecutive men treated with LDR-BT between December 2017 and July 2019 were included in this study, twenty-eight (40%) men had RS insertion according to the preference of the referring urologist, compared to a group of forty-two men (60%) without RS. Descriptive statistics were used to compare the RS safety, dosimetric effects on organs at risk (rectum and urethra), as well as gastrointestinal (GI) and genitourinary (GU) toxicities (assessed using the CTCAEv4) between the two groups. RESULTS The mean prostate-rectal separation with RS at mid prostate was 9.9mm (SD 2.8mm). There were no post-operative complications for RS insertion. There was significantly reduced rectal dosimetry in RS group vs non-RS group, the median RV100 was 0cc (IQR = 0-0.0cc) vs 0.4cc (IQR = 0.1-1.1cc) (P < 0.001) respectively. Mean rectal D1cc and D2cc were 52.4% vs 84.2% (P < 0.001) and 45.7% vs. 70.0% (P < 0.001) for RS and non-RS group respectively. There were no discernible differences in the mean urethral D20, D5, and D1. There were significantly less ≥ grade 1 acute and late GI toxicities in the RS group when compared to the non-RS group (0% vs 24% P = 0.004 for acute GI toxicity: 4% vs 33%, P = 0.003 for late GI toxicity). Strong association remained for RS with late GI toxicity (odds ratios < 0.2) even after accounting for the confounding effects of post-implant RV100, D1cc and D2cc. CONCLUSION Insertion of RS in men treated with LDR-BT is safe and results in significantly lower values in rectal dosimetry. The reduction in rectal dosimetry with RS insertion translates into significantly reduced rates of acute and late GI toxicities.