Rectal spacing, prostate coverage, and periprocedural outcomes after hydrogel spacer injection during low-dose-rate brachytherapy implantation

PURPOSE: To analyze the dosimetric impact and periprocedural outcomes with a bioabsorbable hydrogel rectal spacer injected during low-dose-rate (LDR) prostate brachytherapy implants. METHODS AND MATERIALS: A consecutive series of 80 patients implanted with stranded I-125 LDR brachytherapy seeds were evaluated, of which 40 underwent a transperineal injection of polyethylene glycol (5 cc) in between the prostate and rectum. Same day CT-based dosimetry was compared between patients with and without hydrogel spacer to evaluate for differences in rectal and prostate dosimetry. Physician-reported toxicities were coded with Common Terminology Criteria for Adverse Events (CTCAE) v4. Results: Baseline patient and implant characteristics were similar. There were no acute genitourinary or rectal toxicities attributed to the hydrogel spacer. Comparing patients with and without hydrogel, the mean separation between the prostate and rectum was 13.9 ± 5.2 mm vs. 6.5 ± 5.0 mm (p < 0.0001), respectively. The adjusted mean dose to 1 cc, 2 cc, and 5 cc of the rectum relative to prescription dose was decreased by 32% (p < 0.01), 26% (p < 0.01), and 17% (p < 0.01), respectively. There were no statistically significant differences in prostate coverage: mean V₁₀₀ (92% vs. 91%), V₁₅₀ (45% vs. 48%), and D₉₀ (106% vs. 106%), respectively. At 1 month followup, grade 1 rectal toxicity was 12.5% vs. 17.5% (p = 0.35). No patients developed Grade ≥2 rectal toxicity with hydrogel, although one did without. Conclusion: Hydrogel rectal spacers significantly reduced rectal exposure to LDR brachytherapy seeds without an observable impact on prostate coverage or periprocedural side effects. These outcomes reflect only LDR implants that used stranded seeds.