Introduction: Rectal gel spacers have been shown to reduce rectal side effects in patients receiving prostate radiation. However, concerns remain regarding precise and reproducible gel injection. We report on a novel rectoprostatic spacer balloon which allows potential for controlled, adaptable deployment without time constraints associated with hydrogel injection. To determine efficacy and safety, the study had co-primary hypotheses: 1) Balloon spacer would result in = 25% reduction of rectal V70 in > 75% of subjects, and 2) Implantation procedure-related and rectal = Grade 1 adverse events would be non-inferior in balloon vs control subjects (events with duration = 2 days within 6 months, CTCAE 4.0). Methods: 222 subjects were enrolled at 16 centers. All patients had biopsy-proven T1-T3 prostate cancer without MRI evidence of posterior extraprostatic invasion. Patients receiving brachytherapy or whole pelvic RT were excluded. Randomization was 2:1 (balloon:control) and subject-blinded. All patients underwent transperineal TRUS-guided fiducial marker placement +/- biodegradable balloon, followed by IMRT. For efficacy endpoint comparisons, plans were generated by central core lab on pre- and post-implant CT scans (81 Gy in 1.8 Gy fractions). Adverse events were reviewed by blinded, independent Clinical Events Committee. Results: The primary efficacy endpoint was met, with 97.9% of balloon subjects (139/142) having rectal V70 reduction of >25% (pre- vs post-implant; p<0.001, 95% CI (94.0, 99.6%). Mean V70 was 7.0 % pre- vs 1.1% post-implant. Similar decreases were observed in rectal V40 – V80. Mean prostate-rectal spacing achieved (19 +3.7mm) was maintained through radiation treatment (18+3.9mm). Balloon placement was successful in 99% of subjects. The primary safety endpoint was met with balloon subjects experiencing fewer = Grade 1 events, 18% vs 23% (p < 0.001, 95% CI -15.7%, 5.5%). Event rate was higher in balloon subjects in early post-implant period, but control arm surpassed balloon group at/beyond day 44 post-implant. On pre-defined secondary endpoint of = Grade 2 events, rates were lower in balloon subjects (4.3% vs 6.5%). Complete balloon resorption was observed on 6-month CT in 98.5% (133/135) of subjects. Conclusions: Biodegradable rectal spacer balloon was effective in significantly reducing dose to rectum, and associated with decreased cumulative rectal plus implantation-related adverse events. Balloon resorption was consistently observed by 6 months. SOURCE OF Funding: BioProtect, Ltd.
