Prospective, Randomized Controlled Pivotal Trial of Biodegradable Balloon Rectal Spacer for Prostate Radiation Therapy

Purpose/objectives: Rectal spacers have been shown to reduce rectal side effects in patients receiving prostate radiation. However, concerns remain regarding precise and reproducible gel injection. We evaluated efficacy and safety of a novel rectoprostatic spacer balloon that allows potential for controlled, adaptable deployment. This study tested 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 within 6 months (duration ≥2 days, Common Terminology Criteria for Adverse Events 4.0) would be noninferior in balloon versus control subjects.

Methods and materials: A total of 222 subjects were enrolled at 16 centers. All patients had T1-T3 prostate cancer without magnetic resonance imaging evidence of posterior extraprostatic invasion. Randomization was 2:1 (balloon: control) and subject-blinded. Patients underwent transperineal transrectal ultrasound axial and sagittal-guided fiducial placement ± balloon, followed by Intensity-Modulated Radiation Therapy (81 Gy in 1.8 Gy fractions or biologically equivalent hypofractionated dose). For efficacy comparisons, plans were generated by a central core lab on pre- and postimplant computed tomography scans.

Results: The primary efficacy endpoint was met, with 97.9% of balloon subjects (139/142) having rectal V70 reduction >25% (P < .001). Mean V70 was 7.0 % pre- versus 1.1% postimplant. The primary safety endpoint was met with balloon subjects experiencing fewer ≥grade 1 events, 18% versus 23% (P < .001 for noninferiority). On predefined secondary endpoint of ≥grade 2 events, rates trended lower in balloon subjects (4.3% vs 6.5%, P = .527). Mean perirectal spacing was 19 ± 3.7 mm and maintained through radiation treatment (18 ± 3.9 mm). Balloon resorption was observed on 6-month computed tomography in 98.5% (133/135) of subjects. The Expanded Prostate Cancer Index quality of life instrument was collected throughout study, and did not differ statistically between the study arms.

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.