Rectal Spacer Trial Design: Isolating Its True Impact

In this Urology Times Investigator Perspectives series,Martin T. King, MD, PhD, opens by outlining what sets his randomized trial apart from earlier rectal spacer studies.¹ He points to 3 distinguishing features: the use of a hyaluronic acid spacer product (Barrigel), the choice to evaluate hypofractionated radiation therapy (60 Gy in 20 fractions, reflecting a decade-long shift toward shorter fractionation schemes), and the selection of clinically relevant end points, including grade 2 or higher GI toxicity, alongside dosimetric measures.

He then describes the planning and delivery standards built into the trial. Because the goal was to isolate the spacer's specific impact, the study team held both arms to the same rigorous standard rather than only optimizing care for patients who received the spacer. Every patient benefited from a well-documented radiation planning protocol and prospective core lab review, with some cases requiring multiple planning iterations to reach the best achievable plan.

King explains why this mattered: If the control arm had received less optimized radiotherapy, any benefit seen with the spacer could be attributed to poor-quality planning in the comparison group rather than the spacer itself. By ensuring high-quality, state-of-the-art radiotherapy across both arms, the trial could more confidently attribute any observed differences to the spacer.

He closes this segment by connecting that rigor to the strength of the trial's conclusions. Conducted in the modern radiation era, with advanced imaging and planning tools unavailable a decade or two ago, the study still found a potential benefit for rectal spacing, particularly in reducing rectal toxicity. For King, a well-controlled comparison arm is what allows the field to be more certain that this finding reflects the spacer's effect rather than a gap in radiotherapy quality.

In the next episode, "GI Toxicity Reduction: What the Data Mean for Patients," King moves from trial design into the toxicity findings themselves, detailing what the reduction in GI events actually looked like in the spacer arm.

REFERENCE

1. King MT, Chao M, Svatos M, et al. Prospective randomized controlled trial of hyaluronic acid spacer for hypofractionated prostate radiation therapy: 3-year results (NCT04189913). Presented at: 2026 European Society for Radiotherapy and Oncology Congress. May 15-19, 2026. Stockholm, Sweden. Abstract 3623