Rectal Spacers in Modern, Image-Guided Radiotherapy

Martin T. King, MD, PhD, reflects on how far prostate radiotherapy has advanced over the past decade or two, from 3D planning to intensity-modulated radiation therapy, with both CT and MR simulation now informing more consistent treatment planning through advanced computational techniques. He notes that this progress has raised the bar for rectal spacing to demonstrate added value, yet even with these advancements, the trial still showed a reduction in GI toxicity. In his view, that suggests spacing may still help push outcomes further as the field approaches the limits of what planning alone can achieve.

He then explains the dosimetric mechanics behind that benefit. By increasing the distance between the prostate and rectum, a spacer makes it easier to achieve rectal dose constraints and creates a more uniform dose distribution around the prostate rather than concentrated posterior hot spots, simplifying the dosimetrist's job.

Asked why rectal separation still matters in highly precise protocols, King describes how, at his institution, the planning target volume includes a 5-mm margin around the prostate that, for many patients, extends directly into the rectum, exposing part of it to high-dose radiation. Creating separation reduces that exposure and has been associated with improved toxicity outcomes; he adds that apical spacing specifically may have a stronger association with bowel quality-of-life improvements, although this remains an active area of research.

Finally, he connects spacer use to contemporary techniques such as intensity-modulated radiation therapy (IMRT) and image-guided radiotherapy. Because IMRT delivers dose based on the planning target volume, which for many patients extends into the rectum, the spacer helps ensure full target coverage while minimizing high-dose exposure to the rectum. Combined with image guidance tools like fiducial markers and cone beam CT, the spacer gives added confidence that the rectum remains further from the treatment area.

In the next episode, "Comparing 3-Year Rectal Spacer Outcomes Across Technologies," King puts his 3-year results in context against other rectal spacer technologies and shares the practical factors that guide his own product choice.