Hydrogel spacer protects rectum from radiation during prostate cancer treatment

Injecting a hydrogel spacer is useful for putting some distance between the rectum and prostate during radiotherapy (RT) for prostate cancer, a study reports. As a result, radiation exposure in the rectum is minimized, and late gastrointestinal and genitourinary toxicities are reduced.

Called SpaceOAR, with OAR standing for ‘organs at risk’, the hydrogel spacer is made from absorbable polyethylene glycol (PEG). When injected into the perirectal space, the material pushes the anterior rectal wall away from the prostate.

One interesting feature of the SpaceOAR is that it does not require surgical removal. “The spacer material remains intact during the course of the patient's radiation therapy (approximately 3 months), after which it is absorbed into the body and cleared in the urine,” according to the investigators.

Given the increasing adoption of rectal spacers into clinical practice to address radiation toxicity, the investigators conducted a comprehensive review of studies evaluating the effect of SpaceOAR across the full range of RT modalities for prostate cancer treatment.

The review included 19 studies involving 3,622 patients in total. There was one randomized controlled trial (RCT) in image-guided intensity-modulated RT (IG-IMRT), 18 comparative non-RCTs in external-beam RT (EBRT), brachytherapy, and combinations. No studies in hypofractionation were found.

Regardless of RT type, SpaceOAR meaningfully cut rectal radiation dose. For instance, the average V40 difference relative to no spacer was –6.1 percent in high dose-rate brachytherapy plus IG-IMRT and –9.1 percent in IG-IMRT. [Urology 2021;doi:10.1016/j.urology.2021.05.013]

Meanwhile, the hydrogel spacer produced parallel reductions in gastrointestinal and genitourinary toxicities (1 percent vs 6 percent; p=0.01) and late genitourinary toxicity (15 percent vs 32 percent; p<0.001) in stereotactic body RT.

Improvements occurred in most Expanded Prostate Cancer Index Composite quality of life domains. In low dose-rate brachytherapy plus EBRT, for example, bowel function score decreased by an average of –6.25 at 3 months and –3.57 at 6 months for patients who received SpaceOAR as opposed to no change for patients who did not use a spacer.

“Our review adds to the existing body of evidence on perirectal spacers [and] provides the most comprehensive report on hydrogel spacers, as this includes a wider range of comparative studies than previously reported,” according to the investigators.

The present data are in line with the results of another meta-analysis suggesting that the dosimetric and clinical effects of a hydrogel spacer can be achieved regardless of RT type in real world trials. [JAMA Netw Open 2020;3:e208221]

Nevertheless, the review had several limitations, the investigators acknowledged. First, the non-RCT comparative studies were of relatively low quality, and there were very few studies that examined health-related quality of life in different RT modalities. Also, the review did not consider any resource or cost implications.

“Future research should focus on the development of better designed comparative studies, ideally RCTs, of SpaceOAR in populations that include brachytherapy and hypofractionation,” they said, adding that strategies to protect organs at risk should be recommended as a research priority.