Introduction:

In the past two decades, external beam radiation therapy has rapidly evolved in techniques and fractionated regimens. Newer techniques have allowed for high precision radiation delivery to the clinical target volume while sparing the surrounding organs at risk. However, hypofractionation regimens are also required to provide optimal biochemical outcomes and decrease the risk of distant metastases. Despite the recent evolution of radiation techniques and regimens, the close proximity of the anterior rectal wall to the prostate introduces the trade-off of limiting the total radiation dose below 70 Gy in order to minimize radiation induced rectal toxicity. SpaceOAR (Boston Scientific Corporation, Marlborough, MA, USA), an absorbable polyethylene glycol hydrogel, increases the space between the prostate and rectum. The objective of this systematic review was to evaluate controlled studies on the dosimetric and clinical outcomes of SpaceOAR in men undergoing external beam radiation therapy for localized prostate cancer.

Methods:

The protocol was registered in the International Prospective Register of Systematic Reviews database (PROSPERO: CRD42020181044). PubMed/MEDLINE and The Cochrane Library databases were systematically searched, in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement, for relevant articles in the English language till May 1, 2020. Search strategy included the following terms: (“SpaceOAR System” OR “SpaceOAR” OR “SpaceOAR hydrogel” OR “hydrogel spacer” OR “polyethylene glycol hydrogel” OR “PEG hydrogel” OR “polyethylene glycol spacer” OR “PEG spacer”) AND (“prostate”). Reference lists of included articles and relevant reviews and health technology assessment reports were hand searched to identify additional potentially relevant articles. Randomized and non-randomized studies that evaluated dosimetric and clinical outcomes of external beam radiation therapy in T1-T3 prostate cancer patients, treated with and without SpaceOAR, were deemed eligible.

Results:

Eight studies were included in the systematic review. All of the studies showed SpaceOAR to reduce the radiation dose volume to the rectum over numerous dosimetry levels (Table 1). Of the four studies that assessed toxicity, one reported SpaceOAR to significantly decrease acute Grade 1 diarrhea and two reported SpaceOAR to significantly decrease late Grade 1 and Grade ≥2 rectal toxicities. Two studies assessed cumulative incidence of toxicity at 3 years in which one reported SpaceOAR to significantly decrease urinary incontinence and Grade ≥1 and Grade ≥2 rectal toxicities, and the other reported SpaceOAR to significantly decrease Grade 1 diarrhea and Grade 2 proctitis (Table 2). Moreover, one study reported that fewer SpaceOAR patients experienced 10-point declines in bowel quality of life at 3 years but another study reported no significant difference in 10-point declines in bowel quality of life between the SpaceOAR and control groups at 5 years.

Conclusion:

SpaceOAR effectively reduced the radiation dose to the rectum which resulted in a reduction in mostly Grades 1 and 2 radiation-induced toxicities. However, results for long-term bowel quality of life were inconclusive. Additional well-designed and long-term studies are needed to further evaluate the magnitude of clinical benefits of this promising and novel technology. With the current research available, SpaceOAR may be the most beneficial to those who did not meet the standard rectal dose volume criteria, have higher risk factors of developing rectal toxicities post-radiation, or wish to decrease the length and costs of radiotherapy by increasing the dose of radiation per fraction.

Funding: N/A