A multicenter, double-arm, central core lab, retrospective study was performed to compare the rectal dosimetry of patients implanted with two injectable, biodegradable perirectal spacers, in conventional fractionation (CF), as well as ultrahypofractionation (UH) treatment plans.

Fifty-nine patients were enrolled into the study in five centers: two centers in Europe, which implanted a biodegradable balloon spacer in a total of 24 subjects and three centers in the US, which implanted the SpaceOAR in 35 subjects. Anonymized CTs (pre and post-implantation) were reviewed by the central core lab. For VMAT CF plans rectal V50, V60, V70, and V80 were calculated. For UH plans, a corresponding rectal V22.6, V27.1, V31.37, and V36.25 were established representing 62.5%, 75%, 87.5%, and 100% of the 36.25 Gy prescribed dose.

For CF VMAT, a comparison between the balloon spacer and the SpaceOAR revealed a significant difference of 33.4% decrease in mean rectal V50 (71.9% vs. 38.5%, p < 0.001), 27.7% in mean rectal V60 (79.6% vs. 51.9%, p < 0.001), 17.1% difference in mean rectal V70 (84.1% vs. 67.0%, p = 0.001), and a significant difference of 3.0% (p = 0.019) in mean rectal V80 (87.2% vs. 84.2%). With UH analysis, the mean rectal dose reduction for the balloon spacer compared to the SpaceOAR was 79.2% and 53.3% for V27.1 (p < 0.001), 84.1% and 68.1% for V31.71 (p = 0.001), and 89.7% and 84.8% for V36.25 (p = 0.012), respectively.

Rectal dosimetry is more favorable for treatment with the balloon spacer compared with SpaceOAR. Further research, particularly in the context of a prospective randomized clinical trial design, is needed to assess the acute and late toxicity experience as well as physician satisfaction with achieving symmetrical implantation, and ease of use in light of increasing clinical use.