Hydrogel Spacer Distribution Within the Perirectal Space in Patients Undergoing Radiotherapy for Prostate Cancer: Impact of Spacer Symmetry on Rectal Dose Reduction and the Clinical Consequences of Hydrogel Infiltration into the Rectal Wall
Benjamin W. Fischer-Valuck, MDa; Anupama Chundury, MDa; Hiram Gay, MDa; Walter Bosch, DSCa,b; Jeff Michalski MD, MBAa
b. Washington University Advanced Technology QA Center, St. Louis, Missouri
The purpose of this secondary analysis of the SpaceOAR Hydrogel US Pivotal trial was to assess hydrogel distribution and its impact on rectal dose reduction during radiotherapy. The study included 149 patients with stage T1 or T2 prostate cancer, Gleason score ≤7, and a PSA ≤20 ng/mL who were planning to receive IG-IMRT.
A team of 3 radiation oncologists evaluated the postimplant T2-weighted MRI for all 149 patients and determined hydrogel symmetry using a semiqualitative scoring system. A score of Sym 1 recognizes hydrogel symmetry in all 3 slices (midgland, 1 cm superior and 1 cm inferior to midland). Scores 2–5 represent increasing asymmetry (see Figure 1 below). The symmetry was then correlated to rectal dose reduction using a Student t test.
Overall, a symmetrical spacer application was found in 92 (62%) of the midgland and superior axial slices, and 108 (72%) of the inferior slices. Seventy-three (49.0%) patients had spacer symmetry in all 3 axial slices, with 26 (17.4%), 20 (13.4%), 28 (18.8%), and 2 (1.3%) patients with scores of Sym 2 through Sym 5, respectively.
“Significant reduction of rectal dose can still be achieved even in the setting of asymmetric hydrogel spacer placement.”
Conclusion: As previously demonstrated in the Pivotal trial, SpaceOAR Hydrogel demonstrates significant reduction of rectal dose during IG-IMRT. In this study, asymmetry of hydrogel was found to be common; however, all but the most extreme asymmetry still resulted in significant rectal dose reductions.