Application of a Radiopaque Viscous Hydrogel Spacer for Prostate Cancer Radiation Therapy: A Prospective Phase 2 Study

Purpose: The aim of this study was to evaluate the application of a radiopaque viscous spacer (RVS) for prostate cancer radiation therapy (RT), including injection procedure, toxicity, treatment planning, image guidance, and imaging results up to 12 months after RT. Methods and Materials: RVS (median, 10 mL) was injected between prostate and rectal wall in 30 patients. Cone beam computed tomography (CT) was performed during the course of RT, a magnetic resonance imaging 3 and 12 months after RT. Injection and treatment tolerability were analyzed. The resulting distribution was compared with a control group of 30 patients with an initially fluid spacer. Results: Procedure- or device -related adverse events were not observed. Signs of hydrogel migration were not found in any case. The volume decreased by 25% at 3 months after RT, and small residues were detected at 12 months after RT in 3 cases (10%). The median rectal volume percentage within the 90% isodose was 3.0% (interquartile range, 1.5%-4.5%). Acute and late gastrointestinal toxicities were found in 17% and 3%, respectively (all grade 1). The median distance between prostate and rectum at the base/midplane/apex was greater for RVS in comparison to initially fluid spacer (14/12/11 mm vs 12/10/10 mm, respectively), the gel symmetry (right vs left from midline) was comparable. The application was assessed to be easier to control by the users, and visibility in cone beam CT as good. Conclusions: The injection of a radiopaque viscous hydrogel spacer resulted in a prostate -rectum distance of >10 mm in most cases. The resulting rectum volume within the high -dose region and RT toxicity were very low. Advantages in comparison to the conventional hydrogel spacer are predominantly an improved placement control during the injection process and good visibility on CT. (c) 2023 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.