Qualitative MRI-Based Assessment of Hydrogel Spacer Distribution Predicts Rectal Dosimetry and Acute Toxicity in Prostate Stereotactic Body Radiotherapy

Abstract

Abstract Background Hydrogel Rectal Spacers (HRS) are increasingly used to reduce rectal irradiation during stereotactic body radiotherapy (SBRT) for localized prostate cancer (PCa). However, the clinical relevance of qualitative spacer distribution remains incompletely defined. This study evaluated whether MRI-based qualitative characteristics of HRS placement correlate with rectal dosimetry and acute rectal adverse events. Methods This prospective observational study included patients with localized PCa treated with transperineal HRS implantation followed by prostate SBRT between 2023 and 2025. Post-implant T2-weighted magnetic resonance imaging (MRI) was used to assess spacer distribution according to three qualitative metrics: Spacer Quality Score (SQS), Symmetry Score (SYM), and Rectal Wall Infiltration score (RWI). Associations between spacer characteristics, rectal dosimetric parameters, and acute rectal toxicity, defined as events occurring within 6 months after SBRT, were analyzed. Results Seventy-six patients were included. Median prostate-rectum separation at the mid-gland level was 10 mm (range, 4–20 mm). SQS was significantly associated with rectal Dmax (p = 0.003), D1cc (p = 0.003), V36Gy (p = 0.018), V30Gy (p = 0.002), and V18Gy (p = 0.002). SYM score correlated with rectal V40Gy (p = 0.009). Acute rectal toxicity occurred in 18 patients (23.7%) and was significantly associated with SQS (p = 0.004). Rectal wall infiltration was identified in 20 patients (26.3%), including edema in 2 (2.6%) and superficial infiltration in 18 (23.7%), but did not correlate with acute rectal adverse events (p = 0.672). Conclusions MRI-based qualitative assessment of HRS distribution provides clinically meaningful information beyond spacer thickness alone. Optimal spacer distribution and symmetry were associated with improved rectal sparing and reduced acute rectal toxicity in patients undergoing prostate SBRT.