The Evaluation of Hydrogel Rectal Spacer Movement and Absorbance during the Treatment of Prostate Cancer with Proton Therapy

Background: Hydrogel perirectal spacers are widely used to reduce rectal toxicity during prostate radiation therapy; however, their stability and degradation specifically during proton therapy remain poorly characterized. This study evaluated hydrogel spacer geometry and absorption throughout a course of intensity-modulated proton therapy (IMPT). Methods: Ten patients with localized or recurrent prostate cancer underwent hydrogel spacer placement before IMPT. Eight patients received SpaceOAR. All patients were treated to 70 Gy in 28 fractions using two lateral beams with single-field optimization. Baseline CT and contrast-enhanced MRI were obtained for radiation planning, followed by bi-weekly adaptive CT and MRI scans without contrast. Spacer thickness was measured at the prostate base, mid-gland, and apex on serial MRIs. Changes in spacer size were analyzed per patient and in aggregate. Results: Interpatient variability exceeded intrapatient changes over the treatment course. One patient demonstrated complete spacer disappearance by the third assessment, requiring rectal balloon placement. Three patients exhibited notable reductions in spacer size (18%, 33%, and 72% across all regions). The remaining patients showed a <10% reduction overall. Among the eight patients with SpaceOAR spacers, two experienced early spacer loss within four weeks, one demonstrated significant spacer displacement, and five maintained stable geometry. Conclusion: This study provides the first clinical characterization of hydrogel spacer stability during IMPT. While most patients maintained relatively stable spacer geometry, a subset experienced early degradation or displacement. The limited sample size constrains the interpretation of how often these events arise or whether they meaningfully affect rectal sparing. Our results indicate routine monitoring may help detect unexpected spacer changes; however, given the absence of a photon comparison group, the clinical significance during IMPT courses remains unclear. Larger, prospective research is warranted to define the determinants of hydrogel stability in proton therapy and to evaluate whether enhanced surveillance could improve outcomes.