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Interfractional Monitoring of Spacer Hydrogel Morphology in Patients With Prostate Cancer Receiving SBRT Treatment
Journal article   Peer reviewed

Interfractional Monitoring of Spacer Hydrogel Morphology in Patients With Prostate Cancer Receiving SBRT Treatment

Sravya Koduri, David Barbee, Cheongeun Oh, Tal Sherman and Michael J. Zelefsky
Practical Radiation Oncology, Forthcoming
07 Aug 2026
DOI: https://doi.org/10.1016/j.prro.2026.05.015
PMID: 42565768
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Abstract

The purpose of this study is to explore whether spacer hydrogel morphology changes during the course of stereotactic body radiation therapy (SBRT) radiation to the prostate. A total of 198 patients at our institution each received spacer gel before SBRT on magnetic-resonance-guided linear accelerator (MR-Linac) between October 2023 and March 2025. Each patient received at least 1 simulation magnetic resonance imaging and 5 on-treatment magnetic resonance imaging scans. Average time between simulation and first treatment and between simulation and final treatment was approximately 19.9 days and 31.1 days, respectively, with 2- to 3-day intervals between treatments. A U-Net deep learning model was implemented using nnUNetv2. This model was iteratively trained using inference images, resulting in a Dice similarity coefficient >0.98, and was then applied to detect changes in gel morphology over time. Longitudinal modeling of spacer gel radiomic shape features demonstrated a statistically significant change in flatness that remained significant over false discovery rate correction and all sensitivity analyses. This metric increased by 0.003 units per 10 days (95% CI, 0.001-0.005; false discovery rate q ≈ 0.02), implying a progressively rounder shape over the observation period. All other features showed minimal or inconsistent change. The hydrogel appeared larger on treatment imaging, with a mean percent change of 10.62% (95% CI, 8.68%-12.56%; P < .001). The corresponding absolute volume difference was 1.19 cm³ (95% CI, 1.00 cm³-1.39 cm³; P < .001). Because prostate SBRT planning assumes gel stability, interfractional changes could affect rectal dose estimates. In this cohort, the spacer hydrogel’s overall geometry remained stable across the 5-fraction course, with no meaningful evidence of gel resorption. A small increase in the flatness metric was observed, but without a change in size or compactness. These findings indicate that the gel maintains its rectal-sparing configuration throughout SBRT.

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