S1145
Clinical - Urology
ESTRO 2026
GU (3.7%) toxicities were low and unrelated to spacer use or workflow. Spacer use achieved significantly lower rectal doses (median V105 < 0.03 cc; V95 < 3 cc; V90 < 10%; V80 < 20%; all p < 0.00001), confirming a dose–toxicity relationship.
Digital Poster Highlight 529 Rectal Spacer Mitigates Late GI Toxicity Independent of Adaptive Workflow in MR-Guided SBRT for Localized Prostate Cancer Darren MC Poon 1 , Jing Yuan 2 , Oi Lei Wong 2 , Hiu Yi Wong 2 , Bin Yang 3 , Sin Ting Chiu 4 , Mei Yan Tse 3 , Ben Yu 3 1 Comprehensive Oncology Centre, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong. 2 Research Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong. 3 Medical Physics Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong. 4 Department of Radiotherapy, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong Purpose/Objective: Magnetic resonance–guided stereotactic body radiotherapy (MRgSBRT) enables online adaptive replanning to accommodate daily anatomical changes. The clinical value of rectal spacers in the era of adaptive MR-guided radiotherapy remains uncertain. This study evaluated the impact of rectal-spacer implantation on gastrointestinal (GI) and genitourinary (GU) toxicities in patients undergoing 1.5 T MRgSBRT for localized prostate cancer and assessed whether its benefit persists after adjusting for pelvic irradiation and adaptive workflow. Material/Methods: This prospective analysis included 190 consecutive patients treated on a 1.5 T Elekta Unity MR-LINAC (5 × 7.25–8 Gy to the prostate). In high-risk disease, elective pelvic lymphatic radiotherapy (25 Gy/5 fractions) was concurrently delivered. Online adaptation was performed using either Adapt-to-Position (ATP) or Adapt-to-Shape (ATS) workflows; ATS involved daily MR-based re-contouring and re-optimization. Acute ( ≤ 3 months) and late (> 3 months) toxicities were prospectively graded (CTCAE v5.0). Associations between spacer use and grade ≥ 2 (G2+) toxicities were analyzed using Firth’s logistic regression adjusting for pelvic RT and adaptive workflow. Rectal dosimetry was compared using Wilcoxon tests with Benjamini–Hochberg adjustment. Results: Among 190 patients, 71 (37.4%) received rectal spacers and 71 (37.4%) underwent ≥ 1 ATS-adapted fraction. Median follow-up was 26 months. Baseline clinical and treatment characteristics were comparable between cohorts, including age, risk category, androgen- deprivation therapy use, and pelvic RT. G2+ late GI toxicity occurred exclusively in non-spacer patients (9.2% vs 0%; p = 0.0075). Multivariable analysis confirmed spacer use independently reduced late GI toxicity (OR 0.064; 95% CI 0.0009–0.496; p = 0.0042) without interaction with pelvic RT (p = 0.89). No G2+ acute GI events occurred. Acute GU (11.1%) and late
Conclusion: This study provides the first prospective evidence that rectal-spacer implantation significantly reduces late GI toxicity in 1.5 T MRgSBRT, independent of adaptive workflow and pelvic irradiation. Even with daily online adaptation, spacers are essential to achieve optimal rectal protection in high-precision prostate SBRT. These findings demonstrate that anatomical adaptation and anatomical separation function as complementary—not redundant—strategies in modern MR-guided radiotherapy for localized prostate cancer. References: 1. Poon DMC, Yuan J, Wong OL et al. One-year clinical outcomes of MR-guided stereotactic body radiation therapy with rectal spacer for patients with localized prostate cancer. World J Urol. 2024 Feb 23;42(1):97.2. Wong CH, Ko IC, Leung DK et al. Does biodegradable peri-rectal spacer mitigate treatment toxicities in radiation therapy for localised prostate cancer-a systematic review and meta-analysis. Prostate Cancer Prostatic Dis. 2025 Mar 273. Mariados NF, Orio PF 3rd, Schiffman Z et al. Hyaluronic Acid Spacer for Hypofractionated Prostate Radiation Therapy: A Randomized Clinical Trial. JAMA Oncol. 2023 Apr 1;9(4):511-518 Keywords: Rectal Spacer, MRgSBRT, Gastrointestinal Toxicity
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