S712
Clinical – Lower GI
ESTRO 2026
cancer. Clinical data suggests whole prostate dose escalation increases cancer control with increased risk of late gastrointestinal (GI) toxicity. The main objective of the SABRE trial is to assess the effectiveness of a next generation iodinated perirectal hydrogel spacer in reducing late GI toxicity in subjects undergoing prostate SBRT. Herein we report dosimetry results from the largest, global, randomized trial of perirectal spacing performed to date. Material/Methods: In this prospective, multi-center, multinational randomized control trial (NCT04905069), 500 patients have been randomized 2:1 spacer:control (enrollment completed June 2025). Spacer patients received a transperineal injection of 10 mL of the iodinated PEG hydrogel followed by SBRT (40 Gy to PTV, 5 fractions). Over 5-year follow-up, patients are assessed for GI and genitourinary toxicity, quality of life, tumor control, device deficiencies and concomitant medications. Primary endpoint (late grade 2+ GI toxicity) results will be available 24 months post- SBRT.The PTV equals the CTV (prostate plus or minus seminal vesicles) plus 5mm margin (3mm posterior). No more than 1cc of rectal volume is to receive 38 Gy (V38 < 1cc). Other rectal constraints include: V36 < 10% rectal volume, V32 < 20%, V20 < 40%. Quality assurance includes review of SBRT plan, approval for variations, and standardized documentation. Minor variations to PTV and organ at risk dose are permitted; major variations require approval by the trial steering group. Results: As of October 1, 2025, 454 (308 spacer, 146 control) have completed SBRT planning. Baseline patient characteristics do not differ between study arms. Median mid-gland separation post-spacer placement is 9.2mm (IQR: 7.0-11.0). 90.3% patients have received SBRT on a linear accelerator. Nearly all patients (98.7%) have planned CTV V40 Gy > 95%. The rectal V40, V38, V36, V32 and V20 are significantly lower in the spacer arm compared to control (all p<0.0001). Significantly more patients who received a spacer met the PTV dose constraint (V40 Gy > 95%; spacer 71.7% vs control 44.5%, p<0.0001) and rectum constraint (V38 Gy < 1 cc; spacer 90.2% vs control 46.6%, p<0.0001).
Conclusion: The SABRE study is the largest randomized trial to date measuring the dosimetric benefits of perirectal spacing in prostate SBRT. These results suggest that the spacer reduces the rectal dose while improving PTV coverage. These dosimetric improvements have the potential to reduce late GI toxicity, which is the primary endpoint of the trial. Keywords: prostate cancer, hydrogel spacer, radiotherapy Digital Poster 2433 Anal squamous cell carcinoma: a population level study of patients treated radically with chemoradiation Rosie Hill 1 , Sarah O'Hare 2 , Jin Tee 1 , Stephanie Craig 3 , Allison Irwin 1 , Richard M Park 1 , Gemma Corey 1 , Catherine R Hanna 3 1 Department of Oncology, Northern Ireland Cancer Centre, Belfast, United Kingdom. 2 Department of Specialist Radiographers, Northern Ireland Cancer Centre, Belfast, United Kingdom. 3 Johnston Cancer Research Centre, Queen's University Belfast, Belfast, United Kingdom Purpose/Objective: Anal squamous cell carcinoma (ASCC) is a rare cancer, commonly associated with human papillomavirus (HPV)1. This study evaluated clinical outcomes in a real-world population-based cohort in Northern Ireland (NI) (1.95 million). This study addressed two questions:Do patients achieving early (3 months) complete response (CR) to chemoradiotherapy (CRT) have better progression free survival (PFS) or overall survival (OS) compared with those achieving CR by 6 months?Does the definition of T4 disease (vaginal versus other organ invasion) impact on PFS or OS? Material/Methods: All patients in NI treated with radical CRT for Stage I-III ASCC over 10-years (January 2015-December 2024) were identified. Descriptive statistics were used to summarise clinical variables. Kaplan-Meier (KM) and
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