ESTRO 2025 - Abstract Book

Brachytherapy - Gastro-intestinal, paediatric brachytherapy, miscellaneous

S179

ESTRO 2025

Purpose/Objective: With OPRA (1) and OPERA (2) studies, treatment intensification with brachytherapy or chemotherapy with long course chemoradiation (LCRT) have become standard options for rectal preservation. With the recent emergence of other total neoadjuvant therapy (TNT) options like short-course radiotherapy (SCRT) with chemotherapy (CT) (3) similar to LCRT, varied combinations have emerged in real-world practice. We aimed to determine if using MR-based brachytherapy dose escalation increases the probability of rectum preservation when used with SCRT-CT-based-TNT versus LCRT. Material/Methods: An ambispective cohort of patients (Jan 2021 to March 2023) suitable with IWWD criteria (4) (at baseline) who were non-participants in any ongoing interventional phase II studies was identified. They received either LCRT followed by MR-based brachytherapy (Group A) or SCRT-CT, interdigitated with brachytherapy between 8-12 weeks post-SCRT (Group B) as suitable. MR-based endorectal brachytherapy of 5-7 Gy X 3 fractions was prescribed to the outer surface of the tumour, with tumour-mucosa receiving up to 200% dose. The opposite rectal mucosal dose was limited to <50% by using two inflatable balloons over an indigenously designed 3D-printed six-channel applicator. LCRT or SCRT-CT was selected based on multidisciplinary decisions, patient logistics, and preferences. The primary outcome was organ preservation at 3 years and quality of life (QoL). This study was registered with ClinicalTrials.gov, NCT06252142, and is ongoing. Results: 57 eligible patients were identified, of which 28 received LCRT (Group A) while 29 received SCRT-CT (Group B). Detailed patient demographics, clinico-radiological, and treatment characteristics are compared in Table-1. Patients with larger T-size, higher T-stage, involved ISS, MRF, and higher baseline CEA levels received Group B treatment representing real-world selection bias. Group A patients received brachytherapy more frequently than Group B (96% vs 69%), as some of the Group B patients had cCR or rectal narrowing. At a median follow-up of 18 months, the organ preservation rate was 56 +/- 10.1% for Group A versus 78.4 +/-7.9% for Group B (hazard ratio [HR] 1.26, 95% CI 16.908 - 21.794; p=0.035; Fig-1). Overall, two patients developed distant metastasis (one in each group), one died on adjuvant chemotherapy post-surgery in Group A, and one had pelvic recurrence post-surgery in Group B. On EORTC QLQ-C-30, Group B patients reported significantly more fatigue and constipation. Two-year follow-up outcomes and detailed HRQoL will be available by the time of the meeting and will be presented.