ESTRO 2025 - Abstract Book

S1149

Clinical – Lower GI

ESTRO 2025

243

Digital Poster Evolution of rectal volume on weekly CBCT during neoadjuvant chemoradiotherapy and tumor response in locally advanced rectal cancer. Sasha Benichou 1 , Koceila Amroun 2 , Sofiane Guendouzen 1 , Paolo Torielli 1 , Vignot Stéphane 3 , Claire Carlier 3 , Olivier Bouché 4,5 1 Department of Radiotherapy, Institut Godinot, Reims, France. 2 General, Digestive and Endocrine Surgery Department, CHU de Reims, Reims, France. 3 Department of Oncology, Institut Godinot, Reims, France. 4 Digestive Oncology and Hepato-Gastroenterology Department, CHU de Reims, Reims, France. 5 BioSpecT, UFR Médecine et Pharmacie, Université Reims Champagne-Ardenne (URCA), Reims, France Introduction : Total Neoadjuvant Therapy (TNT) with FOLFIRINOX chemotherapy followed by CAP50 chemoradiotherapy (CRT) is the standard of care for locally advanced rectal cancer (LARC). Response to TNT influences prognosis. A complete tumor response allows organ preservation. Predicting complete response is challenging. This study evaluated the evolution of rectal volume during CRT as measured by weekly cone-beam computed tomography (CBCT). Materials and Methods : All patients treated by TNT for LARC between October 2021 and October 2023 were included. Weekly CBCT scans were performed to verify patient positioning. Rectal volume was measured on CBCT scans every week for all patients. Patients were classified as complete responders (CR) and non-responders (NoCR). Evolution of rectal volume was compared between groups. Results : Fifty-three patients were included. Tumor CR occurred in 17 patients (32.1%). Rectal volume was similar between groups at baseline. From the second week to the fifth week of CRT, rectal volumes were significantly smaller in CR patients. Rectal volume on CBCT was significantly reduced by a median of 10% between the third and fourth week of CRT in CR patients