ESTRO 2025 - Abstract Book

S1225

Clinical – Lower GI

ESTRO 2025

A retrospective study of 16 patients with RC treated with EBRT using VMAT on a type O linear accelerator. MR was delineated on days 1, 6, 11, 16, and 21 on cone-beam CTs (CBCTs) and compared to the simulation CT. The MR was delineated from the anorectal junction to the rectosigmoid junction and reviewed by two radiation oncologists. Variability was measured in mm: anterior (ant) (distance between the anterior edge of the MR and the posterior edge of the bladder), posterior (post) (distance between the posterior edge of the MR and the anterior edge of the sacrum), right (rt) (distance between the lateral edge of the MR and the lateral edge of the femoral head at the midline), and left (lt) (distance between the lateral edge of the MR and the lateral edge of the femoral head at the midline). Differences were analyzed using Bel, Antolak, Stroom, Van Herk, and Parker equations to determine margins. Results: 16 patients with RC (3 women and 13 men, mean age 63 years). Tumor location: 12.5% upper, 12.5% middle, and 75% lower rectum. Stage: 75% T3-T4a N0-N2. Doses: 45Gy/25Fx (12.5%), 50.4Gy/28Fx (75%), 50Gy/25Fx (12.5%). Two patients had a prior colostomy. Radiotoxicity: 81.3% (Cystitis G1: 56.1%, G2: 6.3%, proctitis G1 in 12.5%).