ESTRO 2025 - Abstract Book

S1219

Clinical – Lower GI

ESTRO 2025

1 Radiation Oncology, Hospital Universitario de Navarra, Pamplona, Spain. 2 Radiophysics and Radiological Protection, Hospital Universitario de Navarra, Pamplona, Spain. 3 Radiophysics and Radiological Protection, Universitario de León, León, Spain Purpose/Objective: This is a single institution retrospective analysis, to describe first clinical experience treating locally-advanced rectal cancer patients with CBCT-based online Adapative RadioTherapy (oART): patient selection, clinical and machine workflow steps and decision pathways. ART treatments allows us to modify dosimetry regarding daily anatomic variations: rectal and bladder filling, gases. Staff requirements and the hypothetical increase in machine time are two of the critical issues usually discussed about online-ART. The potential PTV ( Planning Target Volume) margin reduction is one of the main benefits of online-ART, in order to attenuate acute and late RT treatment associated toxicities. Material/Methods: In this paper we report data from 29 consecutive patients (9 female ; 20 male), mean age 64.8 y (47 to 81 years-old), elegible for pre-operative online-ART, treated on Varian/Ethos linear accelerator, between October 2023 and November 2024 . 8 cases were conventional schedule (50Gy/25 fractions); 21 patients underwent short-course RT scheme (25Gy/5fx). A total number of 305 online ART fractions were analysed (105 short course, 200 conventional course). In relation to the tumor location, 38% of cases were lower tract rectal tumors; 14% were upper tract and 48% were middle tract tumors (20% middle-up; 21% middle; 7% middle-low) (Figure 1).