ESTRO 2025 - Abstract Book

S1259

Clinical – Lower GI

ESTRO 2025

3860

Poster Discussion MRI-guided stereotactic radiotherapy: a new frontier for peritoneal oligometastases Angela Romano, Giuditta Chiloiro, Luca Boldrini, Giulia Panza, Rosa Autorino, Francesco Cellini, Gabriele Turco, Marco Rapisarda, Matteo Nardini, Lorenzo Placidi, Maria Antonietta Gambacorta Radiation Oncology Department, Fondazione Policlinico Universitario "A. Gemelli" IRCCS, Rome, Italy Purpose/Objective: Peritoneal carcinomatosis (PC) has historically been linked to a dismal prognosis. Advances in oncologic management now suggest that, in carefully selected cases within the oligometastatic context, a multidisciplinary strategy combining systemic therapies with locoregional modalities such as stereotactic body radiotherapy (SBRT) may confer significant clinical advantages. Magnetic resonance-guided radiotherapy (MRIgRT) introduces the capability for online adaptation (OA), facilitating dynamic adjustments to the radiation dose distribution based on daily anatomical variations. This approach enhances treatment precision and minimizes radiation-induced toxicity, which is particularly critical for patients with PC, where disease nodules often abut radiosensitive structures such as the bowel. This single-institution investigation explores the feasibility and therapeutic efficacy of magnetic resonance-guided stereotactic body radiotherapy (MRIgSBRT) in a cohort of patients affected by PC. Material/Methods: Clinical and dosimetric data were collected for patients with PC treated using MRIgSBRT. The primary endpoints included 1-year local recurrence-free survival (LRFS), progression-free survival (PFS), and overall survival (OS) rates. Objective response rate (ORR) was defined as the sum of complete response (CR) and partial response (PR). Clinical benefit (CB) encompassed the achievement of ORR or stable disease (SD). Acute and late toxicities were evaluated using the CTCAE 5.0 criteria. Results: Between July 2019 and December 2023, 33 oligometastatic PC patients, encompassing 47 lesions, underwent MRIgSBRT. Table 1 summarizes patient and treatment details. Ovarian cancer was the primary diagnosis in 30 (63.8%) patients. Among the 11 (33.3%) patients with multiple nodules, all but one were treated in the same fractions. The median PTV dose was 35 (2-50) Gy across 5 (2-5) fractions, with a median BED of 75.7 (14.8-128.5). Of 212 total fractions, 169 (79.7%) utilized OA protocols. CR, PR, and SD were noted in 26 (55.3%), 9 (19.1%), and 7 (14.9%) lesions, respectively, yielding an 89.4% CB rate. At a median follow-up of 9 months (range: 1-51), the 1-year LRFS, PFS, and OS rates were 84%, 25%, and 100%. PARP inhibitors were given to 10 (29.4%) patients post MRIgSBRT, significantly improving PFS (P=0.0282). LRFS was positively associated with ovarian histology in 30 (63.8%) lesions (P=0.0045) and BED ≥ 80 in 20 (42.6%) lesions (P=0.0055). No acute or chronic toxicities were observed.