ESTRO 2025 - Abstract Book

S1530

Clinical – Mixed sites & palliation

ESTRO 2025

Keywords: PATHY, SBRT, bulky tumors, immunotherapy

2818

Digital Poster Do we need a CTV? Local failure rates and patterns after stereotactic body radiotherapy (SBRT) for non spinal, non-sacral bone metastases. Elisa Bodson 1 , Mathilde Baré 1,2 , Zineb Belbaraka 1 , Younes Jourani 3 , Paulus Kristanto 4 , Robbe Van Den Begin 1 1 Radiation Oncology, Université Libre de Bruxelles (ULB), Hôpital Universitaire de Bruxelles (HUB), Institut Jules Bordet, Brussels, Belgium. 2 Radiation Oncology, EpiCURA, Baudour, Belgium. 3 Medical Physics, Université Libre de Bruxelles (ULB), Hôpital Universitaire de Bruxelles (HUB), Institut Jules Bordet, Brussels, Belgium. 4 Information Management Unit, Université Libre de Bruxelles (ULB), Hôpital Universitaire de Bruxelles (HUB), Institut Jules Bordet, Brussels, Belgium Purpose/Objective: Stereotactic body radiation (SBRT) is commonly used to treat bone metastases. For spinal or sacral metastases, consensus guidelines support anatomically based CTV-margins, shown to reduce local failure (LF) risk in retrospective studies. However, data regarding the use and optimal size of CTV-margins in non-spinal, non-sacral bone metastases (NSBM) remains limited. This study evaluates LF rates and patterns after NSBM SBRT according to CTV-margins and explores additional LF risk factors. Material/Methods: Patients who underwent NSBM SBRT between 2018 and 2022 were retrospectively reviewed. LF was defined as disease progression within the treated bone that would have come into contact with the treatment field with a 20 mm concentric CTV-margin around the GTV. LF patterns were classified as in-field when over 95% of the recurrence volume (Vrec) was within the PTV, out-of-field when none of the Vrec was within the PTV, and marginal for other cases. Marginal/out-of-field failures were considered potentially preventable if over 95% of the Vrec fell within the PTV with a given hypothetical CTV-margin of 5, 10 or 20 mm (see Figure 1).

Results: 156 patients with 221 NSBM were included. A CTV-margin was applied in 25.9% of cases, mostly 5 mm (70%), with a variation ranging from 5 to 20 mm. The PTV-margin varied between 3 to 5 mm. Median follow-up was 31.6 months