ESTRO 2025 - Abstract Book

S1840

Clinical – Upper GI

ESTRO 2025

3682

Digital Poster Matched pair dosimetric comparison of CBCT-guided SBRT versus MR-LINAC in patients with liver malignancies. Sally A. Elkhamisy 1,2,3 , Franciska Lebbink 1,4 , Fabian Funer 1,3,4 , Annika Lattermann 1,3,4 , Chiara Valentini 1,3,4 , Karoline leger 1,3,4 , Fabian Lohaus 1,3,4 , Falk Tillner 1,3,5 , Anna Kornek 1,3 , Steffen Löck 1,3 , Esther Troost 1,3,4 1 a. OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Cal Gustav Carus, TUD Dresden University of technology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. 2 b. Department of Clinical Oncology and Nuclear Medicine, Faculty of Medicine, Mansoura University, Mansoura, Egypt. 3 c. Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany. 4 d. National Center for Tumor Diseases (NCT/UCC), Germany, Germany Cancer Research Center (DKFZ), Heidelberg, Germany; Faculty of Medicine and University Hospital Carl Gustav Carus, TUD Dresden University of technology, Dresden, Germany; Helmholtz Zentrum Dresden – Rossendorf (HZDR), Dresden, Germany. 5 e. Helmholtz-Zentrum Dresden - Rossendorfe. Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany Purpose/Objective: Stereotactic body radiotherapy (SBRT) is an effective treatment for liver malignancies 1 . However, the most commonly used technique, conventional cone-beam computed tomography (CBCT)-guided SBRT, has limitations regarding soft-tissue contrast and lack of real-time tracking capabilities. Magnetic resonance imaging guided adaptive radiation therapy (MRgRT) using an MR-integrated linear accelerator (MR-LINAC) overcome these limitations 2 . This study dosimetrically compared CBCT-based SBRT and MR-LINAC regarding target coverage and doses to organs at risk (OARs). Material/Methods: This retrospective study included 13 patients treated with SBRT (N=7) or MR-LINAC (N=6; 7 lesions) between 2020 and 2024. Different fractionation schedules (5-18.75Gy/ fraction in 3-8 fractions) were evaluated. Patients were matched based on the anatomical location of the hepatic lesions to ensure comparability of nearby OARs. To align the daily varying anatomy with the baseline, image registration was performed using rigid registration, followed by deformable registration, using gross tumor volume (GTV)and planning target volume (PTV) as controlling structures). Dose deformation and accumulation were applied to create accumulated dose distributions. Baseline SBRT plans and three MR-LINAC plans [baseline plan (BP), accumulated predicted plan as a recalculation on the daily MRI without plan adaptation (APP), and accumulated adapted plan (AAP)] were analyzed. Dose-volume histograms were compared for target coverage [PTV coverage by prescribed isodose (PID)] and homogeneity index (HI). For relevant OARs (i.e. stomach, duodenum and colon), the VxGy (volume of OAR receiving xGy, corrected for the fractionation schedule) and D0.5cc (dose of 0.5cc of an OAR) were evaluated according to the institutional guidelines. Results: PID effectively covered the PTV in the SBRT, BP, and AAP plans (90%-97%), while lack of adaptation (APP) showed reduced coverage (50%-70%). The HI of AAP and SBRT were comparable (mean 0.30 vs. 0.29, p=0.2), while superior to APP (0.98, p=0.02). The D0.5cc values for all OARs were statistically non-significantly different for all plans (SBRT, BP, APP, and AAP). In all three MR-LINAC plans, VxGy for OARs were almost 0, except in patients 2, 3, and 4, in whom the PTV was in close proximity to OARs (Figure 1).