ESTRO 2025 - Abstract Book

S3055

Physics - Image acquisition and processing

ESTRO 2025

3499

Mini-Oral Low-dose contrast-enhanced MRI for daily adaptive radiotherapy of glioblastoma Faisal Mahmood 1,2 , Uffe Bernchou 1,2 , Anders Bertelsen 1 , Anne Bisgaard 1 , Rasmus Lübeck Christiansen 1 , Bahar Celik 1 , Elisabeth Kildegaard 1 , Tine Schytte 3,2 , Rikke Hedegaard Dahlrot 3,2 1 Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Odense, Denmark. 2 Department of Clinical Research, University of Southern Denmark, Odense, Denmark. 3 Department of Oncology, Odense University Hospital, Odense, Denmark Purpose/Objective: Glioblastomas are known to evolve during radiotherapy [1]. MRI-linac radiotherapy enables daily MRI-guided adaptation, potentially improving treatment of this disease. Standard of care includes gadolinium-based contrast agent (GBCA)-enhanced MRI to delineate glioblastoma boundaries. While studies suggest GBCAs are chemically stable under irradiation [2], concerns remain about GBCA retention and potential late adverse effects with repeated use [3]. This study evaluates the feasibility of low-dose GBCA MRI in adaptive radiotherapy for glioblastoma. Material/Methods: Patients eligible for post-operative radiotherapy were included under the MOMENTUM study [4] and treated with hypo-fractionation (34 Gy/10 Fx, 5 d/wk) at a 1.5 T MRI-linac. Daily imaging consisted of a clinical T1W 3D gradient echo scan, with GBCA (Gadobutrol) administered according to the following dose scheme: F(1)-N(2)-H(3)-N(4)-F(5)- N(6)-H(7)-N(8)-F(9)-N(10). (F=full-dose, 0.1 mmol/kg; H=half-dose, 0.05 mmol/kg; N=no-dose; fraction number in parantheses). The gross tumour volume (GTV) was adapted by an oncologist at fractions where either full- or half-dose GBCA was administered (fractions 1,3,5,7,9), and the new plan was used as reference for the next fraction without GBCA (fractions 2,4,6,8,10), where the GTV was transferred rigidly without modifications. GTV to Planning target volume (PTV) margin was 3 mm (CTV margin=0). Otsu’s method [5] was applied for automatic thresholding within a 2 mm expansion of the GTV, for objective assessment of the enhancing tumour region. Results: Significant tumour changes were observed in three out of the nine patients included (10-15% GTV increase (n=2), 10-15% GTV decrease (n=1), occurring midway through treatment (example Fig.1A). Auto-thresholding revealed a less sharply defined enhancing region in half-dose images compared to the full-dose images, resulting in increased volumes, whereas volume reduction resulted when the full-dose threshold was used (Fig.2). Half-dose images provided sufficient contrast for GTV surveillance (Fig.1A), however in small lesions, half-dose GBCA led to too low contrast enhancement even for surveillance (Fig.1B). Images without GBCA enhancement were unsuitable for surveillance/adaptation (Fig.1A).