ESTRO 2025 - Abstract Book

S698

Clinical - CNS

ESTRO 2025

centers with varying MRI systems, using the same protocol, to ensure consistent dosimetric planning results. The goal is to confirm whether DTI-based organ-at-risk (OAR) identification can be standardized for intracranial stereotactic radiotherapy (SRT) planning [1-4]. Material/Methods: DTI imaging protocols were harmonized between CHR Metz-Thionville, France (Siemens SOLA 1.5T MRI) and UKS Homburg, Germany (Siemens SKYRA 3T MRI). NIST diffusion phantom (Caliber MRI), DTI resolution, and fiber crossing phantoms (ImagingHQ), as well as data from four healthy volunteers, were used to assess the reproducibility of diffusion metrics, spatial resolution, and fiber tract extraction methods. Longitudinal validity was evaluated through acquisitions performed at three time points, approximately six months apart, between December 2023 and November 2024. Using synthetic CT (MR-Box, Therapanacea), dosimetric treatment plans derived from DTI OARs were compared between centers (FiberTracking 2.0 and Elements 4.0, Brainlab). All DTI exams were corrected for MR distortion (Cranial Distortion Correction, Brainlab). Results: Phantom measurements were highly reproducible, with ADC values showing an absolute deviation ≤ 2%, and consistent spatial resolution and fiber-crossing tracking. All fiber bundles were similar both between sites and over time with an overlap of ~95% when a 2 mm margin was applied between exams. Planning studies produced comparable dosimetric results, particularly in terms of PTV coverage and fiber bundle sparing. Conclusion: This study establishes the groundwork for using DTI to protect white matter tracts in SRT planning, ensuring consistency and reproducibility in multicenter settings.

Keywords: DTI imaging, SRT, Multicentric imaging

References: [1] PMID: 31326352. [2] PMID: 22014949. [3] PMID: 35249816. [4] PMID: 19301968

2401

Mini-Oral Oncological outcome of re-irradiation for recurrent glioblastoma – Results of a prospective multicenter trial (GLIAA, NCT01252459) Ilinca Popp 1 , Wolfgang A Weber 2 , Erika Graf 3 , Michae Mix 4 , Rolf Wiehle 1 , Ursula Nestle 1,5 , Tanja Schimek-Jasch 1 , Maximilian Niyazi 6,7 , Claus Belka 6 , Frank Paulsen 7 , Franziska Eckert 7,8 , Michael J Eble 9 , Liane König 9 , Frank A Giordano 10 , Elena Sperk 11 , Felix Momm 12 , Irina Spehl 12 , Stephanie E Combs 13 , Denise Bernhardt 13 , Rita Engenhart Cabillic 14 , Markus M Schymalla 14 , Martin Stuschke 15 , Christoph Pöttgen 15 , Rainer Fietkau 16 , Anca L Grosu 1,17 1 Department of Radiation Oncology, Medical Center - University of Freiburg, Faculty of Medicine, Freiburg, Germany. 2 Department of Nuclear Medicine, School of Medicine, Technical University Munich, Munich, Germany. 3 Institute of Medical Biometry and Statistics, Medical Center-University of Freiburg, Faculty of Medicine, Freiburg, Germany. 4 Department of Nuclear Medicine, Medical Center - University of Freiburg, Faculty of Medicine, Freiburg, Germany. 5 Department of Radiation Oncology, Kliniken Maria Hilf GmbH Mönchengladbach, Mönchengladbach, Germany. 6 Department of Radiation Oncology, University Hospital, LMU Munich, Munich, Germany. 7 Department of Radiation Oncology, University Hospital Tübingen, University of Tübingen, Tübingen, Germany. 8 Department of Radiation Oncology, Medical University Vienna, AKH, Comprehensive Cancer Center, Vienna, Austria. 9 Department of Radiation Oncology, RWTH Aachen University, Aachen, Germany. 10 Department of Radiation Oncology, University