P03.09.B EANO, EORTC, EPTN AND ESNR CONSENSUS-BASED DEFINITION FOR POST-RADIOTHERAPY MRI ABNORMALITIES IN THE BRAIN: PRELIMINARY FINDINGS FROM AN INTERDISCIPLINARY DELPHI STUDY.

Abstract BACKGROUND Patients treated with radiotherapy for intracranial tumours can develop new abnormalities on follow-up MRI. However, there is significant heterogeneity in the terminology and definitions used to describe these post-radiotherapy alterations across studies. This inconsistency precludes accurate assessment of their incidence, underlying risk factors, and clinical significance. To address this gap, a collaborative initiative from EANO, EORTC, EPTN, and ESNR members aims to develop standardized definitions through a Delphi method. MATERIALS AND METHODS We are currently conducting a web-based Delphi study to reach expert consensus on MRI abnormalities following radiotherapy. In the preliminary round, panellists reviewed and classified three cases drawn from the literature. The questionnaire included items regarding the conceptual scope and applicability of a standardized consensus framework. Expert panellists were selected from the four participating societies (EANO, EORTC, EPTN and ESNR) ensuring a balance of medical specialities involved in neuro-oncology patient care. RESULTS Fifty experts responded to the preliminary Delphi questionnaire (response rate: 86.2%), including sixteen radiologists, sixteen radiation oncologists, eight neurologists, seven medical oncologists, two clinical oncologists and one neurosurgeon from fourteen European countries. Most respondents had over ten years of experience and worked in academic centres primarily treating adult patients. While 82% regularly encountered MRI abnormalities following radiotherapy, only 10% perceived a general consensus on their classification. Across the three clinical cases, a wide range of terms were used to describe the MRI alterations, highlighting current variability in interpretation. Panellists emphasised the importance of longitudinal, clinical, imaging, and treatment data to support accurate classification. There was broad agreement on the need for standardized terminology for both clinical care (82%) and research purposes (88%). The majority of panellists agreed that a consensus framework should be applicable across radiation modalities (84%) within well-defined timeframes after treatment (80%) and evaluated on predefined MRI sequences (88%), incorporating radiotherapy (88%) and other treatment data (88%). CONCLUSION The preliminary round of this Delphi study reveals substantial variability in the description of MRI abnormalities after radiotherapy and highlights the critical need for a standardized framework. The insights gathered will directly inform the design of the subsequent Delphi rounds, ultimately resulting in a robust, multidisciplinary consensus framework. SUPPORT Lieselotte Lauwens is funded by Kom op tegen Kanker (Stand up to Cancer), the Flemish cancer society (ProjectID: 13899).