A Framework for MRI Characterization of Intracranial Metastatic Disease.

Intracranial metastases (IM) are far more common than primary malignant brain tumors, yet remain understudied, representing an important unmet clinical need. Although patients with IM have historically been excluded from clinical trials, thus limiting assessment of intracranial treatment efficacy, their inclusion is increasing; however, patients with suspected leptomeningeal metastatic disease (LMD) remain commonly excluded, likely due to poorer prognosis and anticipated limited treatment response. Despite established treatment response criteria for confirmed LMD, there is limited guidance on how LMD should be diagnosed or excluded in broader IM trials, creating substantial risk for variability and misclassification. This article addresses the gap by proposing MRI-based criteria for classifying IM appearances with respect to LMD suspicion, focusing on the exclusion of LMD, illustrated with real patient examples. IM-related MRI findings are categorized into three groups: (a) imaging consistent with LMD, not requiring cerebrospinal spinal fluid confirmation; (b) equivocal findings warranting further evaluation with cerebrospinal fluid analysis or spinal MRI; and (c) imaging with low suspicion for LMD. Common pitfalls, mimics, and recommended next steps for equivocal findings are discussed. Keywords: Intracranial Metastasis, Leptomeningeal Metastatic Disease, MRI © RSNA, 2026.