Focal Leptomeningeal Metastasis Mimicking Inflammatory Lesion: Role of Serial MRI and MR Spectroscopy.

Background: Leptomeningeal metastasis (LM) is a rare and severe complication of non-small cell lung cancer (NSCLC), occurring when cancer cells spread to the meninges and cerebrospinal fluid (CSF) compartment. Diagnosing LM is challenging due to nonspecific clinical symptoms and routine magnetic resonance imaging (MRI) findings that show meningeal enhancement along the cerebral sulci, which can resemble inflammatory, infectious, or autoimmune conditions. Objective: The aim of this case presentation was to illustrate the diagnostic challenges of LM and the role of advanced MRI, and we present a clinical case of localized LM in a patient with NSCLC, presenting with nonspecific seizures and significant changes on routine and spectroscopic MRI over time. Case report: A 42-year-old male patient with no significant medical history was admitted due to recurrent seizures. The seizures were not accompanied by weakness or speech impairment. Neurological examination showed no focal neurological deficits. Blood tests and biochemical parameters were within normal limits. Initial MRI revealed a hyperintense lesion on T2W and FLAIR sequences in the left middle frontal gyrus cortex, adjacent to the precentral gyrus, measuring approximately 19×9 mm. The lesion had a thin peripheral rim with restricted diffusion on DWI/ADC, meningeal enhancement post-contrast, and grade 1 perilesional edema. MR perfusion showed no increased cerebral blood volume (rCBV) on the perfusion map. MR spectroscopy indicated no increase in choline, a decrease in NAA, with Cho/NAA ratios of 0.503-0.630, Cho/Cr ratios of 1.16-1.49, NAA/Cr ratios of 2.32-2.37, and a lactate peak. Based on routine and advanced MRI findings, a diagnosis of nonspecific inflammatory lesion was initially considered. The patient was followed up and started on anti-seizure treatment with levetiracetam 500 mg twice daily. After treatment, major seizures were controlled, but the patient reported intermittent tongue numbness every 2-3 days. A brain lesion biopsy was performed, revealing large, basophilic, cylindrical cells with broad, eosinophilic cytoplasm, multiple atypical nuclei, arranged in glandular or papillary structures, with scattered calcifications, consistent with metastatic papillary adenocarcinoma to the brain. Subsequent systemic screening identified a primary lung tumor. Conclusion: This overlap can lead to delayed diagnosis and treatment. To address these challenges, a comprehensive approach combining clinical evaluation, MR spectroscopy to assess metabolic changes, and systemic screening to identify the primary tumor is essential. Meningeal biopsy should be considered in unclear cases. The role of radiologists is critical in raising early suspicion and collaborating with clinicians to avoid missing lesions or performing unnecessary biopsies. Treatment options include radiation therapy, chemotherapy, targeted therapy, and immunotherapy, but the prognosis remains poor with a high mortality rate.