A 26-year-old immunocompetent male presented with five weeks of worsening headaches, nausea, vomiting, neck stiffness, fever, and confusion. An initial computed tomography (CT) brain scan showed no abnormalities. Cerebrospinal fluid (CSF) analysis demonstrated hypoglycorrhachia and mononuclear lymphocytic pleocytosis. The patient was empirically treated for a possible infectious meningitis with levofloxacin, meropenem, doxycycline, and acyclovir without significant clinical improvement. Extensive infectious workup from CSF was unrevealing. His mental status progressively deteriorated requiring intubation. Magnetic resonance imaging (MRI) demonstrated diffuse cerebral edema, leptomeningeal enhancement with foci of cortical and subcortical signal intensity abnormalities in the right orbital and parasagittal frontal lobe, and right temporal lobe. Hypertonic saline and later steroids were administered with some improvement. CSF flow cytometry showed no monoclonal B cell populations. CSF cytopathology was consistent with ALK-positive ALCL. PET/CT and bone marrow biopsy revealed no evidence of systemic involvement, consistent with primary CNS ALK+ ALCL. Treatment with high-dose intravenous methotrexate was initiated. Within hours of a methotrexate dose, he developed fluctuations in mental status and left sided weakness; emergent MRI Brain revealed new areas of cortical diffusion restriction along the anterior paramedian frontal lobes and the paramedian right parietal lobe, which was thought to reflect acute effects of methotrexate treatment on malignant cells. His neurological exam improved over two days; he was extubated with only mild dysarthria and subtle fine motor deficits. He has since completed 8 cycles of methotrexate and remains on the ALK inhibitor lorlatinib.

This case demonstrates progressive neurological changes, radiographical findings and CSF profile of primary CNS ALK-positive ALCL,, an atypical and uncommon malignancy. We also describe hyperacute MRI Brain findings associated with acute methotrexate treatment of a CNS malignancy