A 75-year-old man presented with several weeks of intermittent headache that had progressed to severe, persistent headache with right eye pain. He had hypertension, hyperlipidemia, diabetes, and atrial fibrillation. He denied tobacco, alcohol, or drug use. A neurological examination was notable for disorientation to the month, deficits in short-term memory, and an upward drift of the left arm. Cranial nerve, sensory, coordination, and deep tendon reflex examinations were normal. He walked with a slightly wide but steady gait. His serum red and white blood cell counts, coagulation profile, and electrolyte levels were within normal limits. Blood cultures had negative results. A 24-hour electroencephalogram showed right hemispheric slowing and multiple right centroparietal seizures of 1 minute each, without noticeable clinical correlates. A magnetic resonance image (MRI) of the brain was remarkable for a large T2-hyperintense lesion in the right temporoparietal region, with an increased diffusion-weighted imaging (DWI) signal (low apparent diffusion coefficient), a low signal on susceptibility-weighted imaging, increased T1 signal intensity in and around the lesion, and a contrast-enhancing rim with leptomeningeal enhancement (Figure 1). A smaller satellite lesion with a similar MRI profile was noted inferiorly in the right anterior temporal lobe. A computed tomography scan of the chest, abdomen, and pelvis did not show any metastatic disease. A cerebrospinal fluid (CSF) profile showed a normal red blood cell count and protein and glucose levels and a mildly elevated white blood cell count of 11 cells/μL (to convert to cells × 109/L, multiply by 0.001). The fluid was absent oligoclonal bands and had negative bacterial and fungal culture results, a meningitis-encephalitis polymerase chain reaction panel, and a paraneoplastic panel. Cytology and flow cytometry had negative results. The patient then underwent surgical biopsy of the temporoparietal lesion.