Restricted diffusion in a thrombosed anterior cerebral artery aneurysm mimicking a dermoid cyst.

A neurologically intact 37-year-old woman presented with an acute severe frontal headache after a month of intermittent headaches. Multimodal radiological examination including computed tomography scan, magnetic resonance imaging, and conventional angiography demonstrated a 1 cm mass in the anterior interhemispheric region with heterogenous calcifications. Of note, MR revealed restricted diffusion within the mass. The presumptive diagnosis of dermoid tumor was made and the patient was scheduled for surgical resection. On operative exploration, a 1 cm thrombosed aneurysm was revealed. Thrombosed aneurysms must be considered in the differential diagnosis for midline cerebral masses with negative angiogram and restricted diffusion. This distinction has implications for the clinical management of the patient.