A complete thrombotic large aneurysm at the nonbranching segment of the distal anterior cerebral artery mimicking a cavernous malformation: illustrative case

BACKGROUND: Thrombotic aneurysms at the nonbranching segment of the distal anterior cerebral artery (ACA) are extremely rare and difficult to differentiate from cavernous malformations by radiographic features alone. OBSERVATIONS: Computed tomography and magnetic resonance imaging of a 30-year-old female patient with a chronic headache complaint revealed a 22-mm frontal lobe mass. The mass showed heterogeneous mixed intensity and hemosiderin deposits on magnetic resonance images. It was not visualized by conventional angiography, indicating that the mass and ACA/other vessels were not connected. The patient was preoperatively diagnosed with a cavernous malformation. However, during resection, the mass surface was white and smooth, different from a cavernous malformation. Although the mass was adherent to the pericallosal artery branch, no luminal continuity was observed. After detachment, the mass was completely resected. Pathological and immunohistochemical findings indicated a vessel wall and interior thrombus. The patient was rediagnosed with a thrombotic aneurysm at the distal ACA nonbranching segment and discharged 10 days postsurgery without neurological deficits. LESSONS: Because radiographic findings of thrombotic aneurysm and cavernous malformation are similar, mass lesions in contact with major arteries should be differentiated as thrombotic aneurysms, even when the artery lumen appears disconnected from the mass.