Cystic meningioma misdiagnosed as an arachnoid cyst in the early stage of tumor growth: A case report

We report a case of a cystic lesion mimicking an arachnoid cyst, which developed to a large cystic space-occupying lesion with peritumoral cysts. A 56-year-old man who had been experiencing mild weakness in the right hand and mental lethargy over a period of 1 month was admitted to our hospital. Computed tomography (CT) and magnetic resonance imaging (MRI) revealed a round cystic tumor with large peritumoral cysts in the left frontal lobe. The cystic tumor contained a solid component, which appeared to be attached to the cyst wall just under the dura matter. Eight years before the admission, the tumor was misdiagnosed as an arachnoid cyst by the evaluation via plain CT scan. During surgery, a cystic tumor with the solid component was totally removed and the peritumoral cyst walls were subtotally resected. Histological examination indicated that the tumor was a microcystic meningioma and that the wall of the peritumoral cyst was a degenerated arachnoid membrane. This case suggests that cystic meningiomas may be mistaken for arachnoid cysts. The diagnosis of arachnoid cysts on the basis of plain CT images may be problematic, and knowledge of this variability during the differential diagnosis of an arachnoid cyst can help avoid diagnostic pitfalls. Thus, MRI with gadolinium enhancement appears to have diagnostic value in the cases of cystic meningiomas resembling arachnoid cysts.