Background and importance: Arachnoid cysts have not been reported to be located within the brain parenchyma. We present a case of an arachnoid cyst that was contained entirely within the right frontal lobe devoid of communication with the subarachnoid space and ventricle.
Clinical presentation: A 65-year-old woman presented with a 1-year history of progressive headache and nausea. Computed tomographic and magnetic resonance imagining scans showed a well-defined, nonenhancing mass measuring 5 × 5 × 3.5 cm in the right frontal lobe. The mass appeared to be contained entirely within the brain parenchyma. The patient underwent a right frontal craniotomy, at which time the cystic mass was identified in the brain parenchyma without any communication with the arachnoid space. The cyst contained a clear fluid, and its wall was excised. The fluid contents demonstrated a composition similar to that of normal cerebrospinal fluid. Histological and immunohistochemical examinations of the cyst wall were compatible with the diagnosis of an arachnoid cyst. Postoperatively, the symptoms of the patient resolved, and no recurrence was observed up to 6 months after removal.
Conclusion: The present case showed an intraparenchymal arachnoid cyst arising in the frontal lobe. Although the etiology is not known, an arachnoid cyst should be included in the differential diagnosis of primary intracerebral cysts.