Frontotemporal giant extradural dermoid cyst: illustrative case.

backgroundDermoid cyst is a rare benign tumor exhibiting a typical radiological pattern and most commonly located along the midline. Laboratory examination was always normal. However, the features of some rare cases are atypical that can be easily misdiagnosed as other tumors. OBSERVATIONS: A 58-year-old patient presented with tinnitus, dizziness, blurred vision, and gait unsteadiness. Laboratory examination showed the serum levels of carbohydrate antigen 19-9 (CA19-9) were significantly increased (186 U/mL). A computed tomography (CT) scan revealed a predominant hypodense lesion in the left frontotemporal region with a hyperdense mural nodule. The lesion appeared as an intracranial extradural mass with a mural nodule on the sagittal image, displaying mixed signal on T1- and T2-weighted imaging. A left frontotemporal craniotomy was performed for cyst resection. Histological results confirmed a diagnosis of dermoid cyst. No tumor recurrences were observed at the 9-month follow-up. LESSONS: Extradural dermoid cyst with a mural nodule is extremely rare. When a hypodense lesion on CT shows mixed signal on T1- and T2-weighted imaging with a mural nodule, even if it is located in the extradural areas, it is important to consider a dermoid cyst. Serum CA19-9 combined with atypical imaging features may contribute to the diagnosis of dermoid cysts. Only recognition of atypical radiological features can avoid misdiagnosis.