Isodense Epidermoid Cyst in the Pineal Region

A 69-year-old male was admitted complaining of gait disturbances and diplopia, 2.5 years after an episode of serous meningitis. Neurological examination on admission disclosed Parinaud's sign, unsteady gait and dysdiadochokinesis on the left side. A striking finding on the computerized tomography (CT) was the left to right shift of the posterior portion of the third ventricle without visualization of the quadrigeminal and ambient cisterns, which were almost completely occupied by an isodense mass accompanied by high dense flecks and a low dense part. Enhanced CT showed positive enhancement in the vicinity of the pineal calcification. By the suboccipital supracerebellar approach, an encapsulated mass containing brownish yellow fluid was subtotally removed and a histological examination of it revealed epidermoid tissue and hemosiderin deposits in the solid portion. Few reports of isodense epidermoid cysts have so far been found in the literature giving a full explanation for this unusual CT attenuation value. Based on the clinical course and histology of this case, the pathogenesis of the unusual density is discussed along the following lines: The mixture of the low dense factor due to cholesterin and the high dense factor due to prior bleeding is believed to result in the isodense attenuation value in the liquid portion. Also, in the solid part, a microscopically mixed texture of deposited hemosiderin and cholesterin clefts in the inflammatory granulomatous tissue could explain its density on the CT scan.