Simultaneous occurrence of epidermoid and dermoid cysts in the posterior fossa: CT and MR findings.

A 61-year-old woman had bi lateral sensorineural hearing loss and difficulty with balance but no vertigo. Neurologic examination showed a right-sided peripheral facial nerve paresis, nystagmus on right and left lateral gaze, hypesthesia in the territory of the right trigeminal nerve with corneal hypesthesia, deviation to the right at toe walking, and positive Romberg sign. CT (Fig. 1 A) of the brain showed a hypodense lesion in the right cerebellopontine angle with displacement of the brainstem and the basilar artery; administration of IV contrast material produced no enhancement. Differential diagnosis was an arachnoid or epidermoid cyst. A second lesion at the level of the vermis had mostly negative attenuation values (-80 H) but with peripheral calcifications and central dense opacity . This lesion was considered typical for a dermoid cyst. On T1-weighted MR images (Figs. 1 B and 1 C), the hyperintense signal of the vermian lesion confirmed its fatty nature. The central part was almost isointense to brain. On T2-weighted MR images (Fig . 1 D), the fatty component of the lesion was hypointense and the central part was hyperintense. The peripheral calcification was seen as a dark rim. These radiologic findings again were considered typical of a dermoid cyst. The ventral lesion was hypointense on T1-weighted images but had some streaky infiltrations within the lesion. On T2weighted images, the lesion was almost isointense to CSF and was inhomogeneous. This appearance suggested an epidermoid cyst. Both lesions were removed during one operation. The ventral lesion was diagnosed as an epidermoid cyst at pathologic examination. The vermian lesion was a dermoid cyst and contained whorls of hair and fatty tissue. The cyst wall was calcified.