Diagnosis: Epidermoid cyst
Epidermoid cysts are congenital lesions which arise from the inclusion of ectodermal elements in the neural tube during its closure in utero. They represent up to 1% of primary brain neoplasms. 1/2 are located at the cerebellopontine angle. Another common location is in the parasellar region. They tend to engulf the cranial nerves and vessels they come in contact with and often present in middle age. There is no sex predilection. They are isodense with CSF on CT, and may be difficult to detect unless there is mass effect or calcification which is is present in 25% of cases. They may be demonstrated as filling defects within opacified CSF on CT cisternogram since contrast material will not insinuate into the interior of the lesion. Epidermoids have signal characteristics similar to CSF on MR and do not enhance. The major differential diagnostic consideration for an epidermoid is arachnoid cyst which will show contrast material diffusing into the cyst on delayed CT cisternogram. Related Cases
