Giant intradiploic epidermoid cyst of the occipital bone.

A rare case of giant intradiploic epidermoid cyst of the occipital bone with large intracranial extension in the posterior fossa is described. The lesion was discovered when the patient presented with headache and subcutaneous swelling in the occipital region, in the absence of signs of neurological involvement. CT scan showed extensive destruction of the occipital bone, mainly of the inner table, up to the foramen magnum. On MRI the lesion was hypointense in T(1) and hyperintense in T(2)-weighted images; signal inhomogeneity was due to cellular debris and cholesterol crystals. The enhancing rim due to the thickened dura confirmed the extradural location. Complete removal of the cyst was easily accomplished despite its large size. We found only 3 documented cases in the literature of giant intradiploic infratentorial epidermoid cysts, none of which was studied by MRI. The radiological features and differential diagnosis are discussed.