Extradural epidermoid cyst

An ovoid well circumscribed right-sided posterior fossa mass with bony remodeling is noted. It is hypodense with only peripheral enhancement. 

There is an extradural mass in the right posterior fossa. The lesion is T2 hyperintense with multiple internal septations. It demonstrates diffusion restriction with no appreciable contrast enhancement. It causes moderate mass effect, displacing the right cerebellum. It effaces, but does not compress the right transverse and sigmoid sinuses. There is no surrounding edema. The fourth ventricle is patent and there is no hydrocephalus. A 5mm high T2 / low T1 focus in the left post-central gyrus demonstrates susceptibility artefact consistent with calcification on phase and CT suggests an old granuloma or burnt out cysticercosis. There are no other foci of susceptibility artefact. Parenchymal signal is otherwise normal throughout.

Conclusion: Extradural cystic mass in the right posterior fossa is almost certainly a "white" epidermoid.

Sections show a cyst wall lining with a thin layer of stratified squamous epithelium including a granular cell layer, with bland nuclear features and surface keratinization. There is abundant Iamellated keratin debris. The wall is fibrous with areas of infiltration by mononuclear chronic inflammatory cells, foreign body type multinucleate giant cells, in association with cholesterol clefts. The inflammatory process focally extends into bone. There is no brain tissue identified. There are no adnexal skin structures or immature elements. There is no evidence of malignancy. 

FINAL DIAGNOSIS: epidermoid cyst.