Epidermoid Cyst of the Cerebellopontine Angle.

This patient had a right CPA epidermoid cyst excised 10 years previously. They were lost to follow-up until representing with symptoms relating to posterior fossa mass effect.
Epidermoid cysts are congenital lesion caused by inclusion of ectodermal elements during neural tube closure, as opposed to dermoid cysts which have both epidermal and skin appendages such as hair and sebaceous cysts. Epidermoids are the 3rd most common cerebellopontine angle mass, after acoustic schwannomas and menigiomas. Pathologically it is identical to the petrous apex and middle ear congenital cholesteatoma.
On imaging they are similar to arachnoid cysts but can usually be distinguished from them due to restriction on DWI and usually slight deviation from CSF on FLAIR.
For more sequences of this case, and more on epidermoid cysts, visit Radiopaedia.org here.
References:
Neuroradiology: The Requisites (Requisites in Radiology)
Diagnostic Imaging: Brain

Credit: Dr Frank Gaillard
http://www.radpod.org