What is the most likely diagnosis? What is the differential diagnosis?
Epidermoid cyst is almost certainly the diagnosis on the basis of this scan. To be considered are arachnoid cysts (which should follow CSF density exactly) and a cystic tumour (e.g. acoustic schwannoma, or rarely meningioma).
How would you confirm this?
An MRI is required, and diffusion weighted imaging is most useful, able to categorically differentiate it from an arachnoid cyst or cystic tumour.
How do epidermoid cysts develop? To what petrous apex / middle ear lesion are they histologically identical?
They may be congenital (most common) due to epidermal rest cell, or acquired (post surgical or post traumatic implantation). Pathologically intracranial epidermoid cysts are identical to the petrous apex and middle ear congenital cholesteatomas.
What is the management of these lesions?
Surgical excision is the treatment of choice if symptomatic. Complete resection is difficult however as not all tissue can be removed, especially from around cranial nerves and vessels. Recurrence is therefore not uncommon, although growth is typically slow and many years can elapse without new symptoms.
A space-occupying lesion is demonstrated in the left cerebellopontine angle which has density approximating that of CSF. The lesion has scalloped borders indenting the left middle cerebellar peduncle and hemisphere. The fourth ventricle is displaced towards the right, without convincing evidence of hydrocephalus. There is no solid component and it is an isolated abnormality.