Unusual MRI appearance of an intracranial cholesteatoma extension: the 'billiard pocket sign'.

We describe a unique case of an intracranial extension of acquired cholesteatoma. Previous reports have described cholesteatoma extension through the middle fossa plate and into the middle cranial fossa, but to our knowledge ours is the first report of a case in which the sac herniated into the temporal lobe and overlying dura from a site far lateral to the otic capsule. The findings on magnetic resonance imaging were most unusual, and we call the radiologic characteristics of the mass in this case the "billiard pocket sign." We also discuss the possible mechanisms that produced such an image.