Cerebellopontine angle arachnoid cyst
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Right cerebellopontine angle CSF density space with mass-effect on the adjacent cerebellar hemisphere and remodelling of the overlying bone is almost certainly an arachnoid cyst. The only viable differential is that of an epidermoid cyst which is far less likely given the very homogeneously exactly-CSF density and the presence of bony remodelling absent in epidermoid cysts. An MRI would be required to categorically confirm this.
Case courtesy of Bob Cook, MD. Western Memorial Regional Hospital Corner Brook, Newfoundland.