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On CT the mass is of near-CSF density, with a hounsfield density of approximately 0 HU (ROI not shown). It is clearly extra-axial (not the angle it forms with the pons anteriorly) and exerts significant mass effect on the contents of the posterior fossa without bony remodelling or expansion (trumpetting) of the internal acoustic meatus.
The diagnosis of an epidermoid cyst can already be strongly favoured merely on the non-contrast CT imaging appearances although an MRI is required to confirm the diagnosis and definitively exclude other possible (albeit unlikely differentials: arachnoid cyst, cystic acoustic schwannoma, cystic exophytic parenchymal tumour e.g. pilocytic astrocytoma etc...)