Presentation
No presenting complaint available.
Patient Data
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 remodeling or expansion (trumpetting) of the internal acoustic meatus.
The diagnosis of an epidermoid cyst can already be strongly favored 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 tumor e.g. pilocytic astrocytoma etc...)
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MRI confirms the diagnosis. The mass has a characteristic growth pattern whereby it insinuates itself around structures, (note the relationship to the basilar artery) and scalloped margins. Although it is of near-CSF intensity on T1 and T2 weighted images, it does not fully attenuate on FLAIR or FIESTA, instead having a dirty, heterogeneous appearance. There is no contrast enhancing solid component and it does not extend into the internal acoustic meatus.
The imaging diagnosis is confirmed on diffusion weighted imaging (DWI) and (ADC) which demonstrates restricted diffusion (note that it is very bright on DWI due to a combination of T2 shine-through and restricted diffusion).
Case Discussion
This case demonstrates typical appearances of a large cerebellopontine angle epidermoid cyst.
The patient went on to have surgical resection, and the diagnosis was confirmed histologically.
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