What is its main differential?
Arachnoid cyst or an epidermoid cyst are the most likely diagnoses. Epidermoid cyst is more likely in this location.
On MRI, which sequences are going to be most useful in confirming the diagnosis?
DWI and FLAIR. Both sequences (especially DWI, which will demonstrate restricted diffusion) are able to distinguish between an arachnoid cyst and an epidermoid cyst.
What lesion of the petrous bone is histologically identical?
A mass in the right side of the prepontine cistern is largely isodense to CSF but contains some high attenuation strands. It measures approximately 34 x 30 mm in axial diameter and displaces the pons to the left and posteriorly. There is some compression of the fourth ventricle but no dilatation of the lateral or third ventricles. No calcification within the lesion and no haemorrhage.
No intra cerebral lesion elsewhere in the brain. No bone lesion.