What two entities are the main differential considerations?
Atypical meningioma and haemangiopericytoma.
What features suggest that if this is a meningioma, it is atypical (i.e WHO grade II)?
T2 hyperintensity; invasion into skull; irregular contour, with enlarged pial vessels and oedema (although oedema is a soft sign). None of these signs are pathognomonic however, and all can be applied to haemangiopericytomas.
Why is invasion of the dural sinus an important feature to assess and report?
Involvement of the sinus makes it difficult to completely resect the mass, and can lead to brisk bleeding or air embolism during surgery if the flowing sinus is inadvertently entered.
How could you confirm occlusion of the sinus? Why is doing so important?
MRV or angiography. If the sinus is only compressed and stenosed but is still contributing to venous outflow of the brain, resecting it may not only be a bloody affair, but may also result in venous hypertension proximally and potentially a venous infarct / haemorrhage.
MRI of the brain demonstrates a large left sided extra-axial posterior fossa mass. It is isointense to the cerebellum on T1 weighted images, and hyperintense on T2. Following contrast administration it vividly and homogeneously enhances. There is no evidence of calcification or haemorrhage, however it does modestly restrict on DWI.
The mass invades and occludes the transverse and proximal sigmoid sinus as well as invades into the skull, with a single nodular focus passing through the outer table and into the subcutaneous tissues of the scalp. It also extends superiorly through the tentorium into the supratentorial compartement.
The cerebellum is markedly distorted by the mass and has oedema within it. Somewhat surprisingly there is no hydrocephalus despite effacement of the fourth ventricle.