Schwannoma of the hypoglossal nerve

A hypodense well-circumscribed lesion inferiorly within the posterior fossa on the right elevates the cerebellar hemisphere and displaces the brainstem towards the left. A small focus of calcification is present that may be related to the mass or belong to normal choroid plexus. No hydrocephalus.

Within the right inferior aspect of the posterior fossa, there is a mixed cystic and solid lesion centered in cerebellomedullary angle. Nodular solid tissue surrounding this cystic component heterogeneously enhances with elevated CBV and fairly low ADC values only slightly facilitated compared to adjacent brain. The solid component of the tumor demonstrates elevated choline with mildly reduced NAA (not shown). Punctate susceptibility artifacts at the periphery of the lesion are consistent with blood product.

The lesion completely encases the intradural right vertebral artery up to the origin of the basilar artery. Although the fourth ventricle is distorted, there is only minor prominence of the lateral ventricles.

No leptomeningeal or intraventricular enhancement.

Conclusion:

Unusual mass with the main differential being a pilocytic astrocytoma, ependymoma or schwannoma.

The tumor is composed of hypercellular areas with interspersed hypocellular areas. 

In the hypercellular areas (Antoni A), the tumor forms broad sweeping fascicles. The tumor cells are bland with elongated nuclei and smooth chromatin. There is well-developed Verocay body formation. Some areas demonstrate lower tumor cell density with a looser stroma (Antoni B).

Many blood vessels have thick sclerotic walls. There are no mitoses or necrosis.

Immunohistochemistry

The lesional cells are diffusely and strongly positive for S100.

Final diagnosis

posterior fossa tumor: schwannoma (CNS WHO grade 1)

Pathology courtesy of Dr Samuel Roberts-Thomson