Where are sporadic pilocytic astrocytomas most frequently encountered?
Typically in the posterior fossa of children.
When seen in the setting of NF1, what is the classic location of pilocytic astrocytomas?
Optic nerve / chiasm.
MICROSCOPIC DESCRIPTION: The tumour is a moderately hypercellular lesion consisting of a loose fascicular arrangement of piloid cells with coarse, strongly GFAP immunoreactive processes. These cells are admixed with a population of astrocytes with small round nuclei and a paucity of processes. Moderate numbers of Rosenthal fibres are noted. Small calibre thick-walled blood vessels are prominent in some areas of the lesion. No mitotic figures are identified and there is no vascular endothelial cell hyperplasia and no necrosis. No staining for IDH-1 or EMA is seen. The Ki-67 proliferative index is <1%.
FINAL DIAGNOSIS: Pilocytic astrocytoma (WHO Grade I)