Pilocytic astrocytoma, light micrograph. Pilocytic astrocytomas make up only about 2% of primary CNS tumours overall, but they are the爉ost common glioma in children. Vast majority of them (almost 85%) involve燾erebellum. Other commonly involved sites include the燽rain stem, hypothalamus, and optic chiasma. This low power view shows the燾lassic biphasic appearance爓ith燾ellular areas alternating with hypocellular, microcystic areas.