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. hey are considered benign (WHO Grade 1) and are slow-growing tumours. Patients with neurofibromatosis 1 are predisposed to this tumour, especially involving the optic nerve/chiasm. The tumour cells are usually spindle shaped and have long, hair-like (piloid) processes.