Close-up of a gross specimen of cerebellum showing a pilocytic astrocytoma (PA). This neoplasm typically is composed of a large cyst with a mural nodule of solid tumour. Pilocytic astrocytoma is a form of astrocytoma of children and young adults and is the most frequent brain tumour in children. Childhood astrocytomas are often cystic (so-called 'juvenile astrocytomas').