This specimen is from a 9 y/o female who presented with a 4 month history of headaches, diplopia, and ataxia. An MRI scan of brain showed a large cystic enhancing lesion with a mural nodule in midline posterior fossa. A biopsy showed pilocytic astrocytoma. Pilocytic astrocytomas make up only about 2% of primary CNS tumors overall, but they are the most common glioma in children. Vast majority of them (almost 85%) involve cerebellum. Other commonly involved sites include the brain stem, hypothalamus, and optic chiasma. This low power view shows the classic biphasic appearance with cellular areas alternating with hypocellular, microcystic areas.