A 10-year-old boy has had persistent headaches for the past 3 months. On physical examination, he is afebrile. He has an ataxic gait and dysdiadochokinesia. CT scan of the head shows a 4-cm cystic mass in the right cerebellar hemisphere. Cerebral lateral ventricles are enlarged. A lumbar puncture is done. The CSF protein concentration is elevated, but the glucose level is normal. Neurosurgery is performed, and the mass is removed and sectioned. On gross examination, the mass is a cyst filled with gelatinous material. The cyst has a thin wall and a 1-cm mural nodule. Microscopically, the mass is composed of cells that stain positive for glial fibrillary acidic protein (GFAP) and have long, hair-like processes. What is the most likely diagnosis?

Primary malignant neoplasms of the brain in children most often occur in the posterior fossa. The two most common neoplasms at this site are pilocytic (cystic cerebellar) astrocytoma and medulloblastoma. Pilocytic astrocytoma is slow-growing and has a better overall prognosis than glial neoplasms in adults. Both may enlarge and block CSF flow, causing hydrocephalus. Ependymomas can occur in childhood but are most likely to arise in the fourth ventricle. A hemangioblastoma is a rare cystic mass in adults, typically arising in the cerebellum, and may be associated with polycythemia. Medulloblastomas often occur in the cerebellar midline, are composed of primitive round blue cells, and have a poor prognosis. Meningiomas occur in adults; they are circumscribed, solid mass lesions adjacent to the dura, and maybe multiple in neurofibromatosis. A schwannoma typically arises in cranial nerve VIII in adults.