Which posterior fossa tumor in the cerebellum is well-circumscribed and typically cystic with a mural nodule and has a good prognosis?

• A. Medulloblastoma
• B. Pilocytic astrocytoma
• C. Ependymoma
• D. Hemangioblastoma

Answer: (B)

This pattern highlights a tumor that is especially characteristic in children: a cerebellar lesion that is well defined, cystic with a mural enhancing nodule, and a prognosis that is generally favorable after surgery. Pilocytic astrocytoma is the classic example of this combination. It is a low-grade (WHO I) tumor that tends to be well circumscribed and grow slowly, making complete surgical resection often possible. The cyst forms most of the lesion, while a mural nodule at the cyst’s wall shows contrast enhancement on imaging, which is a key radiologic clue.

Why this fits better than the others: medulloblastoma in the cerebellum tends to be a solid, more aggressive tumor that often presents with rapid symptoms and CSF spread, not the typical cyst with a mural nodule pattern. Ependymoma originates near the fourth ventricle and commonly causes obstructive hydrocephalus, with a different imaging appearance. Hemangioblastoma can also be cystic with a mural nodule, but it usually occurs in adults and is associated with high vascularity and sometimes von Hippel-Lindau syndrome. Taken together, the pediatric cerebellar lesion described—well-circumscribed, cystic with a mural nodule, and good prognosis after resection—is most consistent with pilocytic astrocytoma.