This case illustrates a large cystic mass lesion arising from the left inferior cerebellar hemisphere with an enhancing solid component and associated obstructive hydrocephalus. In the patient's age group and with these image features, hemangioblastoma and pilocytic astrocytoma must be considered.

Hemangioblastoma typically occur in the young adult, and although they are the most common posterior fossa mass in a young adult, they are nonetheless uncommon in absolute terms, accounting for only 1-2.5% of all intracranial tumors. They have a peak incidence around 30-60 years of age. One imaging feature that makes this case less likely a hemangioblastoma is the presence of enhancement in the cystic component wall.  

Pilocytic astrocytomas are tumors of young people, with 75% occurring in the first two decades of life, typically late in the first decade (9-10 years). There is no recognized gender predisposition. 

This case was first considered to favor a pilomyxoid astrocytoma (PMA) diagnosis by the histopathology study. However, after the BRAF genetic tests, the diagnosis was confirmed to be a pilocytic astrocytoma due to the absence of genetic fusion or mutations. PMA was originally reported as arising in the hypothalamus or optic chiasm of an infant or young child. However, it is also known that they may also occur in the posterior fossa and in the spinal cord ¹.