The differential diagnosis of posterior fossa mass in a child include pilocytic astrocytoma, ependymoma and medulloblastoma. MRI findings of a predominantly cystic posterior fossa mass with an enhancing solid component is typical for pilocytic astrocytoma. Patient underwent craniotomy for tumor excision which was histologically confirmed.

HISTOLOGY:

Sections show a benign tumor composed of cells disposed in biphasic pattern with cellular areas alternating with hypocellular and microcystic areas. The tumor cells are fairly monomorphic, spindle to oval in shape with hyperchromatic nuclei, moderate eosinophilic cytoplasm and long (piloid) processes. Also seen are eosinophilic granular bodies, Rosenthal fibers and prominent vascular proliferation including glomeruloid bodies. There are also focal areas of necrosis seen.

DIAGNOSIS: Pilocytic astrocytoma.