The patient went on to have the posterior fossa mass resected. 

Histology

MICROSCOPIC DESCRIPTION:

Paraffin sections show fragments of a densely hypercellular tumor. Tumor cells have markedly pleomorphic nuclei, some with delicate processes and are arranged predominantly in diffuse sheets in a fibrous stroma. Scattered Homer-Wright rosettes are noted. An occasional mitotic figure is identified. There is no microvascular proliferation and no necrosis is seen. Attachment of leptomeninges is noted. The tumor cells are diffusely synaptophysin and CD56 positive.

There is patchy staining for NeuN and GFAP. AE1/3 and IDH-1 are negative. The topoisomermase index is about 25%.

DIAGNOSIS: Medulloblastoma (WHO Grade IV)