The patient went on to have a posterior fossa craniotomy and debulking of the tumor. 

Histology

MICROSCOPIC DESCRIPTION:

Paraffin sections show fragments of a moderately hypercellular astrocytic glioma. Tumor cells are a mixture of piloid cells with elongated spindle shaped hyperchromatic nuclei with bipolar processes and cells with round and oval nuclei which are sometimes arranged in fluorettes. The piloid cells are arranged in loose fasciculi. There are prominent Rosenthal fibers. No mitotic figures are identified. There is no microvascular proliferation or necrosis. 

IMMUNOHISTOCHEMISTRY:

• GFAP positive
• IDH-1 R132H negative
• p53 negative
• p16 negative
• Topoisomerase labeling index: Approximately 1%

FINAL DIAGNOSIS:

Pilocytic astrocytoma (WHO Grade I)