The patient went on to have a resection. 

Histology

Microscopic Description:

Sections show a moderately cellular glial neoplasm diffusely infiltrating the grey and white matter. Secondary structures are prominent in the subpial, perivascular and subependymal regions as well as perineuronal satellitosis. The vast majority of tumor cells have round to oval nuclei and perinuclear halos, some with mild degree of nuclear pleomorphism and hyperchromasia. Microgemistocytic forms are focally identified. Mitotic figures are occasionally seen. There are very minor foci of astrocytic differentiation showing more elongated nuclei and appreciable fibrillary processes. There is no evidence of vascular endothelial proliferation or tumor necrosis. GFAP immunostaining shows reactivity of the microgemistocytes and of a small number of cells in the few areas with focal astrocytic differentiation. MIB-1 immunostain shows low nuclear labeling .

The tumor is predominantly oligodendroglial, but a minimal astrocytic component is also noted. 

Final Diagnosis: Oligodendroglioma (WHO grade II).

Note: Unfortunately this case predates the recent (2016) revision WHO classification of CNS tumors, and thus molecular markers (IDH mutation and 1p19q co-deletion) are not available. As such, this tumor would now be classified as an oligodendroglioma NOS.