The patient went on to have a biopsy. 

Histology

MICROSCOPIC DESCRIPTION:

The sections show multiple fragments of white matter in which there is prominent reactive gliosis. Separate fragments are hypercellular and show perivascular accumulation of atypical large mononuclear cells. These have round, oval and angulated vesicular nuclei with conspicuous nucleoli and a variable amount of pale cytoplasm. An occasional mitosis is identified. The atypical cells are mixed with small lymphocytes. Some blood vessel walls are invaded by the atypical cells. A separate fragment of necrotic tissue is noted.

Immunohistochemistry show the large cells to be CD20+, PAX5+, CD3-, GFAP-. The small lymphocytes are CD3+ T cells. No staining for IDH-1 is seen in adjacent white matter.

The features are of non Hodgkin's large B cell lymphoma.

DIAGNOSIS: Non Hodgkin's large B cell lymphoma.