The patient went on to have a brain biopsy (right frontal lobe) 

Histology

The sections of cerebral white matter are mildly hypercellular,  with focally prominent perivascular lymphocytic cuffing.   There are diffusely increased astrocytes through the tissue, as well as a light scattering of macrophages and activation of microglial cells.  The lymphocytes cuffing vessels are predominantly small CD3-positive T cells.  Within some of these vessels are large atypical lymphoid cells with a high proliferative index on the Ki67 stain.   Occasional single large atypical  B-cells are present within perivascular parenchyma.   The features are consistent with a diagnosis of intravascular large B cell lymphoma.
 
Final diagnosis

Right frontal white matter biopsies  - large B-cell non-Hodgkin lymphoma, consistent with intravascular large B-cell lymphoma.