First biopsy of the left frontal lesions showed demyelination.

Repeat biopsy 4 months later:

Paraffin sections show a densely hypercellular tumor. Tumor cells have features of atypical large lymphoid cells with large round and oval vesicular nuclei many with conspicuous nucleoli and a narrow rim of pale cytoplasm. These are arranged in diffuse sheets in a vascular stroma. Frequent mitotic figures and apoptoses are identified. Tumor appears to completely replace brain parenchyma. IMMUNOHISTOCHEMISTRY: Atypical lymphoid cells show strong membrane staining for CD20, strong nuclear staining for bcl-6 and MUM-1 and strong perinuclear staining for bcl-2. Moderate numbers of small CD3+ T lymphocytes are scattered among the large atypical cells. No staining for CD5, CD23 or cyclin D1 is seen in the large atypical cels.

The features are of non Hodgkin lymphoma - diffuse large B cell type.