Patient went on to have surgery.
MICROSCOPIC DESCRIPTION: Paraffin sections show fragments of a densely hypercellular tumour. This is composed of atypical large lymphoid cells with round, oval and angulated nuclei with an open chromatin arrangement, conspicuous nucleoli and a narrow rim of pale cytoplasm.
These show strong membrane immunostaining for CD20, strong perinuclear staining for bcl-2 and strong nuclear staining for MUM-1 and PAX5. These are arranged in diffuse sheets and are admixed with small T lymphocytes (CD4+>CD8+). Frequent mitotic figures are identified.
Similar atypical cells are seen in perivascular spaces in adjacent white matter, extending through vessel walls. The features are of non Hodgkin's diffuse large B cell lymphoma.
FINAL DIAGNOSIS: non-Hodgkin's diffuse large B cell CNS lymphoma