The vast majority of PCNSL in immunocompetent patients are Epstein Barr virus (EBV) negative diffuse large B-cell lymphomas (DLBCL). Other histologies, such as T-cell and small lymphocytic lymphoma, are rare.2 4 5 Unlike other NHLs, it appears that the histological classification of PCNSL does not have prognostic or clinical importance.6 PCNSL usually grows in an angio-centric pattern with sheets of cells that infiltrate adjacent brain parenchyma. Perivascular reactive T-cells are seen in 30% of cases. The CNS is normally devoid of lymphoid tissue; therefore, the site of the cell of origin from which PCNSL develops is likely extraneural. This cell possesses a specific tropism for the nervous system. Both molecular genetic and immunophenotypic studies have suggested that the cell of origin is from the germinal center as BCL6 proto-oncogene mutations or BCL6 protein expression have been reported in the majority of cases.78 Further evidence for this theory is the high proportion of somatic mutations in the clonally rearranged immunoglobulin heavy chains (VH genes) from PCNSL specimens.9 More recent studies have demonstrated a unique immunophenotype and gene expression signature which is distinct from nodal diffuse large B cell lymphoma and which may have both prognostic and potential therapeutic implications.1011

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