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Monocytoid B‐cell lymphoma: morphological variants and relationship to low‐grade B‐cell lymphoma of the mucosa‐associated lymphoid tissue

Twenty‐eight cases of monocytoid B‐cell lymphoma of lymph nodes and 16 lymph node metastases of primary gastric lymphomas, mostly low‐grade B‐cell lymphomas of mucosa‐associated lymphoid tissue (MALT) type were investigated morphologically and immunohistochemically. Both groups showed the same morphological and immunohistochemical features: diagnostically important sites of infiltration were the sinuses and the marginal zones, The tumour cells were either medium‐sized or small. The cytoplasm stained grey with Giemsa and was sometimes rather pale. In imprints the grey colour of the cytoplasm was a characteristic feature. The medium‐sized cell type was more frequent; in one third of the cases it was combined with a prominent lymphoplasmacytic component from the same clone, and it resembled the monocytoid B‐cells of the sinuses. The small cell type was less common, was not combined with a lymphoplasmacytic component and more closely resembled marginal zone cells. The difference was underlined by the negative reaction with the monoclonal antibody Ki‐B3 in the small cell type, which, conversely, was positive in the medium‐sized cell type and in the monocytoid B‐cell reaction of the sinuses. Both of these cell types, however, showed a granular reaction with the new monoclonal antibody Ki‐Mlp. The morphological and immunohistochemical parallels are arguments in favour of the assumption that monocytoid B‐cell lymphoma is the nodal equivalent of low‐grade B‐cell lymphoma of MALT type. This is further supported by the fact that in nine of our 28 cases of monocytoid B‐cell lymphoma, lymphomas were found simultaneously or subsequently in organs of the MALT. Monocytoid B‐cell lymphoma must be differentiated from an infiltration that occurs in the form of clusters of monocytoid B‐cells in other low‐grade B‐cell lymphomas, especially in immunocytoma with a high content of epithelioid cells.