OBJECTIVETo study the clinicopathologic features, immunohistochemical findings and immunoglobulin heavy chain (IgH) gene rearrangement results of primary pulmonary mucosa-associated lymphoid tissue lymphoma (MALToma) and reactive lymphoid hyperplasia.

METHODSTwenty cases, included 13 cases of pulmonary MALToma and 7 cases of pulmonary lymphoid hyperplasia, encountered during the period from 1989 to 2007, were retrospectively analyzed. The samples were paraffin-embedded and stained with hematoxylin and eosin. Immunohistochemical study and semi-nested polymerase chain reaction for IgH gene rearrangement were performed.

RESULTSThe 13 cases of primary pulmonary MALToma were composed of a spectrum of lymphoid cells, including lymphocyte-like cells, centrocyte-like cells and mononuclear B cells with plasmacytoid differentiation. They often had diffuse or marginal zone growth patterns. Lymphoid follicles with neoplastic colonization were apparent. The lymphoma cells spread along alveolar septa and bronchovascular bundles. Vascular invasion was noted in 9 cases, pleura involvement in 6 cases and nodal involvement in 2 cases. Lymphoepithelial lesions (LEL) were identified in 9 cases of pulmonary MALToma. Immunohistochemically, the lymphocytes in LEL were CD20-positive and CD3-negative. On the other hand, LEL was also present in 2 of the 7 cases of lymphoid hyperplasia studied, with a mixture of CD20-positive B cells and CD3-negative T cells. Eight of the 9 cases of primary pulmonary MALToma were positive for IgH gene rearrangement, while all of the 7 cases of lymphoid hyperplasia were negative.

CONCLUSIONSHistologically, the cell population of primary pulmonary MALToma is similar to that of extranodal MALToma occurring in other organs. LEL, though commonly observed in pulmonary MALToma, are not specific and can also be seen in cases of reactive lymphoid hyperplasia. The immunophenotype of intraepithelial lymphocytes in pulmonary MALToma and reactive lymphoid hyperplasia is different. The presence of a monotonous population of CD20-positive intraepithelial lymphocytes supports a diagnosis of MALToma. IgH gene rearrangement study is also useful in differentiating both entities.