Extranodal lymphomas: the MALT concept.

Extranodal lymphomas account for as many as 40% of non-Hodgkin's lymphomas and most arise in the gastro-intestinal tract which is a major lymphoid organ in its own right. Gastrointestinal (gut) associated lymphoid tissue (GALT) and that of other mucosae (MALT), differs both structurally and functionally from nodal lymphoid tissue and low grade B cell lymphomas arising in the gastrointestinal tract and other mucosae have been found to recapitulate the structure and cytological features of MALT. Moreover, these lymphomas are clinically indolent which could be explained by the restricted homing patterns of MALT. Curiously, however, most MALT lymphomas arise in sites, such as the stomach, where MALT is not normally present. Several chronic inflammatory conditions, most of which have an autoimmune component, result in the acquisition of MALT-like lymphoid tissue, and have been identified as necessary precursors for the development of MALT lymphoma. These include Helicobacter pylori induced chronic gastritis, Sjögren's syndrome and Hashimoto's thyroiditis. Histologically, low grade MALT lymphomas are characterized by centrocyte-like (CCL) B cells which surround reactive follicles and form characteristic lympho-epithelial lesions with adjacent epithelium; they frequently show plasma cell differentiation. Specific colonization of reactive follicles by CCL cells often occurs and transformation into a high grade lymphoma may occur. The phenotype of MALT lymphoma CCL-cells is similar to that of marginal zone B cells; no characteristic genotypic features have yet been identified. When lymph nodes are involved by MALT lymphoma their appearance may be indistinguishable from those of so-called monocytid B cell lymphoma, a primary lymph node tumour which, unlike MALT lymphoma, shares the clinical features of other low grade nodal B cell lymphomas.