Epitheliotropism in high‐grade lymphomas of mucosa‐associated lymphoid tissue

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Abstract
Low‐grade lymphomas of mucosa‐associated lymphoid tissue (MALT) characteristically show centrocyte‐like cells, plasmacytic differentiation, follicular colonization and lymphoepithelial lesions (epitheliotropism). High‐grade lymphomas of MALT are thought to lack these features and can only be identified as MALT lymphomas with confidence if they are admixed with residual low‐grade tumours. We studied 23 cases of MALT lymphoma of the thyroid. Six were predominantly low‐grade, 12 were predominantly high‐grade, and five contained both low‐ and high‐grade areas. All cases were stained for cytokeratin, epithelial membrane antigen and laminin using an immunoperoxidase technique. The low‐grade lymphomas all contained lymphoepithelial lesions, generally in the form of clusters of intra‐acinar centrocyte‐like cells. Eleven of the 12 high‐grade MALT lymphomas of the thyroid also showed lymphoepithelial lesions. These appeared in three forms: follicles or clusters of epithelial cells infiltrated by groups of centrocyte‐like cells, intra‐acinar clusters of blast cells, or irregular invasion of islands of epithelial cells by blast cells. The first pattern presumably represented residual low‐grade tumour over‐run by high‐grade lymphoma. The second two patterns indicated that the factors that result in epitheliotropism in low‐grade MALT lymphomas are also prsent in high‐grade tumours. Following this study we identified epitheliotropism in single examples of high‐grade MALT lymphomas of the bronchus and small intestine. Previous failure to recognize epitheliotropism in high‐grade MALT lymphomas of the gastrointestinal tract may be due to loss of the affected epithelium. The localization and relatively good prognosis of high‐grade MALT lymphomas may be related to retention of MALT characteristics including epitheliotropism.