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

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 present 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.