Histopathology

MALT lymphoma

The majority of intestinal lymphomas involving the small bowel are B-cell lymphomas of MALT type, including both low-grade and aggressive types {792, 793, 796}. These so-called 'Western' types are distinct from IPSID and aHCD. The histological features of Western type small intestinal lymphoma are similar to gastric MALT lymphoma, except that lymphoepithelial lesions are less prominent {792}.

In contrast to gastric MALT lymphomas, diffuse large B-cell lymphomas arising in the small bowel are much commoner than low-grade B-cell lymphomas of MALT-type {796}. Some of these lymphomas may have a low-grade MALT component, providing evidence that their histogenesis is related to the mucosal immune system. Precise criteria for defining a MALT lymphoma of large cell type are lacking, as are the criteria for distinguishing transformation within a low-grade MALT lymphoma {383}. When both histologies are evident, the lesion is best described as composite. When small foci of large transformed cells or early sheeting-out of large cells are detected within a background of low-grade intestinal MALT lymphoma, their presence should be noted. Currently, the prognostic impact of these findings and their effect on treatment are undetermined. Diffuse large B-cell lymphomas arising in the small bowel that lack a background of low-grade MALT lymphoma are currently best classified as extranodal diffuse large B-cell lymphoma, not otherwise specified {670}.

IPSID / aHCD

Immunoproliferative small intestinal disease and a heavy chain disease are part of a spectrum of lymphoproliferative diseases prevailing in the Middle East and Mediterranean countries {792}. They are subtypes of small intestinal MALT lym phoma characterized by the synthesis of a heavy chain. The histology is characteristic of MALT lymphoma with marked plasma cell differentiation. Three stages of IPSID are recognized. In stage A, the lymphoplasmacytic infiltrate is confined to the mucosa and mesen-teric lymph nodes, and cytological atyp-ia is not present. Although the infiltrate may obliterate the villous architecture, endoscopic examination appears normal. Resection specimens reveal reactive lymphoid follicles, lymphoepithelial lesions and small clusters of parafollicu-lar clear cells. This phase of the disease is typically responsive to antibiotic therapy. In stage B, nodular mucosal infiltrates develop and there is extension below the muscularis mucosae. A minimal degree of cytological atypia is apparent. This stage appears to represent a transitional phase, can be seen macroscopically as thickening of mucosal folds, and is typically not reversible with antibiotics. The characteristic features of MALT lymphoma are now evident, and follicular colonization may be so marked as to mimic follicular lymphoma. Stage C is characterized by the presence of large masses and transformation to frank large cell lymphoma. Numerous centroblasts and immunoblasts are present. Plasma-cytic differentiation is still evident, but marked cytological atypia is usually found, including Reed-Sternberg-like cells. Mitotic activity is increased. Mesenteric lymph node involvement occurs early in the course of disease, with both plasma cell infiltration of nodal sinuses and marginal-zone areas distended by small atypical lymphoma cells with moderate amounts of pale, clear cytoplasm.

Immunohistochemical studies demonstrate the production of a heavy chain without light chain synthesis {798}. The IgA is almost always of the IgA1 type, with intact carboxy-terminal regions and

deletion of most of the V and all of the CH1 domains. The molecular characterization of individual cases is variable. The small lymphoma cells express CD19 and CD20, but fail to express CD5, CD10 and CD23.

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