Primary gastric lymphoma accounts for 7% of primary gastric neoplasms . H. pylori infection is considered to be the ultimate cause of most primary gastric lymphomas. The normal stomach harbors few lymphocytes, but the inflammatory reaction to H. pylori induces lymphocytic migration into the gastric mucosa . The arriving lymphocytes assemble into an organoid structure resembling a lymph node, with follicular, interfollicular, and intraepithelial populations of lymphocytes. Thus, this lymphocytic population assumes a relationship with the gastric mucosa much like that seen in the constitutive mucosa-associated lymphoid tissue (MALT) of other organs, including the terminal ileum and appendix. Through the proliferative effect of continuous antigenic stimulation, with acquisition of molecular defects , a low-grade lymphoma may arise out of this H. pylori induced gastric MALT. This neoplasm has a characteristic morphology and behavior common to lymphomas arising in other portions of the gut colloquially referred to as “MALTomas.” These often indolent tumors, which have the more formal and descriptive designation of “low-grade B-cell lymphoma of extranodal marginal zone (MALT) type” , are composed of regular, relatively small lymphocytes with light-staining cytoplasm . Expansion of the lamina propria by these cells is the key feature allowing differentiation from an intense H. pylori-induced gastritis . Pathologists are encouraged to avoid being more definitive than the histologic findings warrant; the natural history of these tumors allows the luxury of waiting until the process declares itself morphologically. Once a diagnosis is established, there are several treatment options for these tumors, including eradication of H. pylori; gastric MALTomas will often respond to this measure. Sixty per cent of primary gastric lymphomas are not the indolent, low-grade tumors described above but rather are aggressive, high-grade, diffuse, large B-cell lymphomas , in which the neoplastic cells are large, immature, proliferating
lymphocytes . They may arise in a background of a MALToma or apparently de novo. These high-grade lymphomas often are tumor forming, may be associated with hematemesis or perforation, and can be difficult to distinguish from a poorly differentiated carcinoma both grossly and microscopically. Immunoperoxidase stains for lymphoid and epithelial markers are often diagnostic .
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