To qualify as a low-grade lymphoma of mucosa-associated lymphoid tissue (MALT) type, lymphomas at different extranodal sites must conform to specific morphologic features. Lymphoepithelial lesions are typical, and follicular colonization is common. Regardless of site, many MALT lymphomas are associated with a predisposing condition, infectious or autoimmune, leading to the acquisition of extranodal MALT MALT lymphomas in different regions usually have limited disease and indolent clinical courses in which survival does not substantially differ from healthy controls. There is a tendency for diverse cytologic expressions among different MALT lymphomas, such as small lymphocytes in the lung monocytoid cells in salivary glands, and plamacytic cells in the thyroid and skin, but the site-specific morphologic differences mainly reflect the unique topographic characteristics of the involved organ, and these topographic features also influence the clinical traits. Some of the unparalleled features of MALT lymphomas in different sites include the association of gastric MALT lymphomas with Helicobacter pylori, the lymphangitic distribution of pulmonary MALT cases, and the relationship between MALT lymphomas of the salivary gland and thyroid with Sjogren's syndrome and Hashimoto's thyroiditis, respectively, The ocular cases and those of the skirt, breast, and dura also have characteristic clinical modes of presentation largely owing to the anatomic locale of the lymphoma.
