Typically arising in the mucosa-associated lymphoid tissues (MALT) along the gastrointestinal and respiratory tracts, marginal zone B-cell lymphomas may also originate in the splenic follicles and in some other organs considered to belong to the MALT system, such as the thyroid, conjunctiva, lung, and salivary glands (8,19). These types of lymphomas arise not only in native lymphoid tissues but also in lymphoid tissues acquired in various locations as a result of persistent inflammations (20). Examples of MALT-type lymphomas arising on the background of acquired lymphoid aggregates post chronic inflammation are the marginal zone cell lymphomas developing after myoepithelial sialadenitis and Hashimoto thyroiditis (15,21).

An association between Sjögren syndrome, characterized by lymphoepithelial lesions (LEL) and lymphoma, was reported as early as the 1960s and repeatedly confirmed. These studies report a 44–fold increased risk in patients with LEL, of whom 4% to 7% may develop lymphoma (4,14,16). Lymphomas developing in salivary glands with a LEL background are indolent, characterized by a long latency period and late dissemination. Between 25% and 80% in various studies were reported as low-grade lymphomas that, in more recent and detailed descriptions, appear to fit the type of marginal zone cell lymphoma (4,9,15,17,22). The morphology of MALT-associated lymphomas consists of sheets of cells that lack a definite histologic pattern and include occasional lymphoepithelial formations (Fig. 70A.6). The cells are B cells of monocytoid type—that is, cells with distinct borders, a sizable amount of cytoplasm appearing clear or just palely stained, and centrally located round, mid-sized, nuclei with inconspicuous nucleoli and mitoses (Figs. 70A.2). Alternately, the cells may have smaller, cleaved nuclei of centrocyte type. In both cases, these are B cells in the post germinal-center phase, differentiating into memory cells and/or plasma cells (5). The immunophenotype is CD19+, CD20+ (Figs 70A.3–70A.4), CD23-, CD10-, CD5-, with a light chain restriction. The cells are Bcl-2+ positive in a vast majority (Fig. 70A.5). CD10 negativity is useful in ruling out follicular lymphoma (FL) and CD5 negativity is useful in ruling out mantle cell lymphoma. A few CD5+ marginal zone B-cell lymphomas have been
reported, however, without the overexpression of cyclin D1 (23). In contrast to benign LEL lesions, in which the infiltrating cells are T lymphocytes, the majority of lymphoid cells in the lymphoepithelial formations of MALT lymphomas are neoplastic B cells, CD 20+ and Bcl-2+ (9,15) (Figs. 70A.3, 70A.4, 70A.5, 70A.7).