Extranodal malignant lymphoma arising from mucosa-associated lymphoid tissue.

Four cases of extranodal malignant lymphoma, one each arising in the stomach, salivary gland, lung, and thyroid, are described. These cases have many clinical, histopathologic, and immunohistochemical features in common, and it is proposed that this is because they share a common pattern of histogenesis from mucosa-associated lymphoid tissue (MALT). Clinical features of MALT-derived lymphomas include a tendency to remain localized for prolonged periods, and, thus, to be responsive to locally directed therapy. Histologically, this group of tumors is characterized by a noninvasive lymphoplasmacytic infiltrate within which foci of follicle center cells (FCC) can be seen invading epithelial structures, forming lymphoplasmacytic infiltrate lesions. Immunohistochemistry reveals monotypic cytoplasmic immunoglobulin (CIg) in the plasma cells serving to highlight the CIg-negative lymphoepithelial lesions. The clinical and histopathologic features of these lymphomas can be understood in the context of the behavioral characteristics and morphology of MALT.