Primary malignant lymphoma of the salivary gland: a tumor of mucosa-associated lymphoid tissue.

The clinical, morphologic and immunohistochemical features of 10 cases of the low-grade mucosa-associated lymphoid tissue (MALT) lymphoma of salivary glands are described. Although the initial histologic diagnosis in nine of these cases was myoepithelial sialadenitis, the diagnosis of primary salivary gland MALT lymphoma was based on the demonstration of light chain restriction and on morphologic characteristics. Histologic study showed a characteristic cytology, which included centrocytoid cells (composed of small centrocytes and monocytoid B cells) and a varying degree of plasma cell differentiation; the occurrence of epithelial or acinar invasion by neoplastic centrocytoid cells; and the presence of reactive lymph follicles among the neoplastic cells. Furthermore, multinucleate giant cells resembling Warthin-Finkeldey cells were detected in seven cases. In the light of these findings, cases previously diagnosed as myoepithelial sialadenitis require careful assessment and nine out of 32 cases are, in reality, examples of primary salivary gland MALT lymphomas. Immunohistochemical analysis of paraffin sections revealed the following characteristic immunophenotype of MALT lymphoma: L26, KiB3 and LN2 positive, and a monotypic immunoglobulin pattern (predominantly IgM/kappa). It was of interest that salivary gland parenchyma, infiltrated by neoplastic centrocytoid cells, reacted with LN3 for cells expressing human leukocyte antigen-DR (HLA-DR) antigens. Whereas salivary gland epithelia devoid of a neoplastic invasion were invariably negative for LN3. This suggests a lymphocyte-mediated role in salivary epithelial HLA-DR expression. It appears that HLA-DR expression is an inducible phenomenon in MALT lymphomas of salivary gland.