Extranodal marginal-zone B-cell lymphoma of the salivary gland.

Primary non-Hodgkin's lymphoma of the salivary gland is an uncommon tumor that most often occurs in the parotid gland. The most common subtype is marginal-zone B-cell lymphoma, extranodal, mucosa-associated lymphoid tissue type. This subtype has recently been included in the Revised European-American Classification of Lymphoid Neoplasms, as well as in the upcoming World Health Organization classification of hematopoietic and lymphoid neoplasms. This low-grade lymphoma usually arises in a background of benign lymphoepithelial lesion or myoepithelial sialadenitis that is associated with the autoimmune disease Sjögren's syndrome. It occasionally develops in patients who do not have a history of autoimmune disease. When mucosa-associated lymphoid tissue lymphoma occurs in the salivary gland, as in other extranodal sites such as the stomach, it is usually an indolent neoplasm that tends to remain localized for long periods of time, even without treatment. Eventually, however, the tumor may disseminate or transform to a higher grade. The histologic distinction of myoepithelial sialadenitis from low-grade B-cell mucosa-associated lymphoid tissue lymphoma can be a difficult diagnostic challenge and many of these lesions continue to be ambiguously diagnosed as "pseudolymphoma." Immunophenotypic or flow cytometric analysis may be useful in showing an aberrant phenotype or immunoglobulin light-chain restriction, which helps to support a diagnosis of malignant lymphoma in most cases. Molecular genetic analysis for immunoglobulin gene rearrangements also may be useful in showing monoclonality, although the exact significance of this finding in some cases remains controversial.