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.