Splenic marginal zone lymphoma (SMZL) is a lymphoma comprised of B-cells that replace the normal architecture of the white pulp of the spleen. The neoplastic cells are both small lymphocytes and larger, transformed blasts, and they invade the mantle zone of splenic follicles and erode the marginal zone, ultimately invading the red pulp of the spleen. Frequently, the bone marrow and splenic hilar lymph nodes are involved along with the peripheral blood. The neoplastic cells circulating in the peripheral blood are termed villious lymphocytes due to their characteristic appearance.
|Rappaport||well-differentiated lymphocytic lymphoma|
|Lukes-Collins||small lymphocytic lymphoma|
|Working Formulation||small lymphocytic lymphoma|
|FAB||splenic lymphoma with circulating villous lymphocytes|
Autoimmune thrombocytopenia and anemia sometimes seen in patients with SMZL. Circulating villous lymphocytes are sometimes observed in peripheral blood samples. A monoclonal paraprotein is detected in a third of patients without hypergammaglobulinemia or hyperviscosity.
Reactive germinal centers in splenic white pulp are replaced by small neoplastic lymphocytes that efface the mantle zone and ultimately blend in with the marginal zone with occasional larger neoplastic cells that resemble blasts. The red pulp is always involved, with both nodules of larger neoplastic cells and sheets of the small neoplastic lymphocytes. Other features that may been seen include sinus invasion, epithelial histocytes, and plasmacytic differentiation of neoplastic cells.
The relevant markers that define the immunophenotype for SMZL are shown in the table to the right. The lack of CD5 expression is helpful in the discrimination between SMZL and chronic lymphocytic leukemia/small lymphocytic lymphoma, and the lack of CD10 expression argues against follicular lymphoma. Mantle cell lymphoma is excluded due to the lack of CD5 and cyclin-D1 expression.