Splenic marginal zone lymphoma (SMZL) is a usually indolent lymphoma involving the spleen, bone marrow, and blood.

In this case, the peripheral blood shows abnormal lymphocytosis. The lymphoid cells are slightly enlarged with a moderate amount of cytoplasm. A few cells have fine projections (villous lymphocytes). The bone marrow aspirate is hemodilute but shows an increased number of lymphoid cells of small to medium size, with a moderate amount of cytoplasm. The bone marrow biopsy shows multiple aggregates of lymphoid cells with slight nuclear irregularity. Immunohistochemistry shows CD20 expression highlighting lymphoid aggregates and intra-sinusoidal infiltrative pattern. Additional immunohistochemistry is negative for CD5, CD10, CD23, BCL6, CyclinD1; positive for BCL2.

This slide shows the peripheral blood smear. See Related Content for the bone marrow aspirate and bone marrow biopsy H&E and CD20 stains.

Differential diagnoses include other mature B cell lymphomas: negative CD5/23 staining does not support B cell chronic lymphocytic leukemia; negative CD10 does not favour follicular lymphoma; negative Cyclin D1 does not support mantle zone lymphoma. The cytological and architectural features are not typical of hairy cell leukemia. The lack of prominent plasma cell population does not support lymphoplasmacytic lymphoma. The top differential diagnosis is marginal zone lymphoma. Findings of villous lymphocytes in the peripheral blood and intrasinusoidal infiltrative pattern favour bone marrow involvement by splenic marginal zone lymphoma over nodal or extranodal (MALT) marginal zone lymphoma. A definitive diagnosis of SMZL can only be made with clinical and/or tissue correlation.