Clinicopathologic Features of Waldenström's Macroglobulinemia and Marginal Zone Lymphoma: Are They Distinct or the Same Entity?

Waldenström's macroglobulinemia (WM) is considered in the World Health Organization classification as a clinical syndrome associated with monoclonal immunoglobulin (Ig) M secretion, mainly observed in patients with lymphoplasmacytic lymphoma (LPL) and occasionally with other small B-cell lymphomas. Some authors consider it a rare distinct lymphoproliferative disorder with primary bone marrow infiltration and IgM monoclonal gammopathy. As LPL shares important morphologic and immunophenotypic overlaps with marginal zone B-cell lymphomas (MZLs) in cases showing plasmacytic maturation, it remains unclear if they constitute unique or distinct entities. Both diseases are composed of lymphocytes, lymphoplasmacytoid cells, and tumoral plasma cells with a surface (s) IgM-positive sIgD+/- cytoplasmic IgM-positive CD19+ CD20+ CD27+/- CD5- CD10- CD23- phenotype, without a specific marker. Extranodal mucosa-associated lymphoid tissue (MALT) lymphoma, nodal MZL (NMZL), and splenic MZL (SMZL) are distinct entities displaying common morphologic, immunophenotypic, and genetic characteristics. MALT lymphoma is clearly distinct from LPL, although bone marrow infiltration and IgM paraprotein are not rare. Splenic MZL and NMZL are incompletely characterized, but a plasmacytoid/plasmacytic differentiation, autoimmune manifestations, and monoclonal component are frequent in both diseases. Bone marrow involvement is constant in SMZL and present in 60% of NMZLs. Molecular IgVH gene analysis has confirmed this heterogeneity, particularly within SMZL, with mutated and unmutated cases. Further studies are needed to clarify the pathogenesis of these MZLs and their relationship with LPL.