Waldenstrom macroglobulinemia (WM) is a rare indolent B-cell lymphoma classified as lymphoplasmacytic lymphoma (LPL) with bone marrow involvement plus IgM monoclonal gammopathy of any concentration. Annual incidence is 0.4 per 100,000 in the US (rarer in Asians), with median age 70 years and male predominance (2:1). Pathophysiology involves clonal proliferation of small mature B-lymphocytes, plasmacytoid lymphocytes, and plasma cells producing IgM monoclonal immunoglobulin. Driver mutations: MYD88 L265P present in >90-95% of WM (toll-like receptor pathway activation leading to BTK and IRAK1/4 signaling), CXCR4 mutations in 30-40% (warts hypogammaglobulinemia infections myelokathexis WHIM-like), with prognostic and therapeutic implications.
Diagnosis requires bone marrow biopsy showing >=10% lymphoplasmacytic infiltrate plus serum IgM monoclonal protein (any quantity); flow cytometry shows CD19+, CD20+, CD79+, surface IgM+, CD5- (negative distinguishing from CLL), CD10- (negative distinguishing from follicular lymphoma), CD23-, and may be IgM-restricted; MYD88 L265P testing essential for diagnosis and treatment selection; CXCR4 mutation testing useful for treatment selection. Differential diagnosis includes IgM monoclonal gammopathy of undetermined significance (IgM-MGUS, no bone marrow LPL), other low-grade B-cell lymphomas with IgM, and multiple myeloma with IgM (rare). Risk stratification uses International Prognostic Scoring System for WM (IPSSWM) integrating age, hemoglobin, platelet count, beta-2 microglobulin, and IgM concentration.
Treatment is indicated for symptomatic disease per IWWM consensus criteria (hyperviscosity symptoms, hemoglobin <10 g/dL, platelets <100,000, bulky/symptomatic lymphadenopathy or splenomegaly, IgM-related neuropathy, cryoglobulinemia, cold agglutinin disease, AL amyloidosis, kidney involvement, B-symptoms). First-line options include: BTK inhibitors (zanubrutinib 160 mg twice daily preferred for CXCR4-mutated and CXCR4-WT due to efficacy and tolerability per ASPEN trial; ibrutinib 420 mg daily preferred for CXCR4-WT, less effective for CXCR4-mutated; acalabrutinib alternative); chemoimmunotherapy with bendamustine-rituximab (BR) or rituximab-dexamethasone-cyclophosphamide (DRC); proteasome inhibitor regimens (bortezomib-rituximab-dexamethasone BDR for high IgM with hyperviscosity, hematologic disease); rituximab monotherapy for IgM-related neuropathy or cryoglobulinemia. Rituximab can cause IgM flare with worsening hyperviscosity (avoid in untreated high IgM, plasmapheresis first if needed). Plasmapheresis emergent therapy for hyperviscosity (IgM >40-50 g/L with symptoms). Allogeneic stem cell transplantation reserved for highly refractory young patients.