A 57-year-old man presented with 2 months of weakness, fatigue, gingival bleeding, and cachexia. Examination revealed right mydriasis, bilateral retinal hemorrhages, diffuse lung crackles, a tense abdomen, and normal lymph nodes. Increased blood viscosity prevented laboratory draw before hydration. Blood count demonstrated 14.6 × 109/L leukocytes with 85% lymphocytes, 8.2 g/dL hemoglobin, and 130 × 109/L platelets; a blood smear showed lymphocytosis and rouleaux formation. Serum viscosity was 11.0 centipoise (normal, 1.5-1.9). A 2.6-g/dL immunoglobulin M κ (IgM-κ) paraprotein was detected. Positron emission tomography/computed tomography showed increased uptake of [18F]fluorodeoxyglucose in the enlarged spleen and in bone marrow. Splenic marginal zone lymphoma (SMZL) was diagnosed after bone marrow examination; the MYD88 L265P mutation was absent.

A 57-year-old man presented with 2 months of weakness, fatigue, gingival bleeding, and cachexia. Examination revealed right mydriasis, bilateral retinal hemorrhages, diffuse lung crackles, a tense abdomen, and normal lymph nodes. Increased blood viscosity prevented laboratory draw before hydration. Blood count demonstrated 14.6 × 109/L leukocytes with 85% lymphocytes, 8.2 g/dL hemoglobin, and 130 × 109/L platelets; a blood smear showed lymphocytosis and rouleaux formation. Serum viscosity was 11.0 centipoise (normal, 1.5-1.9). A 2.6-g/dL immunoglobulin M κ (IgM-κ) paraprotein was detected. Positron emission tomography/computed tomography showed increased uptake of [18F]fluorodeoxyglucose in the enlarged spleen and in bone marrow. Splenic marginal zone lymphoma (SMZL) was diagnosed after bone marrow examination; the MYD88 L265P mutation was absent.

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Although Waldenström macroglobulinemia syndrome is associated with lymphoplasmacytic lymphoma (LPL), other indolent lymphoproliferative diseases can present with similar manifestations, including splenomegaly, spontaneous mucosal bleeding, and IgM paraproteinopathy. SMZL can be distinguished from LPL by circulating lymphocytes with moderate-to-abundant pale cytoplasm (panel A; Wright-Giemsa stain, original magnification ×1000) and microvillous-like cytoplasmic projections (panel A inset). Marrow infiltrates are intrasinusoidal (panel B; CD20 immunostain, original magnification ×200), and random and paratrabecular aggregates may contain reactive germinal centers (panel C; hematoxylin and eosin stain, original magnification ×200) colonized by lymphoma cells (panel D; CD23 immunostain, original magnification ×200). Plasmacytoid lymphocytes are not typically present. The MYD88 L265P mutation, found in most cases of LPL, is rare in SMZL.

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