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An 85-year-old man with an outside diagnosis of mantle cell lymphoma (MCL) presented with acute onset cough, weakness, severe macrocytic anemia (MCV 123 fL, Hgb: 7.4 g/dL), and hypergammamlobulinema with a monoclonal spike (IgG >4000 mg/dL). (A) Flow cytometry identified a monoclonal population of plasma cells that were positive for CD38 and CD138 (top), cytoplasmic Kappa chain (middle), and positive for CD20 but negative for CD19 (bottom). Other positive markers included CD56, CD117, and CD45 (data not shown). (B) Hematoxylin and eosin stain (40X) of the bone marrow biopsy demonstrated marked hypercellularity consisting of >90% plasma cells arranged in focal clusters and diffuse sheets. Morphologically, the plasma cells were small in size, mature in appearance with slightly eccentric nuclei. Immunohistochemistry (all at 40X) demonstrated that the plasma cells were intensely positive for CD138 (C), BCL1 (D), and CD20 (E) and negative for CD3 (not shown). Concurrent cytogenetics revealed a normal 46X,Y karyotype without t(11;14) translocation. While the co-expression of CD20 and BCL1 are found on >96% of MCL cases¹, the clinical and histopathologic findings found in this patient were most compatible with a diagnosis of a multiple myeloma, small cell subtype, with atypical co-expression of CD20 and BCL1.

1.         Boyd SD, Natkunam Y, Allen JR, Warnke RA. Selective Immunophenotyping for Diagnosis of B-cell Neoplasms: Immunohistochemistry and Flow Cytometry Strategies and Results. Appl Immunohistochem Mol Morphol. 2013;21(2):116–131.