Classic hairy cell leukemia and concomitant smoldering myeloma Free

An 80-year-old woman had a 10-year history of thrombocytopenia, which recently progressed to pancytopenia, with diffuse adenopathy, and splenomegaly. Serum protein electrophoresis detected immunoglobulin G-κ paraprotein. The peripheral blood smear showed rare atypical lymphocytes with cytoplasmic projections (panel A; Giemsa stain, 100× objective). Monocytopenia was apparent. The bone marrow biopsy was hypercellular with abnormal lymphocytic infiltrate (panel B; hematoxylin and eosin stain, ×40 objective) expressing CD20 (panel C; CD20 stain, 20× objective) and CD25, CD103, CD123, annexin, and BRAF-V600E (panel D; BRAF-V600E stain, 40× objective), supporting the diagnosis of hairy cell leukemia (HCL). Cyclin D1 (panel E; cyclin D1 stain, 40× objective) was weakly and variably expressed by HCL cells, but also strongly stained plasma cells (10%-15%), corresponding with CD138⁺ cells (panel F; CD138 stain, 20× objective). Bone marrow flow cytometry showed a monoclonal B-cell population (14%) expressing CD20 (bright), CD200 (dim), and surface κ light chain, as well as CD138 (bright), light-chain indeterminate plasma cells (6%). Fluorescence in situ hybridization testing was positive for IGH::CCND1 translocation, correlating with the strong cyclin D1 expression in the plasma cells.

HCL in this case was incidental because flow cytometry did not include an expanded marker panel. The presence of the CCND1 rearrangement and serum monoclonal paraprotein suggest a plasma cell neoplasm (PCN); however, the dominant finding (explaining the patient’s pancytopenia) is HCL. Complete morphologic and immunophenotypic evaluation is important to accurately differentiate between overlapping features of PCN and HCL.

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