Abbreviations: HCL, hairy cell leukemia; TRAP, tartrate-resistant acid phosphatase.

ogy, the leukemic presentation, the lack of monocytopenia, the aspirable bone marrow, and the morphologic appearance of the cells in the bone marrow biopsy. There is a blastic variant of HCL in which patients have massive splenomegaly, peripheral lymphadenopathy, and pancytopenia.53 These cells, interestingly, stain positively with the TRAP stain and negatively with myeloperoxidase.

Splenic lymphoma with circulating villous lymphocytes can be difficult to distinguish from HCL, but the cells have more basophilic cytoplasm and the cytoplasmic projections tend to be polar and more subtle in the majority of circulating lymphocytes (Figures 8-1 and 8-4). Circulating plasmacytoid cells are frequently noted.27 TRAP staining is negative or very weak. Immunophenotyping identifies cells with strong staining for CD11c, but are CD103 (Bly-7)-negative. The bone marrow typically is only minimally involved, with the lymphocytes appearing as small and round and having plasmacytoid features. Peripheral monocytopenia is usually absent. Sections of spleen show predominantly white pulp involvement.

Marginal zone lymphoma/monocytoid B-cell lymphoma involving the spleen can have histopathologic characteristics that may be confused with HCL54 but are TRAP stain-negative. Strong staining for CD11c and Bly-7 suggests HCL rather than these splenic lymphomas. Aplastic anemia needs to be differentiated from pancytopenia due to the hypoplastic variant of HCL, especially when there is no associated splenomegaly. Bilateral bone marrow examinations with special stains or even diagnostic splenectomy may be necessary in selected patients to make the appropriate diagnosis. Hairy cell leukemia must also be differentiated from mast cell disease, which can mimic HCL, especially when there are infiltrates composed of spindle-shaped cells. Mast cells contain metachromatic granules with the Giemsa stain, and the granules are also positive for chloroacetate esterase.55 Immunohistochemistry demonstrates positive staining with KPI (CDBP), a marker of macrophages, and negative staining with CD20 (L26). Hairy cell leukemia also needs to be considered in the differential diagnosis of any disorder that results in cytopenias and splenomegaly, for example, myelofibrosis.

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