Hairy cell leukemia variant with features of intrasinusoidal bone marrow involvement

Hairy cell leukemia variant (HCL-V) is a rare lymphoproliferative disorder. We report a case of HCL-V with an intrasinusoidal pattern of bone marrow involvement without interstitial or diffuse infiltration that is typical of HCL and its variant. The peripheral blood and bone marrow aspirates demonstrated abnormal lymphoid cells with cytoplasmic projections that were weakly positive for tartrate-resistant acid phosphatase cytochemical staining. Immunostaining of the bone marrow biopsy specimen showed that these cells were strongly positive for CD20, located within bone marrow sinusoids, and weakly positive for DBA44. By flow cytometry, these cells were positive for CD19, CD20, CD11c, and CD103, exhibited lambda light chain restriction, and were negative for CD25. The patient was initially diagnosed as having splenic lymphoma with villous lymphocytes (SLVL) or splenic marginal zone lymphoma (SMZL) (World Health Organization designation) and treated with fludarabine followed by splenectomy with simultaneous liver biopsy. The pathologic analysis of the spleen revealed infiltration of red pulp by the critical cells without white pulp involvement, which is characteristic of HCL and HCL-V but not of SLVL (SMZL). This case illustrates an atypical marrow presentation of HCL-V and emphasizes the need to correlate all clinical and pathologic data, including tissue biopsy, in reaching a diagnosis.