Characterization of tissue findings in bone marrow biopsy specimens with small monoclonal B-cell populations

Objectives: Bone marrows (BMs) with incidentally identified, small monotypic B-cell populations (MBPs) were evaluated. Methods: BM aspirates with MBPs representing 5% or less of total events by flow cytometry, less than 5.0 × 10⁹/L B cells in blood, and no history of lymphoma or MBP with a different phenotype from prior lymphoma were selected. Clinical, immunophenotypic, and histologic findings were evaluated. Results: Forty-one of 3,052 BMs had MBPs at 5% or less of total events (median, 1%); 17 were females and 24 were males aged 30 to 87 years (median, 73 years). The MBPs were CD5- in 24, CD5+ resembling chronic lymphocytic leukemia (CLL) in 13, and CD5+ unlike CLL in four. Eighteen of 40 had lymphoid aggregates (LAs) with mostly T cells or a mixture of B and T cells, but three cases had B-cell-rich LAs. Conclusions: Unlike monoclonal B lymphocytosis in blood, MBPs in BMs were more commonly CD5-. Forty-five percent of BMs had LAs; none were interpreted as lymphoma, although three were suspicious for B-cell lymphoma.