Practical diagnostic approaches to composite plasma cell neoplasm and low grade B‐cell lymphoma/clonal infiltrates in the bone marrow

Composite plasma cell neoplasm (PCN) and low grade B‐cell lymphoma (B‐NHL) in the bone marrow are uncommon and raise the differential diagnosis of B‐NHL with plasmacytic differentiation and PCN with lymphoplasmacytic morphology. This can be a challenging differential diagnosis, and the distinctions are important because of differences in management. We report five cases of composite PCN with B‐NHL or clonal B‐cell infiltrates involving the bone marrow. By using multiple different diagnostic modalities, including immunophenotyping by flow cytometry and immunohistochemistry, cytogenetic analysis and IGH gene rearrangement studies by polymerase chain reaction, we were able to distinguish two distinct clonally unrelated neoplasms in all cases. We describe the utility and pitfalls of these different diagnostic modalities. Flow cytometric analysis with a panel of antibodies that includes CD19, CD56, CD138, CD45 and other aberrant markers commonly expressed by PCN will allow identification of clonally unrelated PCN and B‐NHL in a composite neoplasm, and distinguish them from B‐NHL with plasmacytic differentiation and PCN with lymphoplasmacytic morphology. Cytogenetic and molecular analyses can give false‐negative or false‐positive results. In summary, a multimodal approach utilizing these different tools, including clinical data, should be used to arrive at the correct diagnosis. Copyright © 2014 John Wiley & Sons, Ltd.