IgM Multiple Myeloma Has Unique Attributes Distinguishing It from Other Plasma Cell Neoplasms and Lymphoplasmacytic Lymphoma
Rebecca L King, Matthew T Howard, Janice M Hodnefield, William G Morice. Mayo Clinic, Rochester, MN
Background: It is recognized that in a minority of multiple myeloma (MM) cases the abnormal plasma cells (PC) produce an IgM paraprotein; however, the bone marrow (BM) pathology of IgM MM is not well characterized. This knowledge deficit is significant, as bone marrow biopsy is often obtained to help discriminate IgM MM from lymphoplasmacytic lymphoma (LPL), particularly if some clinical attributes of MM (CRAB-hypercalcemia, renal insufficiency, anemia, and bone lesions) are absent. This distinction is critical, as therapies are vastly different for the two diseases. To address this issue, the morphologic, immunophenotypic, and genetic features of BM involvement in well-established IgM MM was studied, and the findings compared to LPL and other types of MM.
Design: BM biopsies from 16 cases satisfying all clinical MM criteria with isolated IgM serum paraproteins were studied. In each, the BM pathology and immunohistochemistry (IHC) were reviewed individually and by multiple authors together.
Results: All 16 patients (mean age 65 years) had an IgM paraprotein (Median: 2.5, range: 0.3 – 5.1 g/dL) and CRAB symptoms (7 with lytic bone lesions) as well as at least 10% clonal BM PC. The BM PC showed a spectrum of cytologies ranging from large, atypical forms to those with a smaller, lymphoplasmacytoid appearance. The latter had features overlapping with LPL, however no cases had monotypic B cells by flow cytometry or IHC. All cases were positive for the PC–associated antigens CD138 and MUM1 by IHC; however our study showed an unusually high frequency of B cell antigen expression as compared to other MM.