IgM plasma cell myeloma in the era of novel therapy: a clinicopathological study of 17 cases.

IgM plasma cell myeloma (PCM) is a rare subtype of myeloma, and its response to novel therapies has not been fully characterized. We describe clinicopathological features and outcome of 17 patients with IgM PCM (11 men and 6 women) with a median age of 63 years. Patients presented with serum hyperviscosity (77%), bone lesions (71%), anemia (65%), renal dysfunction (53%), and hypercalcemia (35%). Median serum IgM level was 6.4 g/dL (0.7-12.1 g/dL). Bone marrow plasma cells (median, 80%; range, 20%-90%) were frequently of lymphoplasmacytic type (8/17; 47%). Immunophenotypically, the myeloma cells were positive for CD38, CD138, CD20 (5/16; 31%), CD56 (4/16; 25%), and CD117 (2/12; 17%); negative for CD19; and decreased or absent CD27 and CD81 in all cases. Seven (41%) patients had a complex karyotype, and fluorescence in situ hybridization showed CCND1-IGH (13/16; 81%), and deletions of 17p13/TP53 (29%) and 13q14/RB1 (38%). No MYD88 L265P mutation was detected. Most patients (94%) received proteasome inhibitor with or without immunomodulatory drug, 62% of patients required multiple regimens because of refractory disease, and 11 (65%) of 17 patients underwent autologous stem cell transplant (ASCT). The median OS was 67 months. After a median follow-up of 38 months (range, 3-106 months), only 5 patients achieved complete remission, 5 had persistent disease, and 7 died (2 progressed to plasma cell leukemia and 1 to blastic variant). In summary, IgM PCM is highly associated with t(11;14) and lymphoplasmacytic morphology. Patients are refractory to novel therapy and progression to high-risk myeloma is common, suggesting a need for alternative novel therapies.