Malignant transformation in IgM monoclonal gammopathy of undetermined significance.

Monoclonal gammopathy of undetermined significance (MGUS) is a frequent disorder characterized by the presence of a small serum M-protein in individuals with no evidence of multiple myeloma (MM), Waldenstrom's macroglobulinemia (WM), or primary amyloidosis (AL). Although one fourth of these individuals will develop a malignant disease, there are no well-established predictors of outcome, particularly in the IgM type MGUS. Among 434 patients diagnosed with MGUS from 1970 to 2001 with a minimum follow-up of 1 year, 52 (27 men and 25 women; median age, 67 years) of IgM type were identified. After a median follow-up of 5 years, five patients (9.6%) have developed WM. The risk of transformation was 13.3% (95% confidence interval [CI], 0 to 27) and 27.7% (95% CI, 0.3 to 55.1) at 10 and 20 years, respectively. The variables significantly associated with transformation were the proportion of bone marrow plasma cells (BMPC) and the percentage of bone marrow lymphocytes (BML). No significant differences in the risk of transformation were found between IgM MGUS and the remaining MGUS types. Thus, in IgM MGUS the rate of transformation was similar to the risk observed in other MGUS types, the percentage of BMPC and BML being the features significantly associated with evolution into WM.