[Clinical courses and risk factors for progression of smoldering multiple myeloma: a nationwide cohort study in Japan].

We carried out a cohort study of smoldering multiple myeloma (SMM) in Japan. The clinical data of 207 patients with SMM, median age 69 years (range: 27-90), were collected from 63 institutions. The subtype of myeloma was IgG type in 168, IgA type in 30, and Bence Jones type in 9 patients. At a median follow-up of 39 months, 53% of the patients had progressed to symptomatic MM (within 5 years of the initial diagnosis). As previously described, a serum free light chain ratio>8 or <0.125, along with the number of bone marrow plasma cells being 10% or more and the serum M-protein level being at least 3 g/dl, was a significant predictor of rapid progression. We found that the rate of increase in serum M-protein levels correlated negatively with the risk of progression. The probability of progression within 5 years was 100% in patients whose rate of serum M-protein level increase was 2 mg/dl/day or higher. This indicates that the serum M-protein level increase rate might be a useful predictor of disease progression in SMM.