Background: Bence Jones proteinuria is a disorder that is defined by the excretion of monoclonal light-chain protein. About 15-20% of patients with multiple myeloma secrete monoclonal light chains only, without expression of the normal immunoglobulin heavy chain, which constitutes light-chain multiple myeloma. The definition, prevalence, and progression of these premalignant phases of light-chain multiple myeloma have not been fully characterised. We aimed to identify a subset of patients with idiopathic Bence Jones proteinuria who had a high risk of progression to light-chain multiple myeloma analogous to that seen in patients with smouldering multiple myeloma. Methods: In this retrospective cohort study, we studied all patients seen at the Mayo Clinic (Rochester, MN, USA) within 30 days of diagnosis of idiopathic Bence Jones proteinuria between Jan 1, 1960, and June 30, 2004. Inclusion criteria were monoclonal light chain in the urine (≥0.2 g/24 h), absence of intact monoclonal immunoglobulin (M protein) in the serum, and no evidence of multiple myeloma, light-chain amyloidosis, or other related plasma-cell proliferative disorders. The primary endpoint was progression to symptomatic multiple myeloma or light-chain amyloidosis. We examined the cumulative probability of progression and the association of potential risk factors on progression rates to identify patients with a high risk of progression to multiple myeloma or light-chain amyloidosis. Findings: We identified 101 patients with idiopathic Bence Jones proteinuria. During 901 total person-years of followup, 27 (27%) patients developed multiple myeloma and seven (7%) developed light-chain amyloidosis. The major risk factors for progression were amount of urinary excretion of M protein per 24 h, proportion of bone marrow plasma cells, presence of a markedly abnormal free-light-chain ratio (<0.01 or >100), and reduction of all three uninvolved immunoglobulins. Based on the risk of progression, monoclonal light-chain excretion of 0.5 g/24 h or greater or at least 10% bone marrow plasma cells, or both, in the absence of end-organ damage was used to define light-chain smouldering multiple myeloma. The cumulative probability of progression to active multiple myeloma or light-chain amyloidosis in patients with light-chain smouldering multiple myeloma was 27.8% (95% CI 14.2-39.2) at 5 years, 44.6% (27.9-57.4) at 10 years, and 56.5% (36.3-70.2) at 15 years. Interpretation: Light-chain smouldering multiple myeloma as defined in this study is associated with a high risk of progression to symptomatic light-chain multiple myeloma, and this subset of patients needs careful observation and could benefit from clinical trials of early intervention.
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