Cancer risk in patients with monoclonal gammopathy of undetermined significance.

To assess the cancer risk of monoclonal gammopathy of undetermined significance (MGUS) we identified 1229 cases of MGUS in the period 1978 to 1993. Data on cancer occurrence in the MGUS cohort were obtained from the Danish Cancer Registry. The expected numbers of cancer cases were calculated from age-, sex-, county-, and period-specific cancer incidence rates. In the MGUS cohort 64 new cancers with a known association with M-components were diagnosed versus 5.0 expected giving a standardized incidence ratio (SIR) of 12.9 (95% confidence interval, 9.9-16.5). The relative risks of developing multiple myeloma (SIR 34.3), Waldenström's macroglobulinemia (SIR 63.8), and non-Hodgkin's lymphoma (SIR 5.9) were significantly increased and independent of time passed from detection of the M-component. The relative risk of chronic lymphocytic leukemia was not significantly increased, SIR 2.7 (0.5-7. 7). Among cancer sites without known association with M-components 141 cases were observed versus 94.6 expected giving a SIR of 1.5 (1. 3-1.8). This enhanced risk was seen for several non-hematological cancer sites but for most cancer sites the risk was dependent on time passed from detection of the M-component, indicating a bias rather than a causal role of MGUS. Copyright 2000 Wiley-Liss, Inc.