Malignant transformation of monoclonal gammopathy of undetermined significance among out-patients of a community hospital in southeastern Netherlands.

Patients with a monoclonal gammopathy without evidence of lymphoproliferative or plasma cell malignancy within a year are still at risk for malignant transformation to multiple myeloma, Waldenström's macroglobulinaemia or non-Hodgkin's lymphoma. In a prospective study performed at the Mayo Clinic, the cumulative incidence of malignant transformation was 29% in 14 years. We conducted a retrospective study to determine the frequency of malignant transformation among 334 unselected out-patients with monoclonal gammopathy of undetermined significance (MGUS) from a community hospital in Southeastern Netherlands. The cumulative incidence of malignant transformation was 11% in 14 years (95% confidence interval 6-17%). The long-term survival of patients with MGUS was slightly lower than that of the average regional population. In a nested case-control study, presence of a kappa light chain was found to be a risk factor for malignant transformation (70% of patients who developed malignant transformation compared to 30% of the control group, P < 0.01). Likewise, an initial high gamma globulin level was also found to be a risk factor (18.7 g/l v 13.7 g/l in the control group, P < 0.01). As neither risk factor has been described before, the significance of these factors for definition of a high-risk group among patients with monoclonal gammopathy remains to be determined.