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Monoclonal gammopathy in Iceland: a population‐based registry and follow‐up

Summary. The term monoclonal gammopathy (MG) signifies the benign or malignant clonal growth of B lymphocytes. In the present study, monoclonal gammopathy of unknown significance (MGUS) was defined as those patients with no identified haematological malignancy. A database was constructed of all 713 MG patients in Iceland between 1976 and 1997 and compared with the Icelandic Cancer Registry. The age‐standardized incidence per 100 000 of MG was 10·3 for males and 8·6 for females, calculated for the whole period, rising steadily from 5·8 (men) and 4·9 (women) during the 5‐year period 1976–80 to 14·7 (men) and 12·5 (women) during the last 5 year period. Age‐standardized incidence rates were very low for subjects under 50 years of age, then increased with age from 11 and 17 per 100 000 at 50‐54, to 169 and 119 per 100 000 at age 80–84, for men and women respectively. No association was detected between MG and non‐haematological malignancies, neither retrospectively nor prospectively. Haematological malignancy was diagnosed in 209 (29·3%) cases before the recorded finding of MG or within the same calendar year, leaving 504 (70·7%) patients diagnosed with MGUS. Of these, 51 (10%) progressed to multiple myeloma or Waldenström's macroglobulinaemia after a mean interval of 3·8 years; mean follow‐up was 7·4 years, median 6 years. The most common immunoglobulin (Ig) class was IgG (55%), followed by IgM (32%) and IgA (13%). MGUS was a highly significant risk factor for developing haematological malignancies and the risk was significantly greater for MG of the IgA class compared with either IgG or IgM.