INCIDENCE OF MULTIPLE-MYELOMA IN OLMSTED COUNTY, MINNESOTA - 1978 THROUGH 1990, WITH A REVIEW OF THE TREND SINCE 1945

Purpose: Incidence rates of multiple myeloma appear to have increased in recent decades, but it is difficult to compare the incidence rates over time because of changes in laboratory procedures, differences in diagnostic criteria, and variations in indexing of medical records. However, Olmsted County, Minnesota, provides a rare opportunity to minimize these limitations, because the medical care has been provided mainly by the Mayo Clinic. Patients and Methods: Records of Olmsted County residents with a diagnosis of multiple myeloma from January 1, 1978 through December 31, 1990 were reviewed. Criteria for diagnosis and residency were the same as in previous Olmsted County studies. Results: The incidence rate per 100,000 age-adjusted to the 1950 United States white population was 5.4 for males and 2.8 for females; the overall rate was 4.1. The rates were age-adjusted to the 1950 United States population for comparison with earlier data from this population. The rates increased with age for both sexes. There was no significant change in incidence rates from 1945 through 1990. A comparison of the median age for Olmsted County patients with the age of those referred to the Mayo Clinic from elsewhere indicated that the local patients were appreciably older (74 v. 62 years, respectively). Thirty-two of 55 patients (58%) with multiple myeloma from 1978 through 1990 had a monoclonal plasma- cell proliferative process before the diagnosis of multiple myeloma. Conclusion: The incidence rate of multiple myeloma in Olmsted County, Minnesota, has not changed significantly during the past 46 years. We believe that reports of increasing rates over time are mainly due to improved case ascertainment.