Incidence of Chronic Lymphocytic Leukemia in Olmsted County, Minnesota, 1935 Through 1989, With Emphasis on Changes in Initial Stage at Diagnosis

To determine whether the stage at the time of diagnosis of chronic lymphocytic leukemia (CLL) had changed during a 55-year period. We conducted a study of the cohort of residents of Olmsted County, Minnesota, who had been diagnosed as having CLL during the period from 1935 through 1989. By analysis of medical records, patients with CLL were characterized by Rai stage, absolute lymphocyte count, age at diagnosis, need for therapy, and reported cause of death in nonsurvivors. Trends for these variables were analyzed by decade throughout the study period. The overall annual incidence rate of CLL per 100,000 population in Olmsted County increased from 2.6 in the 1935 through 1944 period to 5.4 in the 1975 through 1984 period; however, the increasing rate was found only for those 50 years of age or older and was especially dramatic for those 75 years old or older. Analysis of Rai stage over time demonstrated an increase in the proportion of cases diagnosed as Rai stage 0. In addition, the median absolute lymphocyte count decreased, the median time to initiation of therapy increased, and the median age of patients with Rai stage 0 CLL at the time of diagnosis increased over time. Overall, 54% of patients had received therapy for CLL by the time of last follow-up. Among the nonsurvivors, CLL was documented as the underlying or a contributing cause of death in 69%. The overall increase in CLL was thought to be due to enhanced methods of early diagnosis and improved health care for the elderly population. Thus, artifact may best explain the observed trend, although we cannot exclude the possibility of an actual increase in incidence rates over time.