Chronic Lymphocytic Leukemia

The epidemiology of chronic lymphocytic leukemia (CLL) is hampered by two features of the condition. First, it is often found as a chance diagnosis and this in turn can be a reflection of medical care in a particular area, rather than a true representation of the totality of the disease. Second, in both descriptive and analytic epidemiological studies little attention is given to the quality of diagnostic definitions. Most epidemiological studies do not attempt to distinguish B-cell disease and most will include prolymphocytic leukemias and possibly lymphocytic lymphomas as part of what they term "CLL."

There have been no large-scale comprehensive and diagnos-tically sound epidemiological studies of CLL published in the past decade.

Some African populations have a slight age-specific peak in the middle years (35 to 50), possibly due to the PLL cases noted above.2

The concept of a genetic basis of risk is reinforced by the observation that Japanese migrants to the United States and their descendants retain low rates.4 Studies of CLL subtypes in Japan suggest relatively few have the "typical" disease (with 90 percent small lymphocytes), only 7 in a series of 41, the rest being a variety of conditions, particularly large and prolym-phocytic cell types.5

There is some evidence that CLL declined in incidence in the United States in both White and Black populations6 between 1973 and 1990. The decline is modest and may reflect diagnostic changes rather than any fundamental change. A similar study in the United Kingdom shows very little change in incidence from 1984 to 1989.7

CLL has not shown any evidence of close case aggregation or clustering,8 but some evidence of heterogeneity of geographic distribution was uncovered in a study of U.K. cases from 1984 to 1988.3 This observation may well reflect the difficulties in achieving a uniform standard of case ascertainment even in developed countries.

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