Descriptive Data Incidence

Leukemia is the most common malignancy diagnosed in childhood, comprising approximately 30 percent of all diagnoses under the age of 15 years.7 About 3000 children under the age of 20 years will develop leukemia each year in the United States. There are some notable international differences in the incidence of childhood acute leukemia. For childhood ALL, the highest incidence rates occur in Costa Rica, Denmark, Sweden, U.S. Whites, New Zealand (non-Maori), and Los Angeles His-panics; lowest rates occur in India, Israel, China, and Africa.8 For childhood AML, the highest rates occur in Costa Rica, Denmark, Japan, and Australia; lowest rates occur in Kuwait, Scotland, and Thailand. Although there is some difficulty in directly comparing international incidence rates as there may be differences in case ascertainment and/or coding practices, these observations can help in hypothesis generation.

For most of the developed world, there is a pronounced age peak in childhood ALL that occurs between the ages of 2 and 5, which became apparent in non-Hispanic White children in the 1930s; it is absent in many developing countries.8-11 In the United States, the age peak was not noted in Black children until the 1960s.12 Importantly, the incidence rate of childhood ALL in Black children is consistently about half the rate in White children. Higher socioeconomic status (e.g., income, education) has been associated with an increased risk of childhood ALL in several studies.13 This observation, coupled with the lack of a childhood age peak in developing countries, has led to the speculation that factors associated with modernization may account for the peak.14,15 In contrast to childhood ALL, childhood AML peaks in infancy, and then decreases until about the age of 4, where the incidence remains relatively constant throughout childhood.7 There is about a threefold excess risk of AML in Whites during the first few years of life compared to Blacks; however, Black children have slightly higher rates of childhood AML after the age of 3.

Trends

Recent data from the United States suggest a modest increase in the incidence of childhood leukemia during the period of 1975 to 1995, which was largely due to an inexplicable increase during the period 1983 to 1984.16 Rates have decreased slightly since 1989. Site-specific analyses suggest a slight increase in the incidence of childhood ALL over the past few decades.7,17 Gurney et al.17 reported an average annual percentage change (AAPC) of 1.6 percent (95 percent confidence interval [CI] = 0.2 to 2.3) for ALL during the period 1974 and 1991, which was most notable in the youngest age group (diagnosed < 2 years of age). They also noted a striking decreasing trend for leukemias classified as "other" during this same time period (AAPC = -5.0 percent, 95 percent CI = -6.9 to -2.9). Increasing ALL incidence has also been reported in the North Western Regional Health Authority area of England between 1954 and 1988.18 However, no increased trend has been observed in Germany.19 A recent report from the Manchester

Children's Tumour Registry suggests an annual increase of approximately 3 percent in the incidence of pre-B-cell ALL during the period 1980 to 19 98.20 Diagnostic coding changes in part may explain some of the increases observed, but it is possible that other factors either artifactual or real (e.g., changes in the environment) could be important. Thus, ongoing surveillance and monitoring of childhood leukemia trends is important. In contrast to ALL, there has been little fluctuation in AML incidence rates during the past few decades, remaining generally around 5 to 8 cases per million children.7

Below are described some of the risk factors that have been investigated with respect to childhood acute leukemia. More recent studies are highlighted.

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