An ecologic study of childhood leukemia and population mixing in Ontario, Canada

Objective: To examine the association between population mixing and the inc idence of childhood leukemia, specifically the acute lymphocytic leukemia ( ALL) subtype among young children. Methods: This ecologic study was based on incidence rates of leukemia in ch ildren aged 0-14 years. The Ontario Cancer Registry was used to identify th e residence of 1394 leukemia cases between 1978 and 1992. Ecologic units we re composed of census subdivisions in a 5-year period. Percent population c hange, determined from the Census of Canada, was employed as a measure of p opulation mixing. The relationship between population mixing and childhood leukemia was examined separately after stratifying by the level of geograph ic isolation, defined according to urban-rural status. Analyses were also c onducted separately in specific age groups and for the ALL subtype. Results: Population growth in rural areas was associated with an increased incidence of leukemia, particularly for the ALL subtype in children aged 0- 4 years (rate ratio = 1.8, 95% confidence interval 1.1-2.8, for a greater t han 20% population change relative to no increase in population). In contra st, an elevated risk due to population mixing was not observed in urban are as. Conclusions: Results from this study are consistent with results from simil ar studies conducted in the United Kingdom, which are suggestive of a role for an infectious agent in the etiology of childhood leukemia, as proposed in the Kinlen hypothesis.