Breast cancer mortality among immigrants in Australia and Canada.

BACKGROUND: By moving between geographic regions with differing levels of breast cancer risk, migrant populations of women provide a unique opportunity to examine the impact of exposure to new environments and lifestyles on breast cancer risk. Breast cancer incidence and mortality rates for the majority of migrant groups originating from countries with low breast cancer risk have been found to increase toward the rates observed in destination countries with populations at higher risk for this disease. Because very little information exists on migrants from high- to low-risk countries, it is not known whether rates for these groups decrease or whether migrant groups generally experience increases in breast cancer rates.

PURPOSE: To address these questions, we determined the breast cancer mortality rates for women from both lower and higher risk countries who had immigrated to Australia and Canada and compared these rates with those exhibited by the population in the origin country and by the destination native-born population.

METHODS: Individual mortality records covering the years 1984 through 1988 and 1986 census data for Australia and Canada were obtained. Direct age-standardized mortality rates and rate ratios (and their 95% confidence intervals) were calculated for immigrant groups in Australia and Canada. Age-standardized rate ratios by length of residence in Australia were calculated. Weighted regression analyses of observed and expected mortality changes were performed.

RESULTS: In Australia, the mortality rates for 12 (75%) of 16 immigrant groups from lower risk countries and 10 (71.4%) of 14 groups from higher risk countries shifted toward the rate of native-born Australians. In Canada, the rates for 12 (60%) of 20 immigrant groups from lower risk countries and four (80%) of five groups from higher risk countries converged to the rate of native-born Canadians. Overall, the extent of convergence (shift of immigrant's mortality rate in origin country toward rate of native-born population) was 50% for immigrants in Australia and 38% for immigrants in Canada. Although there was not a consistent pattern of convergence with length of residence in Australia, after 30 or more years, the mortality rates of 15 (83.3%) of 18 immigrant groups had shifted toward the rate of the native-born Australians. Because of the small number of deaths in many of the immigrant groups studied, the observed differences in the breast cancer mortality age-standardized rates between the origin country and immigrant group, although often substantial, were seldom statistically significant.

CONCLUSIONS: Breast cancer mortality rates among women in the majority of immigrant groups shifted from the rate observed in their country of origin toward the rate of the native-born population in the destination country.

IMPLICATIONS: These findings indicate that environmental and lifestyle factors associated with the new place of residence influence the breast cancer rates of immigrants and also suggest that, since most migrants migrate as adults, the risk of breast cancer can be altered in later life.