Birth Cohort and Calendar Period Trends in Breast Cancer Mortality in the United States and Canada

Abstract

Background: Previous studies of regional and temporal variation in U.S. breast cancer mortality rates have been confined largely to analyses of rates for white women. Purpose: Breast cancer mortality rates from 1969 through 1992 for white women and black women in four regions of the United States and for all women throughout Canada were compared to identify racial, regional, and temporal differences. Differences and trends in the rates were evaluated in view of breast cancer risk factors and relevant medical interventions. Methods: Age-period-cohort models were fit to the data, and changes in birth cohort trends (suggesting a change in a breast cancer risk factor or protective factor) and calendar period trends (suggesting, in part, the impact of new or improved medical interventions) were examined. Results: Breast cancer mortality rates for white women were significantly higher in the Northeast than in any other region of the United States (two-sided t tests; P<.005); the rates for black women were not. Birth cohort trends for all women were similar until about 1940, with a moderation of mortality risk beginning around 1924. A marked moderation of risk by 4-year birth cohorts was observed for U.S. white women born after 1950, whereas stable or slightly decreasing trends were observed for U.S. black women and Canadian women. For women born from 1924 to around 1938, fertility rates increased for all three groups; after 1950, they declined uniformly. Looking at temporal effects, we found that the slope of the mortality calendar period trend increased in the 1980s compared with the 1970s for all women. In the last calendar period, 1991-1992, a trend of decreasing mortality rates was found for white women in the United States and for Canadian women. Implications: Widespread environmental exposures are unlikely to explain the higher relative breast cancer mortality rates observed for U.S. white women in the Northeast, since the rates for black women in this region were not higher than in other regions. The moderation of breast cancer mortality rates for women born between 1924 and 1938 coincides with increased fertility rates following World War II. Stable or decreasing mortality rates for U.S. women and Canadian women born after 1950 were not expected in view of declining fertility rates, suggesting a change in a breast cancer risk factor or protective factor. The increase in calendar period trend slope in the 1980s likely reflects the coincident rise in breast cancer diagnosis via mammography. The recent decline in calendar period trend for white women in the United States and for Canadian women may be the result of earlier detection and increased use of adjuvant therapy.