BIRTH COHORT AND CALENDAR PERIOD TRENDS IN BREAST-CANCER MORTALITY INTHE UNITED-STATES AND CANADA

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 facto rs and relevant medical interventions. Methods: Age-period-cohort mode ls were fit to the data, and changes in birth cohort trends (suggestin g a change in a breast cancer risk factor or protective factor) and ca lendar period trends (suggesting, in part, the impact of new or improv ed medical interventions) were examined. Results: Breast cancer mortal ity rates for white women were significantly higher in the Northeast t han 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 ri sk beginning around 1924. A marked moderation of risk by 4-year birth cohorts was observed for U.S. white women born after 1950, whereas sta ble or slightly decreasing trends were observed for U.S. black women a nd Canadian women. For women born from 1924 to around 1938, fertility rates increased for all three groups; after 1950, they declined unifor mly. Looking at temporal effects, we found that the slope of the morta lity calendar period trend increased in the 1980s compared with the 19 70s for all women. In the last calendar period, 1991-1992, a trend of decreasing mortality rates was found for white women in the United Sta tes and for Canadian women. Implications: Widespread environmental exp osures are unlikely to explain the higher relative breast cancer morta lity rates observed for U.S. white women in the Northeast, since the r ates for black women in this region were not higher than in other regi ons. The moderation of breast cancer mortality rates for women born be tween 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 o r protective factor. The increase in calendar period trend slope in th e 1980s likely reflects the coincident rise in breast cancer diagnosis via mammography. The recent decline in calendar period trend for whit e women in the United States and for Canadian women may be the result of earlier detection and increased use of adjuvant therapy.