Female breast cancer incidence among Asian and Western populations: more similar than expected

Abstract

Background: Previous reports suggested that female breast cancer is associated with earlier ages at onset among Asian than Western populations. However, most studies utilized cross-sectional analyses that may be confounded by calendar-period and/or birth cohort effects. We, therefore, considered a longitudinal (forward-looking) approach adjusted for calendar-period changes and conditioned upon birth cohort.

Methods: Invasive female breast cancer data (1988-2009) were obtained from cancer registries in China, Hong Kong, South Korea, Taiwan, Singapore, and the United States. Age-period-cohort models were used to extrapolate longitudinal age-specific incidence rates for the 1920, 1944, and 1970 birth cohorts.

Results: Cross-sectional age-specific incidence rates rose continuously until age 80 years among US white women, but plateaued or decreased after age 50 years among Asian women. In contrast, longitudinal age-specific rates were proportional (similar) among all Asian countries and the United States with incidence rates rising continuously until age 80 years. The extrapolated estimates for the most recent cohorts in some Asian countries actually showed later ages at onset than in the United States. Additionally, over successive birth cohorts, the incidence rate ratios (IRRs) for the longitudinal curves converged (narrowed) between Asian and US white women.

Conclusions: Similar longitudinal age-specific incidence rates along with converging IRRs indicate that the age effects for invasive breast cancer are more similar among Asian and Western populations than might be expected from a solely cross-sectional analysis. Indeed, the Asian breast cancer rates in recent generations are even surpassing the historically high rates in the United States, highlighting an urgent need for efficient prevention and treatment strategies among Asian populations.

Figure 1.

Cross-sectional age-specific breast cancer incidence rates. The age-specific incidence rates are plotted as log rates by log age over the entire study period.

Figure 2.

Trends in age-standardized incidence rates of breast cancer by age group from 1988 through 2009.

Figure 3.

Longitudinal age-specific breast cancer incidence rates and corresponding incidence rate ratios. A) 1920 birth cohort. B) 1944 birth cohort. C) 1970 birth cohort. The longitudinal rates are plotted as log rates by log age, adjusted for calendar-period effects and conditioned upon the reference birth cohort. Shaded areas represent the age groups for which each reference cohort was directly observed in the corresponding cancer registries; see text for further details. D) 1920 birth cohort. E) 1944 birth cohort. F) 1970 birth cohort. The IRR calculation was based upon the central age group in each shaded area with the US whites arbitrarily assigned an IRR of 1.0. *1922 birth cohort. IRR = incidence rate ratio.