Shifting breast cancer trends in the United States.

Purpose: United States breast cancer incidence rates declined during the years 1999 to 2003, and then reached a plateau. These recent trends are impressive and may indicate an end to decades of increasing incidence.
Methods: To put emerging incidence trends into a broader context, we examined age incidence patterns (frequency and rates) during five decades. We used age density plots, two-component mixture models, and age-period-cohort (APC) models to analyze changes in the United States breast cancer population over time.
Results: The National Cancer Institute's Connecticut Historical Database and Surveillance, Epidemiology, and End Results program collected 600,000+ in situ and invasive female breast cancers during the years 1950 to 2003. Before widespread screening mammography in the early 1980s, breast cancer age-at-onset distributions were bimodal, with dominant peak frequency (or mode) near age 50 years and smaller mode near age 70 years. With widespread screening mammography, bimodal age distributions shifted to predominant older ages at diagnosis. From 2000 to 2003, the bimodal age distribution returned to dominant younger ages at onset, similar to patterns before mammography screening. APC models confirmed statistically significant calendar-period (screening) effects before and after 1983 to 1987.
Conclusion: Breast cancer in the general United States population has a bimodal age at onset distribution, with modal ages near 50 and 70 years. Amid a background of previously increasing and recently decreasing incidence rates, breast cancer populations shifted from younger to older ages at diagnosis, and then back again. These dynamic fluctuations between early-onset and late-onset breast cancer types probably reflect a complex interaction between age-related biologic, risk factor, and screening phenomena.