Declines in invasive breast cancer and use of postmenopausal hormone therapy in a screening mammography population

Whether a recent large decline in use of postmenopausal hormone therapy after the release of the Women's Health Initiative findings in July 2002 and/or a decline in screening mammography use is related to a recently reported decline in breast cancer incidence in the United States is controversial. We prospectively collected data from four screening mammography registries from January 1997 through December 2003 for 603411 screening mammography examinations performed on women aged 50-69 years. Of these women, 3238 were diagnosed with breast cancer within 12 months of a screening examination. We calculated quarterly rates of self-reported current postmenopausal hormone therapy use and of invasive breast cancer, ductal carcinoma in situ (DCIS), and estrogen receptor (ER)-positive invasive breast cancer adjusted for age, registry, and time between screening examinations. All statistical tests were two-sided. Between 2000 and 2002 and between 2002 and 2003, annual rates of postmenopausal hormone therapy use declined by 7% and 34%, respectively (P_trend<.001 for both). Between 2000 and 2003, annual rates of invasive cancer declined by 5% (P_trend =.003). Between 2001 and 2003, annual rates of ER-positive invasive breast cancer declined by 13% (P_trend =.002). Rates of DCIS were stable during the study period. Our finding of a statistically significant decline in the rate of ER-positive invasive breast cancer in a screening mammography population after the start of a concomitant substantial decline in postmenopausal hormone therapy use suggests that a decline in screening mammography rates is unlikely to account for the recent decline in US breast cancer incidence.