For the first time in recorded history, the United States has experienced a dramatic and sustained decline in breast cancer incidence. After decades of rising incidence, this marked reversal has attracted widespread scientific and media attention [1
]. A number of hypotheses have been proposed to account for this phenomenon, including a reduction in the number of women taking postmenopausal hormones, lower utilization of screening mammography, and a natural decline following the screening-related increase in incidence (due to depletion of the pool of prevalent cancers detectable by screening) [1
]. The relative contributions of each have been widely debated and are yet to be resolved.
Prior to 1980, breast cancer incidence increased modestly, 0.5–2% per year, and responded to changes in reproductive patterns and public awareness [6
]. As screening mammography was introduced in a widespread manner in the 1980s, the incidence of breast cancer rose rapidly (). Consistent with an impact of screening, the increase in incidence was largely limited to in situ
and localized tumors, rather than regional or distant disease [8
]. Increases in breast cancer incidence continued until reaching a plateau in the late 1990s. National data for breast cancer incidence have revealed a decreasing pattern since 1999 (). Incidence of invasive breast cancer in 2003–2007, the last 5 years for which national data are available, are the lowest since 1985 [10
Female breast cancer incidence, 1975–2007
Epidemiologic evidence has long suggested that postmenopausal hormone therapy is associated with an increase in breast cancer risk. A meta-analysis of 51 studies in 1997 indicated that the risk of a breast cancer diagnosis increased by 2.3% for each additional year of postmenopausal hormone use [11
]. In 2002, the Women’s Health Initiative (WHI) published the results of their large, placebo-controlled, randomized trial of estrogen plus progestin, finding that the risks of use outweighed the benefits [12
]. After approximately five years of follow-up, women assigned to the estrogen plus progestin arm were 26% (95% CI: 0–59%) more likely to develop breast cancer than women in the placebo arm.
Following the publication of this report in July 2002, there were rapid and dramatic reductions in postmenopausal hormone use in the United States [2
]. Nationally, it is estimated that prescriptions for postmenopausal hormone therapy fell by 38% in the first year following publication of the WHI results [16
]. Reports from a number of populations have demonstrated that the decline in breast cancer incidence since 2002 coincided with this drop in postmenopausal hormone use [2
]. Ravdin et al. observed that breast cancer incidence in the United States fell by 6.7% in the one year following the results of the WHI [1
Given the temporal association between trends in postmenopausal hormone use and breast cancer incidence, particularly between 2002 and 2003, it has been widely speculated that cessation of hormone use accounts for the observed decline in incidence [1
]. Though the descriptive data for trends in breast cancer incidence according to demographic and tumor characteristics offer both support and challenges to this theory, very little quantitative analysis has been conducted. In this paper we estimate the influence of cessation of hormone use in late 2002 on the United States breast cancer incidence rate in 2003 and compare this to what was actually observed.