Abstract LB-170: Breast cancer incidence and hormone therapy use before and after the Women’s Health Initiative (WHI) estrogen (E) plus progestin (P) trial by race/ethnicity
Authors: Chlebowski Rowan T. , Aragaki Aaron K. , Anderson Garnet L. , Prentice Ross L. 
Published: August 2019

Abstract
We examined breast cancer incidence in US women, 50 years or older, from the Surveillance, Epidemiology and End Results (SEER) program through 2015 in relation to estrogen (E) plus progestin (P) and E alone use before and after 2002, when adverse E plus P breast cancer effects in the WHI trial were initially reported. Age-adjusted annual breast cancer incidence rates for all US women averaged over the five-year intervals of 1983-1987, 1988-1992, 1993-1997, 1998-2002, 2003-2007, 2008-2012, 2013-2015 (3 year interval) were 330, 358, 368, 387, 345, 352 and 357 breast cancers per 100,000 women. After early quinquennial-to-quinquennial interval increases in breast cancer incidence, the surprising and sustained reduction in breast cancer incidence rates seen beginning in 2003 (Ravdin et al NEJM 2007) has been largely attributed to the substantial (70%) reduction in E plus P use over the same period (Jewett et al Obstet Gynecol 2014). However, differences in breast cancer incidence trends by race/ethnicity have emerged. Over the same quinquennial intervals, among White women, there were 343, 374, 383, 407, 359, 362 and 367 breast cancers per 100,000 women, following the same pattern of increases followed by sustained decrease beginning in 2003. In contrast, Black women experienced no substantial reduction in breast cancer incidence after 2003 with subsequent sustained increases through 2015 with quinquennial intervals rates of 277, 302, 327, 329, 328, 350, 356 breast cancers per 100,000 women with most recent incidence rates now comparable to White women. Based on reported findings of the WHI hormone therapy clinical trials and the sustained reduction seen in the US for both E plus P and E alone use (the latter for women with prior hysterectomy), we propose a hypothesis that the discordant trends in breast cancer incidence between White and Black women can largely be explained by the sustained changes in hormone therapy use beginning in 2003. In the WHI randomized trials, E plus P and E alone had opposite effects on breast cancer risk, E plus P initiation increasing breast cancer risk, further increasing with duration, with increased risk sustained for over a decade following cessation, and E alone decreasing risk at initiation and cessation (Chlebowski et al JAMA Oncology 2015). In 2003, E plus P use was higher in White compared to Black women. Thus, the reduction in E plus P use, cessation and decreased imitation, in White women was associated with prolonged reduction in breast cancer incidence. In contrast, with hysterectomy more common in Black women, decisions not to initiate E alone use could have increased their breast cancer risk. Thus, consideration of differential use of these two hormone regimens in Black and White women provides a plausible explanation for differential changes in breast cancer incidence seen in these two populations over recent quinquennial periods.
Citation Format: Rowan T. Chlebowski, Aaron K. Aragaki, Garnet L. Anderson, Ross L. Prentice. Breast cancer incidence and hormone therapy use before and after the Women’s Health Initiative (WHI) estrogen (E) plus progestin (P) trial by race/ethnicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-170.