Recent breast cancer incidence trends according to hormone therapy use: the California Teachers Study cohort
1 Department of Epidemiology, School of Medicine, University of California, Irvine, 224 Irvine Hall, Irvine, CA 92697, USA
2 Northern California Cancer Center, 2201 Walnut Avenue, Suite 300, Fremont, CA 94538, USA
3 Department of Health Research and Policy and Stanford Cancer Center, Stanford University School of Medicine, 875 Blake Wilbur Drive, Stanford, CA 94305, USA
4 Department of Preventive Medicine, University of Southern California Keck School of Medicine, Los Angeles, CA 90089, USA
5 Department of Cancer Etiology, Division of Population Sciences, City of Hope Comprehensive Cancer Center, 1500 East Duarte Road, Duarte, CA 91010, USA
6 Department of Nutrition, University of Oslo, PO Box 1046, Blindern, 0316 Oslo, Norway
7 Department of Obstetrics and Gynecology, University of Southern California Keck School of Medicine, Women's and Children's Hospital, 1240 N. Mission Road, Los Angeles, CA 90033, USA
Breast Cancer Research 2010, 12:R4 doi:10.1186/bcr2467
See related editorial by Banks and Canfell, http://breast-cancer-research.com/content/12/1/103Published: 8 January 2010
Recent, international declines in breast cancer incidence are unprecedented, and the causes remain controversial. Few data sources can address breast cancer incidence trends according to pertinent characteristics like hormone therapy use history.
We used the prospective California Teachers Study to evaluate changes in self-reported use of menopausal hormone therapy (HT) between 1995 to 1996 and 2005 to 2006 and age-adjusted breast cancer incidence among 74,647 participants aged 50 years or older. Breast cancer occurrence was determined by linkage with the California Cancer Registry.
During 517,286 woman years of follow up, 565 in situ and 2,668 invasive breast cancers were diagnosed. In situ breast cancer incidence rates in this population did not change significantly from 2000 to 2002 to 2003 to 2005, whereas rates of invasive breast cancer declined significantly by 26.0% from 528.0 (95% confidence intervals (CI) = 491.1, 564.9) per 100,000 women in 2000 to 2002 to 390.6 (95% CI = 355.6, 425.7) in 2003 to 2005. The decline in invasive breast cancer incidence rates was restricted to estrogen receptor-positive tumors. In 1996 to 1999 and 2000 to 2002 invasive breast cancer incidence was higher for women who reported current HT use especially estrogen-progestin (EP) use at baseline than for never or past users; but by 2003 to 2005 rates were comparable between these groups. For women who were taking EP in 2001 to 2002,75% of whom had stopped use by 2005 to 2006, incidence had declined 30.6% by 2003 to 2005 (P = 0.001); whereas incidence did not change significantly for those who never took HT (P = 0.33).
Few data resources can examine prospectively individual HT use and breast cancer diagnosis. Stable in situ breast cancer rates imply consistent levels of screening and suggest recent declines in invasive breast cancer to be explained predominantly by changes in HT use.