The study, published online today in the Journal of the National Cancer Institute, is the latest entry in a controversy about declining U.S. breast cancer rates.
Some experts say the drop in breast cancer rates is partly due to the fact that fewer women have been getting routine mammograms to screen for breast cancer. Without those mammograms, breast cancers may just be going undiagnosed, or so the theory goes.
Other researchers credit the fact that many menopausal women turned away from hormone replacement therapy after July 2002. That's when the Women's Health Initiative, a long-term women's health study, linked HRT to increased risk of breast cancer.
The new study favors the HRT explanation, not the mammography theory.
The researchers included Karla Kerlikowske, MD, of the University of California at San Francisco and the San Francisco Veterans Affairs Medical Center.
Kerlikowske and colleagues studied data on more than 600,000 mammograms performed on U.S. women ages 50-69 between 1997 and 2003.
The researchers picked those years because they span a few years before and after the Women's Health Initiative findings on breast cancer and HRT came out.
The study shows a nearly simultaneous drop in the women's HRT use and estrogen-sensitive breast cancer, which is the kind of breast cancer that's been linked to HRT use.
For instance, between 2002 and 2003, HRT use fell by 34%. That lines up with a 13% drop in the women's rate of estrogen-sensitive breast cancer.
The researchers point out that since they only studied women who got routine screening mammograms, the downturn in breast cancer rates simply can't be due to missed mammograms.
Why didn't breast cancer rates fall further, given the drop in HRT use? Perhaps breast cancer risk falls gradually after ceasing HRT, suggest Kerlikowske and colleagues.
Will this study settle the debate? That remains to be seen, since the topic is "controversial," as the researchers write.
The drug company Wyeth, which makes the postmenopausal hormone therapy drug Prempro, emailed WebMD a statement about the study.
"Wyeth cautions against drawing the conclusion made by the study authors because the decline in breast cancer incidence cannot be determined from this type of study and there may be other factors that could have an impact on breast cancer incidence rates. In addition, Wyeth notes that the product labeling for Wyeth’s and other postmenopausal hormone therapies advises about an increased risk of breast cancer and suggests women taking therapy do monthly breast self exams and have routine mammography screenings," states Wyeth.