The decreased incidence of invasive breast cancer in the United States seen in 2002 and 2003 did not extend to women of African ancestry, researchers from the University of Chicago Medical Center report at the 2008 annual meeting of the American Association for Cancer Research in San Diego.
Breast cancer incidence rates in U.S. women over 50 declined sharply during 2002 and 2003. But this decline, the researchers report, was confined to Caucasians.
For Caucasian women aged 50 to 69, the rate of invasive breast cancer, which was stable from 2000 to mid-2002, decreased by more than two percent every three months during the second half of 2002 and all of 2003, then stabilized in 2004. For African Americans there was virtually no change.
"The benefit was not equal across racial groups," said Dezheng Huo, PhD, research associate (assistant professor) of health studies at the University of Chicago Medical Center. "Only Caucasians saw this reduced incidence of invasive cancers."
In mid-2002, many women who had been taking replacement hormones stopped taking them in response to the Women's Health Initiative trial, which showed that the supplements increased coronary heart disease and breast cancer risk.
Saturation of screening may also account for part of the decline. Screening mammogram use increased substantially during the 1990s, which temporarily raised the rates of new cancer diagnosis. But mammography use stabilized and then slightly decreased after 2000.
Much of the racial disparity, the researchers suspect, may have resulted from differences in biology. "We suspect," Huo said, "that the widespread discontinuation of menopausal hormone use had a greater effect on Caucasians."
African Americans are less likely to use hormone replacement therapy and less likely to develop breast cancers that are receptive to estrogen, Huo said, so they were harmed less by taking hormones and benefited less by discontinuing them.
African Americans had "a similar magnitude of reduction in hormone therapy use," he added, "yet it did not lead to any benefit, suggesting that genetic variations in estrogen and progesterone metabolism may play a role in how women of African ancestry respond to hormonal therapies. Alternatively, the causes of breast cancer may vary in different populations."
Almost 80 percent of breast cancers in Caucasians are estrogen receptor positive, a type of tumor that depends on estrogen to grow, said Huo, compared to about 60 percent in African Americans. That proportion drops to about 30 percent among Nigerian women.
For this study, the researchers looked at rates of breast cancers that were invasive as well as breast cancer in situ--early lesions that have not invaded surrounding tissues--in women aged 50 to 69. They used data from 17 large cancer registries, covering about 26 percent of the U.S. populations.
They found that the incidence rate of invasive breast cancers for Caucasians declined soon after April 2002, when many women stopped taking replacement hormones after studies showed that HRT increased cancer risk. Invasive cancer rates for Caucasian women fell from about 86 cases per quarter per 100,000 women in mid-2002, to about 74 per 100,000 by the end of 2003.
African American women had fewer invasive cancers, about 71 per 100,000 per quarter, but their numbers did not decline in 2002-2003. Rates for Asian women, lower still, declined slightly, but stayed in the mid-60s per 100,000. Native American invasive breast cancer rates remained quite low, about 33 per 100,000, but did not decline.
Rates for early, non-invasive breast cancers, did not follow the same patterns. They remained nearly steady for all ethnic groups, except Asian women, for whom the rates increased from 2000 to mid-2002 then slowly declined, probably because of slowly growing use of mammography in this population.
"The finding that the rate of invasive breast cancer, but not in situ cancer, decreased "suggests that estrogen serves as a promoter rather than an initiator of breast cancer," say the study authors.
The research was supported by Breast Cancer SPORE grant from the National Cancer Institute. Olufunmilayo Olopade, MD, of the University of Chicago Medical Center, also participated in the study.