Huo and his colleagues gathered the study data from 17 cancer registries, covering about 26 percent of the U.S. population.
The researchers also looked at the effect of declining HRT use on estrogen-receptor-negative tumors, and found a slight reduction for whites and Hispanics, but a slight increase in black women. According to the researchers, about 80 percent of tumors in white women are estrogen-receptor-positive, compared to about 60 percent in black women. Women from Nigeria, for example, have estrogen-receptor-positive tumors only about 30 percent of the time, according to the researchers.
Ward said it's important to realize that the differences in tumor types might not be due to race, but due to environmental and social factors.
Huo's team also examined the effect of stopping HRT on very early breast cancers, and found virtually no change.
At the end of 2004, there was a slight increase in the rates of invasive breast cancer for Asian/Pacific Islanders, which Huo said might be attributable to an increased rate of mammography in this group of women.
"This is an area of a great deal of interest -- we've known that black women have a lower incidence of breast cancer, but higher mortality rates. No one really understands why," said Ward, who added that all women should get a mammogram every year. "Mammography can detect breast cancer early and can decrease the risk of dying early," she said.
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