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NEW YORK - For a woman contemplating taking hormone replacement therapy (HRT), the increase in breast cancer risk she faces may depend on her individual body type, race and ethnicity, according to new US research.
In a large new study of women on HRT, slim and normal-weight women with dense breast tissue saw a nearly 50 per cent increase in breast cancer risk, while women who were black or overweight with less-dense breast tissue showed no risk increase at all.
"With many medications, the effects aren't the same for everybody, either for prevention or treatment," Dr. Dezheng Huo of the University of Chicago, one of the new study's authors, told Reuters Health.
For decades, menopausal women routinely took estrogen alone or estrogen with progestin to cope with hot flashes and night sweats. Conventional medical wisdom held that HRT, because it kept estrogen levels high, would also protect the heart and maintain bone strength.
But the Women's Health Initiative (WHI), a large, long-term clinical trial, found that HRT did not reduce women's heart attack risk and actually raised the risk of stroke. Moreover, that study showed a 24 per cent increased risk of invasive breast cancer in women who took estrogen and progestin.
Since 2002, when the WHI findings were released, the number of women using HRT has fallen sharply, but it remains the most effective treatment for menopausal symptoms and many doctors still prescribe it for short-term use.
And several recent studies have suggested that breast cancer risks associated with hormone therapy may vary based on factors like body mass index (BMI) - a measure of weight in relation to height - and ethnicity.
To investigate further, Huo and his team analysed data from the National Cancer Institute's Breast Cancer Surveillance Consortium, which includes 1,642,824 screening mammograms and 9,300 breast cancer cases.
In their analysis, they looked at women 45 years old and older and considered the women's ethnicity and BMI, as well as their breast density. Greater breast density means a woman has more breast tissue and connective tissue than fatty tissue, while lower density means she has more fatty tissue relative to breast and connective tissue.
Women with extremely dense breasts, who represent about 15 per cent of the general female population, are known to have a greater risk of breast cancer.
The researchers found that HRT use was linked with an increased risk of cancer for white, Asian and Hispanic women, but not for black women. The highest risk was seen for women with normal to low BMI and extremely dense breasts, who were 49 per cent more likely to develop breast cancer on HRT than women with the same traits not taking HRT.
However, women who were overweight or obese and had less-dense breasts - who were about 20 per cent of the study population - had no increased risk, "suggesting a large subgroup of women may not be at elevated risk for breast cancer with HRT use," Huo and his colleagues write in the Journal of the National Cancer Institute.
Among the ethnic groups, the risk associated with HRT was highest for Asian women, who were at 58 per cent increased risk relative to counterparts not on HRT, although the relatively small number of Asian women in the study (just 3.2 per cent of the group) limits the reliability of that estimate, according to the researchers.
The study did not look at the amount of time that women were on HRT, the authors note, so it is possible that breast cancer risk could be further increased with long-term HRT use among women in higher-risk subgroups.
"The big contribution from this study is the fact that it also had data on breast density," said Dr. Mary Beth Terry, a professor of epidemiology at Columbia University in New York who co-wrote an editorial accompanying the study.
"A lot of data now, both observational as well as clinical trials, have supported that certain subgroups of women may not have an increased risk from hormone replacement therapy," she told Reuters Health.
One drawback of the study is that it only included one measurement of breast density, which can change over time, Terry added. "Breast density is something you can measure on a regular basis, every time you get a mammogram. It's more important how you change over time versus just a single measure."
The findings "would suggest that if you have low breast density in general, then that may be something you want to factor in if you're someone who's thinking about doing hormone replacement therapy," Terry said.