There's a tangle of information about the pros and cons of using hormones to relieve the symptoms of menopause, but a new analysis of data generated by the Women's Health Initiative confirms that one cause of concern can be laid to rest: There is no evidence that taking estrogen alone increases the risk of breast cancer in postmenopausal women.
The findings come three years after a related WHI study found that a different type of hormone therapy - which combined estrogen and progestin - did result in a higher risk of breast cancer among postmenopausal women.
Marcia Stefanick, PhD, a Stanford University School of Medicine researcher who oversaw the latest WHI findings, said she hopes the results provide some clarity.
"The clinical trial results for estrogen-alone and for estrogen-progestin are very different - WHI is not flip-flopping," said Stefanick, professor of medicine at the Stanford Prevention Research Center and lead author of the study to be published in the Journal of the American Medical Association. "The use of estrogen-progestin clearly showed an increased risk for breast cancer; estrogen-only therapy shows no evidence of an increased risk."
The WHI is a 15-year, broad-based look at the causes and prevention of diseases affecting older women. In addition to hormone therapy, other WHI studies have examined heart disease, breast and colon cancer, and osteoporosis.
For the estrogen-alone arm of the study, which was stopped in 2004, researchers tracked nearly 11,000 women nationwide for about seven years. The women were between the ages of 50 and 79, and had previously undergone a hysterectomy. Half of the women were given a form of estrogen known as conjugated equine estrogens, while the other half were given a placebo.
In the latest analysis, researchers found no evidence that estrogen increased the risk of breast cancer. The women taking estrogen had fewer breast tumors (28 per 10,000 cases per year) than those in the placebo group (34 per 10,000 cases), but the difference of six fewer cases per 10,000 women per year was not deemed statistically significant, meaning that it could have occurred by chance.
Still, Stefanick added that other aspects of the WHI findings suggest women taking estrogen should have additional mammograms and breast biopsies to confirm that no breast cancer is present.
These results are markedly different from those arising from the estrogen-progestin arm of the study, in which a different group of postmenopausal women taking the combined hormone therapy were found to have a 26 percent increase in their risk for breast cancer.
Stefanick noted that it's important for women and physicians to distinguish between the two studies: the women in the combined estrogen-progestin trial still had their uteruses. Because estrogen-alone therapy increases the risk for endometrial cancer, women who haven't had a hysterectomy and who need relief from menopausal symptoms are also given progestin, which offsets the risk of that cancer.
The estrogen-progestin arm of the WHI study was called to an early halt in 2002 - three years ahead of schedule - when evidence from the trial showed that in addition to a higher risk of breast cancer, the women taking the combined hormone therapy also had a greater risk of stroke, blood clots and, in the first year of treatment, heart attack.