Stanford researchers have published the results of a study that reinforces the link between postmenopausal hormone replacement therapy and increased incidence of breast cancer.
According to the study results, women who take combined estrogen plus progestin hormone therapy for at least five years after menopause, have twice the annual risk of developing breast cancer. The risk of breast cancer is not associated with women who only take estrogen.
The scientists looked at two groups of women, from two different studies. One group included 15,000 women from an original study, halted three years early by the NIH who sponsored the study. The women either were given a placebo, or combined hormones. By 2002, no incidences of breast cancer had emerged, but the researchers continued to follow the women, monitoring frequency of mammogram and new cases of breast cancer. The women were instructed to stop taking their pills.
Data from a second group included 41,449 women, starting in 1994, who made their own decisions about whether or not to use hormone replacement therapy. At the start of the study, 40 percent had been taking hormones, an average of 6.9 years. At the end of the study, in 2002, the women were received a letter, but no special instructions.
The researchers found similar results in both groups. The women who took hormones had a much higher incidence of breast cancer, leading up to 2002. The number of women diagnosed with breast cancer dropped by 28 percent when the hormones were stopped. Women who continued hormone therapy had more than double the annual risk of breast cancer.
Co-author of the study, Marcia Stefanick, PhD, professor of medicine at Stanford University School of Medicine says, "This is very strong evidence that estrogen plus progestin causes breast cancer. You start women on hormones and within five years, their risk for breast cancer is clearly elevated. You stop the hormones and within one year, their risk is essentially back to normal. It's reasonably convincing cause-and-effect data."
The researchers also say that mammogram screening is more effective without hormone therapy, because breast tissue returns to normal. Dr. Stefanick says, "Our data suggest that although you are picking up more tumors in those women, you're probably missing a lot as well because of the problems in using mammography in women taking hormones."
The study reinforces the fact that postmenopausal hormone therapy is linked to breast cancer. The researchers hope the current study will convince scientists who suggest that women who take hormones are just most likely to get mammograms, improving breast cancer detection.
New evidence of hormone therapy causing breast cancer, Stanford professor says