MONDAY, Oct. 18, 2004 (HealthDayNews) -- A new study adds more weight to the cancer risks posed by hormone therapy by finding that the menopausal treatment also makes breast cancer harder to detect.

Over the last few years, several studies have strongly suggested that hormone replacement therapy (HRT) raises a woman's risk of developing breast cancer. The major blow took place in July 2002, when U.S. researchers halted the massive Women's Health Initiative (WHI) study, which discovered that combined HRT (estrogen plus progestin) increased the risk of breast cancer and cardiovascular events.

The latest research, also from the WHI, finds that combined HRT can double breast density, a known risk factor for breast cancer, and lead to a fourfold increase in abnormal mammograms.

These findings are from data collected on 413 postmenopausal women, aged 50 to 79. These women participated in the WHI estrogen-plus-progestin trial, which included more than 16,500 women.

In this study, the women were randomly assigned to receive estrogen plus progestin or a dummy drug. The women were all given a mammogram at the start of the study and again two years later.

The researchers measured breast density using a computer-aided method that calculates the amount of dense, or white-appearing, tissue on mammography images.

"Postmenopausal women who take combination estrogen-plus-progestin hormone replacement therapy for one year experience a twofold increase in breast density and a quadrupled risk of having an abnormal mammogram," said lead researcher Dr. Anne McTiernan, the director of the Cancer Prevention Research Program at the Fred Hutchinson Cancer Research Center in Seattle.

This is one more reason for women to reconsider taking hormone therapy and to avoid hormone therapy if they are really concerned about mammograms being the best they can be for diagnosing breast cancer, McTiernan added at a news conference Monday.

McTiernan noted the density of breast tissue can increase the risk of breast cancer and also make it more difficult to diagnose. Since cancer also appears white on a mammogram, increased breast density can make it harder to see tumors and other abnormalities.

"Increased breast density decreases the ability of radiologists to detect breast cancers," she said. "Increased density is also associated with increased risk for breast cancer."

McTiernan was to present her findings Oct. 18 at the American Association for Cancer Research meeting in Seattle.

Earlier research has found that women under 50 with dense breasts are four times more likely to develop breast cancer compared with similar women who have little or no breast density.

While breast density is largely a function of age and genetics, McTiernan's study shows that other factors can play a role. "For several reasons, women should limit the amount of time they are on HRT," she said. "If they are concerned about their risk of breast cancer, they might want to avoid HRT."

Her group is now looking at the effect of estrogen alone on risk of mammogram density. "We and others are also looking at the effects of other medications or lifestyle factors on mammogram density to see if we can find ways to lower density," she said.

"This is important for women of all ages, but especially for premenopausal women who tend to have high density breasts, which translates into lower effectiveness of mammogram screening for them," she stressed.

"The word is already out that HRT raises breast cancer risk somewhat, and that HRT overall is not useful for disease prevention," said Dr. David L. Katz, an associate clinical professor of public health and director of the Prevention Research Center at the Yale University School of Medicine.

Since hormone replacement therapy for postmenopausal women tends to be used for the treatment of symptoms only, there is nothing in this study that calls for a change in clinical practice, he said.

"If more postmenopausal women are free of HRT, the interpretability of mammograms in this population may improve," Katz said. "If radiologists cannot interpret mammograms reliably, breast cancer may be over-diagnosed, leading to unnecessary biopsies, or, more importantly, missed tumors. Better interpretation of mammograms could in turn mean more detection of early stage breast cancers, and fewer false positive readings."

"We have known for some time that HRT increases breast density in some women," said Robert Smith, director of cancer screening at the American Cancer Society.

Because increased density makes mammograms more difficult to read, radiologists may see this increased density as something they need to worry about, Smith said, because it "does increase the risk of a false-positive."

In another report from the same meeting, Erin Aiello from the Group Health Cooperative's Center for Health Studies in Seattle looked at whether all HRT increased breast density. Among 46,436 women, they found that all types of HRT increased breast density. Women taking estrogen only had a smaller increase in breast density compared with women taking combined HRT, Aiello said.

Aiello noted that breast density usually decreases with age. However, in their study her team found that HRT negated this natural process. "When women stop HRT, breast density decreases," she said.

More information

The American Cancer Society can tell you more about breast cancer.

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