April 8 (HealthDay News) -- Women who took a common form of estrogen as hormone replacement therapy during menopause more than doubled their risk for certain types of benign breast disease compared with women who took a placebo, researchers say.

One such condition -- benign proliferative breast disease -- is relatively common and has been linked with an increased risk of breast cancer.

The clinical implication for women contemplating the use of supplemental estrogen "doesn't really change," said study author Dr. Tom Rohan, chairman of the department of epidemiology and population health at the Albert Einstein College of Medicine in New York City.

"It's the idea that women need to balance the benefits and risks," he said. This new finding does point to an added risk, however, Rohan added.

His team published the study online April 8 in the Journal of the National Cancer Institute.

Until now, there have been no randomized studies on the effect of conjugated equine estrogen (CEE), a type of hormone replacement therapy (HRT), on the risk of developing benign proliferative breast disease. Randomized trials are considered the gold standard of science.

The current data comes from the Women's Health Initiative CEE trial, in which almost 11,000 postmenopausal women were randomly assigned to receive CEE or a placebo.

Women taking CEE had more than double the risk of developing benign proliferative breast disease over an average follow-up of almost seven years, the team found.

It is thought that invasive breast cancer can result from a succession of conditions, with benign proliferative breast disease without atypia (cell abnormalities) sometimes being followed by proliferative disease with atypia before the development of in situ (localized) cancer.

This study did not see an actual increase in the risk of developing invasive breast cancer but the progression from benign proliferative breast disease to invasive breast cancer could take a decade, in which case this study was not long enough to note that change.

The progress of the women in the trial is still being followed, the researchers noted.

The Womens Health Initiative has also found that women taking estrogen plus another hormone, progestin, had an increased risk both of breast cancer and of benign proliferative breast disease after only about five years of follow-up. This may suggest that progestin plays a role in breast cancer development, the authors stated.

"This makes perfect sense," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "Estrogen does stimulate breast tissue activity and my concern is that, if you do move along a continuum as the hypothesis states, as you stimulate the breast are you going to be able to predict which women are going to develop into something worse and women who are going to stay in a benign state. I can't predict which of those women will develop [breast cancer] down the line but we know that this agent is highly stimulatory to breast tissue so the question is, do you want to be taking an agent that stimulates breast tissue?"

More information

The U.S. National Heart, Lung, and Blood Institute has more on the Women's Health Initiative.

SOURCES: Tom Rohan, M.D., Ph.D., chairman, department of epidemiology and population health, Albert Einstein College of Medicine, New York City; Jay Brooks, M.D., chairman, hematology/oncology, Ochsner Health System, Baton Rouge, La.; April 8, 2008, Journal of the National Cancer Institute online