A University of Washington, Seattle, research team finds that this increased risk is small, but significant. The risk of ductal cancer -- the most common form of breast cancer -- increases by 50% in women who have used HRT for five recent years.
"If our results are correct, then nonusers of HRT would have an incidence of ductal cancer of about 230 per 100,000 women per year, whereas women with five years of recent HRT use would have a rate of 349 per 100,000 women per year," write Chi-Ling Chen, PhD, and colleagues.
The risk of lobular cancer -- a much less common form of breast cancer -- increases even more with long-term HRT. Women not using HRT have a lobular cancer risk of 23 cases per 100,000 women per year. Women with five years of recent HRT use have 70 cases per 100,000 women per year.
The scientists studied medical information collected from post-menopausal women age 50-74, all enrolled in the same health care plan. They compared 705 women with breast cancer with 692 women without breast cancer.
The risk of having breast cancer was about the same regardless of whether women received estrogen alone or estrogen plus another female hormone called progestin as their treatment. The study included women who used hormone pills and creams but did not include women who exclusively used hormone patches or injections.
The findings on lobular cancer may be particularly important, as this type of cancer is hard to detect by manual examination. However, Chen and co-workers note that there isn't enough information yet to make screening recommendations.