SEATTLE--In recent years, scientists have discovered that hormone therapy (HT), specifically estrogen plus progesterone therapy, increases breast density, a change that makes it harder to detect breast cancer on mammograms. But it hasn't been clear whether this increase happens for all women, or whether it differs based on various breast-cancer risk factors, the type of HT a woman uses, and the length of time she takes it.

A new study, conducted by Erin Aiello, MPH, and colleagues at Group Health Cooperative's Center for Health Studies, examined the link between HT and breast density among women in a broad range of categories. Her study found:

• Estrogen-only therapy is associated with a small increase in breast density; however, the increase is not as large as the increase in women taking HT with progesterone.
• Longer duration of HT use and older age at first use increase breast density, regardless of the type of HT a woman takes.
• Body Mass Index (BMI), race, age at first menstruation, and family history of breast cancer do not appear to affect the link between breast density and hormone therapy use.
• Use of HT appears to eliminate the known protective effects against breast density that certain characteristics have--namely, older age, having one or more children, and being older at the birth of a first child. This association was strongest among current users of estrogen plus progestin (vs. estrogen only).

"Recent research on hormone therapy has raised lots of questions about how these drugs affect breast density and the sensitivity of mammograms," said Aiello. "Having a better understanding of that link and how it relates to various risk factors may help women and their physicians to better evaluate whether HT is the right choice for them."

Aiello's study is the largest population-based study of its kind, using data from 46,436 women who had received more than 100,000 screening mammograms
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Contact: Warren Froelich
communications@aacr.org
206-219-4772
American Association for Cancer Research
18-Oct-2004

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