On average, participants were 62 years old and 12 years post-menopause at the beginning of the study. The majority of the women were non-Hispanic white (42 percent) or African-American (35 percent), 16 percent were Hispanic and 6 percent were Asian/Pacific Islander.
Breast density was measured by a computer-aided technique that calculated the amount of dense, or white-appearing, tissue on digitized mammography images.
A woman's risk of breast cancer and her chances of getting an accurate cancer diagnosis depend largely on the density of her breast tissue.
Density is measured according to the ratio of connective and epithelial tissue, which appears on a mammogram as white; as compared to fatty tissue, which appears as black. Because cancer also appears as white on a mammogram, a high degree of breast density can make it difficult to spot tumors and other abnormalities.
In addition to being harder to image, dense breast tissue also appears to be more biologically active and susceptible to malignancy. Previous research at Fred Hutchinson has shown that women under 50 with predominantly dense breasts are four times more likely to develop breast cancer than those with little or no breast density. While density is determined largely by age and genetics, this study underscores the fact that other factors can play a part as well. In addition to hormone-replacement therapy, weight and physical activity also have been found to have an impact on breast density.
"By increasing breast density, the use of combination estrogen-plus-progestin hormone therapy may increase breast-cancer risk as well as decrease the sensitivity of screening mammography," McTiernan said. "Our results suggest that avoiding such hormone therapy may help improve the sensitivity of mammograms for detecting early breast cancers at a stage when they are most treatable.
Contact: Warren Froelich
American Association for Cancer Research