Dense breasts have less fatty tissue and more non-fatty tissue. Breasts that aren't dense have more fatty tissue and less non-fatty tissue. Mammograms can help you and your doctor determine how dense your breasts are.
A large study suggests that women with dense breasts are more likely to be diagnosed with breast cancer and the breast cancer is likely to be more aggressive. The results were published online in July 2011 in the Journal of the National Cancer Institute.
Other research has shown that dense breasts:
- can be twice as likely to develop cancer as nondense breasts
- can make it harder for mammograms to detect cancer; breast cancers (which aren't fatty) are easier to see on a mammogram when surrounded by fatty tissue
This study used information from the very large and ongoing Nurses' Health Study (NHS), which includes nearly 122,000 female nurses who were between the ages of 25 and 42 when they started in the study. Researchers have been tracking a number of their health factors for more than 20 years.
Researchers identified 1,042 women who were diagnosed with breast cancer after going through menopause and a similar group (the matched group) of 1,794 postmenopausal women who weren't diagnosed with breast cancer.
Using information from mammograms, the researchers compared the breast density of the women who were diagnosed with breast cancer to the breast density of the women who weren't diagnosed. The researchers also looked for any links between breast density and more aggressive cancer characteristics in the women who were diagnosed.
Each woman's breast density was classified in one of four categories:
- less than 10% density (least dense)
- 10% to 24% density
- 25% to 49% density
- 50% or higher density (most dense)
The denser a woman's breasts, the greater her risk of being diagnosed with breast cancer. Compared to women with the least dense breasts (less than 10%), a breast cancer diagnosis was:
- more than 3 times more likely in women with the most dense breasts (50% or more)
- more than 2 times more likely in women with breast density in the 25% to 49% range
In women diagnosed with breast cancer, denser breasts were linked to a greater likelihood of more aggressive breast cancer characteristics:
- larger cancer at time of diagnosis
- high-grade cancer; the grade of the cancer (1, 2, or 3) tells you how different the cancer cells' appearance and growth patterns are from those of normal, healthy breast cells (grade 1 cancers tend to be less aggressive, while grade 3 cancers tend to be more aggressive)
- hormone-receptor-negative; hormone-receptor-negative cancers tend to be more aggressive than hormone-receptor-positive cancers
Researchers aren't sure why denser breasts are more likely to develop cancer and have the cancer be more aggressive. In the future, tools that determine a woman's breast cancer risk might be more accurate if they included information about breast density.
If you're getting screening mammograms, you may want to ask your doctor about what your mammograms suggest about your breast density and how breast density might affect your breast cancer risk. Whether your breasts are dense or not, taking steps to minimize your personal risk of breast cancer is very worthwhile. In the Breastcancer.org Lower Your Risk section, you can learn much more about diet and lifestyle choices you can make to help keep your risk of breast cancer as low as it can be.