What makes one woman more likely than another to develop breast cancer ? According to the Gail model, created in 1989 to estimate breast cancer risk, early age at menarche and having a mother or sister with breast cancer are among the factors that increase risk. Since the Gail model was defined, other possible risk factors—including breast density—have emerged.

Breast density reflects the makeup of the breast. Dense breasts have a greater proportion of glandular and connective tissue, while less dense breasts contain more fat tissue. On a mammogram , fat tissue appears clear while dense tissue appears white. Tumors also appear white, making it more difficult for doctors to spot tumors in dense breasts. While this characteristic may increase the risk of an inaccurate mammogram result, it does not explain why dense breasts are more prone to develop cancer.

Two research groups updated the Gail model to include breast density. Their reports, in the September 6, 2006 Journal of the National Cancer Institute , reveal that breast density is a strong predictor of breast cancer risk and may be even more predictive than family history.

About the Study

Researchers from the Seattle-based Group Health Cooperative examined mammograms from 1,007,600 women ages 35-84. At the time of the mammogram, each woman completed a questionnaire on potential risk factors for breast cancer, and a radiologist classified the density of each woman’s breast using a standardized system of 1-4 (1=least dense; 4=extremely dense). The researchers counted the number of women who developed breast cancer in the year after their mammograms. By comparing the women who did and did not develop cancer, the researchers created a model to estimate breast cancer risk.

In the year after their mammograms, 11,638 women were diagnosed with breast cancer. When the women were classified into two groups based on their menopausal status, different risk factors proved to be significantly associated with cancer (Table 1). Increased age and dense breast tissue, however, were the two most highly predictive factors in either group. After adjustments for the influence of age, women with the densest breasts were 3-4 times more likely to develop breast cancer than those with the least dense breasts.

Table 1. Significant risk factors for breast cancer, based on menopausal status

Postmenopausal women Premenopausal women
Age, breast density, family history of breast cancer (mother or sister), prior breast procedure, body mass index (BMI), race & ethnicity (white and black women at greater risk than Hispanic, Asian/Pacific Islander, or Native American women), age ≥30 at birth of first child or having no children, natural menopause, current use of hormone therapy, false-positive result on a previous mammogramAge, breast density, family history of breast cancer (mother or sister), prior breast procedure

In the second study, researchers from the National Cancer Institute (NCI) updated the Gail model to include breast density by evaluating data from several national surveys on risk factors, incidence rates, and death rates due to breast cancer. Their results also showed that breast density is a strong predictor for breast cancer risk. Both the Seattle and the NCI studies reported that breast density was more predictive of risk than family history of breast cancer.

These studies report a significant association between breast density and breast cancer risk. Besides surgery, there is currently no way to reduce breast density; and even if there were, it is unknown whether such an intervention would translate into a lower cancer risk.

An important limitation of the NCI model is that it is based on white American women and may not apply to women of other races or nationalities.

How Does This Affect You?

Should your doctor check the density of your breasts? Maybe, but only as one part of a comprehensive evaluation of your breast health. While both studies found that breast density can predict cancer risk, the models are designed to study large groups of women and cannot be relied upon to accurately predict the risk of cancer in an individual.

Keep in mind, too, that dense breasts are fairly common, especially in younger women. In addition, a previous study found that digital mammography, as opposed to the film-screen method, is better suited for women with dense breasts.

To reduce your risk of breast cancer, the American Cancer Society recommends the following:

  • Exercise regularly
  • Maintain a healthy body weight
  • Avoid alcohol
  • Avoid hormone therapy after menopause , if possible, or use the smallest possible dose for the shortest time
  • Follow screening guidelines:
    • In your 20s and 30s, have a clinical breast exam by your doctor every three years
    • In your 40s, if you are at normal risk, have a mammogram and clinical breast exam every year
    • At any age, if you are at high risk, ask your doctor about more aggressive screening methods