Breast density and cancer risk

Answered by Alison Conlin, M.D., medical oncologist and medical director of Providence Breast Care Clinic.

What does breast density have to do with breast cancer risk?

While the reasons aren't yet understood, having dense breasts increases a woman's risk of developing breast cancer.

Before the alarm bells go off, it helps to know that about half of the female population has dense breasts. This is not a rare condition – it just means that the breasts are made up of less fatty tissue and more nonfatty tissue. In addition, breast density generally declines as women age, so if you have dense breasts to begin with, they may not necessarily stay that way. In one study of more than 7,000 women, nearly three-quarters of those in their 40s had dense breasts, compared to 57 percent in their 50s, 44 percent in their 60s, and 36 percent in their 70s.

On its own, breast density is not considered a high risk factor for developing breast cancer, but it is one factor that a woman should consider along with her other risks and protective factors to get a clearer picture of her overall risk.

The other issue with dense breasts is that they make it more difficult to spot cancer on mammograms. That's because dense breast tissue and cancerous tissue both show up as white areas on mammograms (fatty tissue fades into the darker areas). If a part of the breast is very dense, it could make it harder to see a developing cancer in the same area. Women with very dense breasts – especially if they have other risk factors, too – may want to have a discussion with their doctors about the pros and cons of supplemental screening with ultrasound or MRI in addition to mammography.

How can you tell how dense your breasts are?

Breast density isn't something that you can feel. It's only visible on a mammogram. When radiologists read mammograms, they look at the ratio of white to gray/black in the densest part of the breasts and classify them according to four standardized categories:

  • Almost entirely fatty
  • Mostly fatty with scattered areas of density
  • Mostly ("heterogeneously") dense
  • Extremely dense

About 10 percent of women have extremely dense breasts, and 40 percent have mostly dense breasts. The rest have breasts that are not considered dense. In Oregon and several other states, providers are required by law to notify women if their breasts are found to be dense.

How high is the risk?

The American Cancer Society classifies dense breasts as a "moderate" risk factor. According to, "The risk for the 40 percent of women with mostly dense breasts is only about 1.2 times greater [than the risk for women with average breast density], and the risk for the 10 percent of women with extremely dense breasts is only about two times greater."

But I caution women against focusing on one risk factor in isolation. Other risks – as well as protective factors – play a role in estimating a woman's overall risk, and all should be looked at together to paint a balanced picture.

What should you do if you have dense breasts?

Women with dense breasts should do the same things that all women should do – even women who consider themselves to be in a low-risk category, because we know that breast cancer still sometimes happens to the women we least expect to get it. No woman is completely free of risk, but all women can minimize their risks by taking these protective measures:

  • Think about avoiding hormone replacement after menopause – estrogen replacement is a risk factor for breast cancer.
  • Limit the amount of alcohol you drink – having more than one drink per day is associated with higher risk.
  • Lose weight if you need to – being overweight increases the risk of developing breast cancer, as well as the risk of a poor outcome if cancer is found.
  • Eat more vegetables and fruits – many studies show that a plant-based diet rich in vegetables reduces the risk of breast cancer and other cancers.
  • Move more – physically active people have lower rates of cancer.

Curious or concerned about your risks?

If your breast density is causing you a lot of concern – even if it's your only known risk factor – consider a visit to Providence's Breast Care Clinic. We see a lot of women who are concerned about dense breasts. When we go over all of their risks with them and point out protective factors that are working in their favor – such as having babies, having them young, breastfeeding and keeping a healthy body weight – many often learn that their risk isn't nearly as high as they thought it was. If it turns out that a woman's risks are on the higher side, we can help her decide whether she should consider supplemental screening, risk-reducing medicines or genetic testing.

Breast health, like a woman's personal history, hopes and fears, is a completely individualized thing. If dense breasts are a concern for you, talk with your doctor about the options that feel best for protecting your health and peace of mind.