How Breast Density Affects a Mammogram

Elaine K. Howley, Contributor
March 21, 2017

Individual variations in breast tissue density can elevate your risk of developing breast cancer.

Breast density is dependent on several factors, including weight, genetics, age, family history and number of births. (GETTY IMAGES) Breast density is dependent on several factors, including weight, genetics, age, family history and number of births. (GETTY IMAGES)

It’s easy to think of breasts as simply mounds of fat on a woman’s body, but they’re much more complex structures than that. Within the breast are a network of glands and ducts that produce and move milk to the nipple for feeding a baby. There’s also lots of fibrous and fatty tissue in between these structures, and how much of this fatty tissue you have can play a part in whether you develop breast cancer.

If you have less fatty tissue, your breasts are considered dense, and density is both a risk factor of its own and can be a hindrance for diagnosing cancer in some women. According to the Susan G. Komen foundation, high breast density is a common occurrence in American women; between 40 and 50 percent of women ages 40 to 74 in the U.S. have dense breasts.

Regardless of the individual genetic and lifestyle factors that contribute to a woman having dense breasts, the Susan G. Komen foundation reports that these “women with high breast density are four to five times more likely to get breast cancer than women with low breast density.”

How Density is Measured reports that “no one method of measuring breast density has been agreed upon by doctors. Breast density is not based on how your breasts feel during your self-exam or your doctor’s physical exam.” Although there isn’t an exactly quantifiable measure for what makes a breast dense, there are gradations of density that can be determined by looking at a mammogram. The Breast Imaging Reporting and Database Systems, a reporting database supported by the American College of Radiology, classifies density into four categories: mostly fatty, scattered density, consistent density and extremely dense.

Dr. Tari King, chief of breast surgery at Dana-Farber/Brigham and Women’s Cancer Center and the associate division chief for breast surgery at Brigham and Women’s Hospital in Boston, says “it’s important to understand that there’s different degrees of breast density. Women with extremely dense breasts represent a relatively small group of about 10 to 15 percent of all of women having screening mammograms. And that is the group that’s really at increased risk of developing breast cancer as compared to someone with fatty breasts.”
What Causes Dense Breasts?
Breast density is dependent on several factors, including weight, genetics, age, family history and number of births. King says that breast density is highly correlated with body mass index. “Women who are obese or overweight are less likely to have extremely dense breasts than women who are lean or underweight. Part of the density measurement is the (ratio) of glandular tissue to fatty tissue, and overweight women with more fat will have less dense tissue because of fat tissue distribution.” But that doesn’t mean that women with higher BMIs can’t also have dense breasts. “Certainly, there are some overweight women with dense breasts, but there is a strong relationship” between weight and breast density.

King says “having dense breasts means you have a higher percentage of ducts, glands and connective tissue and a lower percentage of fat in the breast, and there’s two problems with having dense breasts. One is the so-called ‘masking effect,’ which makes it harder to see [tumors, lumps and precancerous lesions] on a mammogram.”

The Masking Effect
Dr. Sara Hurvitz, associate professor in the division of hematology-oncology at the David Geffen School of Medicine at University of California–Los Angeles, says that because “the breast is comprised mostly of fat,” an X-ray mammogram will show the bulk of the breast as a soft gray tone on the image. At least in theory, any abnormalities should show up as white spots on the mammogram. But this doesn’t always work, and dense breast tissue can obscure or hide worrisome spots because the mammogram simply can’t see them through the denser tissue. “It’s like trying to see the car ahead of you in a thick fog,” Hurvitz says.

“White on a mammogram means density,” says Dr. Otis Brawley, chief medical officer at the American Cancer Society. That density can either mean simply dense tissue or a mass that requires further investigation.

But breast density, as with many aspects of cancer, is not a singular thing; rather, it’s a spectrum, and breast density can change over time. As women age, their breasts tend to become fattier and less dense, which Hurvitz says is part of the reasoning behind holding off on mammograms until after age 40 or 50. The guidelines don’t recommend using mammograms when a woman is in her 20s or 30s because younger women tend to have very dense breasts because the cycle of hormones at work in the breast keep the tissue from becoming fattier until after menopause. Younger breast tissue is usually too dense to reveal a good picture during a mammogram.

Brawley further explains that in mammograms conducted on young women, the breast tissue often shows up as all white, obscuring the white highlight of an abnormality. “As the woman ages, however, the picture a mammogram can present comes into better focus. Aging is like turning the contrast knob on a black-and-white TV. From age 20 to about 45, I’m talking about 50 shades of gray, excuse the pun. When I do a mammogram on a 40-year-old woman, I’m still looking for white on a very light gray image. Some of those 40-year-old women will have darker gray,” but it can still be challenging to find a spot in the image. But as women age, the breast density falls, making the tissue appear darker on a mammogram, thus making it easier to spot a white abnormality. “That’s why mammography is a better test for older women,” Brawley says.

Density as Risk Factor
The second problem that women with dense breasts face is that breast density “is an independent risk factor for breast cancer,” King says. “Even if you have the best tool in the world that can see through the density, just having increased density in and of itself is a risk factor.” Although researchers don’t know exactly why or how dense breast tissue increases a woman’s cancer risk, it’s an active area of investigation.

The Susan G. Komen foundation reports that some states now require health care providers to notify patients after a mammogram shows that they have dense breasts. The foundation also endorses this action, although this is a somewhat controversial area; a 2015 article in The Atlantic reported that some doctors and advocates are concerned that states sending a letter to women informing them of their breast density can cause unnecessary fear and harm, particularly if that state then requires additional screening.

Finding out whether you have dense breasts requires a mammogram. But if you’ve had one and your doctor has said you have dense or extremely dense breasts, it doesn’t automatically mean you’re going to develop cancer. And, there are a few things you can do to lower your risk of developing breast cancer.

First, maintaining a healthy lifestyle – limiting alcohol consumption, eating right, exercising, controlling your weight, never smoking or quitting if you do smoke, and getting enough sleep have all been correlated with a reduced risk of developing breast cancer. These findings hold for women with dense breasts.

Second, King says that women with dense breasts should avoid post-menopausal hormone replacement therapies. “Stopping HRT therapy typically results in reduced risk,” she says.

Although breast density is in part genetic or inherited, and not something that women have much control over, King notes that breast density may be modifiable to a certain extent in some women with the use of certain drugs, such as tamoxifen. “It’s been well documented that tamoxifen, which is used for the treatment of breast cancer and prevention in women who are at high risk, does reduce breast density in many women who take it,” King says. reports that a 2008 study of more than 7,000 women at high risk for breast cancer who’d never been diagnosed with the disease found that, after taking tamoxifen for 12 to 18 months, 45 percent of these women experienced a 10 percent or greater drop in breast density. These women were 63 percent less likely to develop breast cancer compared to women taking a placebo. Therefore, it has been posited that using drugs like tamoxifen prophylactically may result in a reduction in risk for developing breast cancer in some women.

Lastly, women with dense breasts are usually encouraged to undergo more frequent screening. recommends working with your doctor to develop a personalized screening plan if you have dense breasts. This screening program may include mammograms, digital mammograms, breast MRIs, ultrasounds and physical examinations.