When a woman gets her mammogram results, what she’s most interested in hearing from her doctor is, “Everything looks fine.” But even if the test doesn’t show anything suspicious, there’s another piece of information that women need to know.

In addition to showing breast changes—such as small white spots (known as calcifications) and/or lumps or tumors (called masses), mammograms indicate a woman’s breast density, a measure of fibrous and glandular tissue versus fatty tissue in the breasts. 

Why breast density matters: About half of women ages 40 to 74 have dense breasts, which can’t be felt when a woman does a self-exam or when a doctor palpates her breasts. This is vital information because dense breasts make it harder to find malignancies on a mammogram.

Even more important, women with dense breasts are four to six times more likely to develop breast cancer than women without dense breast tissue. In fact, high breast density surpasses other known risk factors for developing breast cancer, such as family history or later-in-life childbirth, according to research published in JAMA Oncology

Another important finding: In ­women with dense breasts, tumors tend to be larger when detected compared with women without dense breasts, according to a study of women ages 50 to 69 published in Radiology

The Detection Challenge

On mammograms, the fibrous and glandular tissue that indicates breast density appears white. To distinguish the degree of breast density, radiologists use four categories—extremely dense (75% or more of the mammogram ­appears white)…heterogeneously dense (50% to 75% appears white)…scattered areas of density (25% to 50%)…and mostly fatty (0% to 25%). If a woman’s mammogram falls within one of the first two categories, her breasts are considered dense.

Even though dense breasts can make mammograms more difficult to read, the test is crucial because many cancers may still be visible. Breast density, along with other risk factors, including personal and family history of breast cancer, should be taken into account when determining mammogram frequency. Women should talk to their doctors to come up with a personalized screening schedule—for both the frequency and types of tests performed.

For women with dense breasts, screening may include one or both of these tests… 

• 3-D mammography. A traditional mammogram is printed on film and is two-dimensional, with two X-ray images taken of each breast—top-to-bottom and side-to-side. With 3-D mammography, also called digital tomosynthesis, the machine takes multiple X-rays, or “slices,” and combines them into a 3-D image that the radiologist reads on a computer screen, where breast tissue can be viewed one layer at a time. The resulting images are finer and clearer, showing abnormalities that might otherwise be difficult—or impossible—to see. Both types of mammography require the use of plates that compress the breasts.

3-D mammography delivers only very slightly more ionizing radiation than traditional mammography, but it’s still considered safe by the FDA. There are about 40% fewer false positives with 3-D mammography compared with 2-D, preventing unnecessary anxiety in patients and repeated testing to confirm results.  

• Ultrasound. This technology uses high-frequency sound waves to identify breast cysts or masses. A layer of gel is applied to each breast, and a handheld device called a transducer is placed directly on the skin, transmitting ultrasound waves into the body. A mass will block the sound waves’ pathway, signaling a potential cancer. Ultrasound is noninvasive, creates no ionizing radiation and is typically used as a follow-up to any abnormal mammogram. 

Important finding: When 3-D mammography and/or ultrasound was given to more than 3,200 women with “dense” or “extremely dense” breasts whose conventional screening mammograms had found no cancer, the additional tests found 24 breast cancers, according to a study published in Journal of Clinical Oncology

One of these cancers was detected by 3-D mammogram alone…11 were detected by ultrasound alone…and 12 were detected by both 3-D mammogram and ultrasound.

Caveats: As part of the same study, there were 53 false-positive results for 3-D mammograms…and 65 false positives for ultrasound. However, researchers considered this disparity to be statistically insignificant. 

But do these additional tests actually save lives? That’s debatable. Even though the additional screenings have been shown to identify breast cancers that would have otherwise been missed, modern breast cancer treatment is so effective that even if the cancer is detected a bit later, it’s still often curable. Even so, the earlier a cancer is detected, the less likely a woman will need chemotherapy. So while an ultrasound may not save lives, it could minimize treatment…or make a treatment regimen less taxing.  

The screening decision: This can be a challenging question because professional medical groups have no established screening guidelines based on breast density. These groups do not consider the additional cancers that are detected by ultrasound to be numerous enough to warrant the additional monetary—and potential emotional—cost.

Insurance coverage also has been spotty for both 3-D mammography and ultrasound, so consult your insurer before getting these tests. For those paying out of pocket, a 3-D mammogram costs about $100 more than a 2-D mammogram (about $380 versus $290). An ultrasound can cost several hundred dollars. But if you’re one of the women whose cancer is detected with additional testing, it’s money well spent. 

Note: Women with a strong family history of a BRCA mutation, which increases breast cancer risk, are often advised to also get an MRI, but this scan is typically not recommended for women with dense breasts. For that reason, I usually do not recommend MRI for breast density alone.

Other Breast Cancer Factors

You can’t change certain risk factors, such as your family history of breast cancer, but other risk factors are under your control. Most important steps to take: Strive for a healthy weight, achieved through 150 minutes of moderate-intensity physical activity a week plus a diet rich in plant-based foods and with less than 10% of calories from saturated fat. 

There’s strong evidence supporting the benefits of these strategies. For instance, nearly a quarter of breast cancers could be avoided by obese and overweight women attaining a body mass index under 25 (for example, a body weight of 140 pounds for a 5’4″ woman). That’s because fat cells produce estrogen—and an excess of this hormone increases risk for certain types of breast cancer. 

Also: Don’t smoke…limit alcohol use (to one drink a day or, even better, less)…and be judicious about using hormone replacement therapy, avoiding it if possible or taking it for the shortest course possible if you need it to alleviate menopausal symptoms. Fortunately, most women’s breasts become non-dense after menopause.