‘My cancer was not in those pictures’: how breast density affects mammograms

In 2017, 55-year-old Leslie Ferris Yerger underwent a routine mammogram and breast ultrasound. Both came back clear. Two months later, she had an unrelated Dexa scan to measure her bone density. The technician noticed an abnormality in her hip. After additional testing, Yerger was diagnosed with stage four breast cancer – an advanced stage of the disease in which the cancer has spread beyond the breasts.

The mammogram and ultrasound had not picked up the cancer because, Yerger would learn, of her dense breast tissue.

“My cancer was simply not in those pictures,” she says. “I also learned it’s not an uncommon story.”

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According to the National Cancer Institute, roughly 50% of women aged 40 or older are found to have dense breasts, a term that refers to the ratio of fatty tissue to more dense fibrous and glandular tissues in a person’s breast. During a mammogram, dense tissue can make it difficult for technicians to see potentially cancerous masses.

While some US states did require mammography reports to include information about a patient’s breast density and how it could potentially affect their results, there has been no federal rule until this year.

In September, the FDA began requiring mammogram reports to include information about breast density and how it may affect patients’ results. “Dense tissue makes it harder to find breast cancer on a mammogram and also raises the risk of developing breast cancer,” the disclosure reads. The report says that “in some people with dense tissue, other imaging tests in addition to a mammogram may help find cancers”.

The ruling represents an inflection point in the field of breast cancer detection and treatment. But there is no agreement yet on what these results mean for screening in the US or abroad. In the UK, for example, the NHS does not include information about breast density on patients’ mammogram reports.

Going forward, experts say, there needs to be more research and testing, as well as self-advocacy from patients. Here’s what you need to know.

What are dense breasts?

Breasts are composed of three types of tissue: fatty breast tissue, fibrous connective tissue and glandular tissue. These last two, referred to collectively as “fibroglandular tissue” are more dense than fatty tissue. The ratio between fibroglandular tissue and fatty tissue determines whether or not breasts are considered dense; a person with dense breasts has more fibroglandular tissue relative to fat. This determination is not based on a specific ratio, but the radiologist’s subjective assessment, explains Dr Habib Rahbar, professor of radiology at the University of Washington.

There are four categories of breast density. The first two – entirely fatty breast tissue and scattered fibroglandular breast tissue – are considered low density. The other two – heterogeneous breast tissue (which roughly 40% of women have) and extremely dense breast tissue (which about 10% of women have) – are considered dense.

Despite its prevalence, “there’s a lot that’s not understood about breast density and why some people have more dense breasts than others,” says Dr Angela DeMichele, professor of medicine and co-leader of the breast cancer program at the University of Pennsylvania’s Abramson cancer center.

People from some racial groups, such as Asian Americans and Black Americans, are more likely to have dense breast tissue, says Dr Arif Kamal, chief patient officer of the American Cancer Society. It can also run in families.

Additionally, an individual’s breast density can vary from year to year depending on age, hormone levels, menstrual cycle and medication. While roughly 50% of women in their 40s have dense breasts, that number drops to about 25% in women over 60, says Dr Maryam Lustberg, chief of breast medical oncology at the Yale Cancer Center.

How do you find out if you have dense breasts?

To find out if you have dense breasts, you need to have a mammogram. A radiologist will then examine the results and determine your level of breast density. As a result of the new FDA ruling, these findings will be included on your mammogram report, along with guidance about whether or not to seek additional testing.

Breast density is not something one can self-diagnose, says Kamal. “It’s saying, ‘When I look at the screen [of a mammogram] your breasts are dense.’ It doesn’t mean that you can feel the difference.”

How does breast density affect cancer screening?

A higher density of this fibroglandular tissue can make reading mammograms more difficult. Mammograms are X-rays, and X-rays pass more easily through soft tissue, like fat, making these areas appear dark on the resulting image. Higher density tissue appears more white and obscures the field of the mammogram, making it harder for a radiologist to identify suspicious masses, which also appear white. Lustberg likens it to trying to see through a “snowstorm”.

For people with dense breasts, doctors often strongly recommend other imaging in addition to mammograms. This may include a whole breast ultrasound, which uses sound waves rather than X-rays to create an image of the breast. “It’s a different modality, so if there is a mass hiding behind something, [the sound wave] is going to bounce a little bit differently,” explains Lustberg. However, experts disagree about how useful breast ultrasounds are; one described them as having “limited value”.

Some doctors may also recommend patients with dense breasts undergo an MRI, which is an effective way of spotting cancer even in extremely dense breasts. But MRIs can result in false positives. Lustberg says this risk may be justified for those with an elevated risk of breast cancer. MRIs can also help those concerned about whether a mammogram is accurate. But for those with an average risk, MRIs can result in “unnecessary biopsies” and “increased anxiety”.

Kamal says balancing these factors about additional testing comes down to “the risk tolerance of the patient”.

“It’s very confusing,” says DeMichele. “We have not had a definitive study that showed us exactly the right test to do in [people] with dense breasts that could [detect] cancers without also leading to more false positives and more biopsies.”

Further complicating the issue is uneven access, says Rahbar. Such testing is not always covered by insurance, and some patients live in so-called “medical deserts” where advanced or even basic imaging technologies may not be available.

Are mammograms still useful?

Does all of this mean that mammograms are useless for people with dense breasts? Not at all, say experts.

Besides detecting dense masses, mammograms also detect calcifications in the breast. “This is one of the best ways to find a very small cancer,” says DeMichele, particularly cancers that don’t usually show up as densities. While these calcifications are somewhat harder to see in mammograms of dense breasts, they are still “very visible”, she says.

Does breast density affect one’s cancer risk?

Breast density itself is a cancer risk factor, though experts aren’t entirely sure why.

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People with dense breasts are two to three times more likely to develop breast cancer in their lifetime, says Lustberg. There are a number of hypotheses as to why this might be, though none have been conclusively proved. “Dense breasts do seem to have a little bit different biology than fatty breast tissue,” says Lustberg.

Experts emphasize that a person’s breast density is merely one potential risk factor. Breast cancer risk assessment tools, some of which are available online, take into factors such as a person’s age, how old they were when they first got their period, how old they were when they first had a child and their family history of cancer.

“I think it’s every woman’s right to know her own personalized risk of breast cancer,” says Lustberg.

How can patients advocate for themselves when it comes to breast density?

Without any clear, standardized guidelines for screening breast density, much of the responsibility falls on patients to get the information they need.

“We should not wait for the scientific community to figure out what it wants to do in a unified voice. There’s no time for that,” says Kamal. Instead, he says, patients need to insist on conversations with their clinicians to understand their breast cancer risks, and advocate for the care and testing that makes them feel comfortable.

Yerger founded the advocacy group My Density Matters in 2021. Groups like this have been raising awareness of breast density for years. They also provide information about breast density as well as questions to ask healthcare providers, additional screening options and advice about how to address potential pushback from doctors and insurance agencies.

“What we ask [patients] to do, it’s not easy,” says Yerger. “Right now, it’s what we have to do.”