How Changes in Breast Density Over Time May Influence Breast Cancer Risk

Having dense breasts is already a recognized risk factor for breast cancer, but a new study showed breast density—and risk—is nuanced.

Dense breasts are composed of a higher amount of fibrous or glandular tissue compared to fatty tissue, which creates more white areas on a mammogram image—the same color as tumors.

According to Debbie Bennett, MD, section chief of breast imaging at Washington University in St. Louis, and one of the new study’s co-authors, most women don’t start getting mammograms until age 40, the recommended age to start regular screening for breast cancer in most people. For this reason, experts don’t know much about breast density before then. 

What is known is that breasts naturally lose density after menopause, but the rate at which that happens is different for everyone. The new study also revealed that a person’s breasts may lose density at different rates—which could change the way risk is understood. 

“Typically when we talk about having dense breast tissue and that being a risk factor for developing risk cancer, we’re talking about both breasts together. But what we found is you can see differences in how density changes over time in the breasts individually,” Dr. Bennett explained. 

The new study included almost 1,000 women who had mammograms every one to two years for up to 10 years. About 15% were Black women, who are more likely than white women to be diagnosed with breast cancer at later stages. The average age of people in the study was 57. 

During the length of the study, there were 289 cases of breast cancer. The authors noted that although breast density decreased significantly in everyone, the rate at which density decreased was much slower in women who developed cancer.

The researchers concluded that the slower breasts are to lose density, the longer the person has an elevated risk for breast cancer. They also found something more novel: That density between breasts may be different, too. The study found that if one breast was losing density less quickly than the other, that breast was more likely to develop cancer.

The National Cancer Institute estimates that around 50% of women have dense breasts. Understanding how density changes not only between people, but individual breasts, could help radiologists and oncologists catch breast cancer in its early stages—when it’s easier to treat. 

“You can potentially flag that something is wrong before it shows up on a mammogram if one of the breasts is not losing density as quickly as the other,” Dr. Bennett noted.

Still, it’s unknown why this may occur, she added. There’s also no rule of thumb for how quickly breast density should wane, and many factors, including lactation, pregnancy, hormone therapies, and rapid weight changes can temporarily influence breast density. 

“Breast density is a risk factor and this is yet another piece that supports that. The longer you have dense breasts, the higher risk you have,” Jessica W.T. Leung, MD, professor and deputy chair of breast imaging at the University of Texas MD Anderson Cancer Center in Houston, Texas told Health. “But, of course, there are many considerations.”

According to Dr. Leung, who was not involved with the new study, all mammogram readings are subjective, since people are reading the scans and making a judgment call about whether or not the tissue is dense. 

Mammograms are still very effective screening tools and since the cost to operate mammography equipment is lower than other scans that could potentially be used, such as MRIs, mammograms remain the most important school in routine breast cancer screening.

Mammogram technology in the United States is almost exclusively digital, which provides a more clear image—a type of imaging called tomosynthesis, which combines many different images to create a nearly three-dimensional image.

However, having more nuanced information about breast density could help physicians determine which patients may benefit from supplemental screening, usually an ultrasound.

“It’s not that breast mammography is going to go away, or that it doesn’t work in dense breasts,” Dr. Leung clarified. “It’s been proven to save lives and it’s cost-effective. But now we want to determine which women would benefit from supplemental tests.”

The finding also challenges the way physicians read mammograms. 

“We look at all past mammograms to compare, but what our eyes are looking for is anything that shows up that is new,” Dr. Bennett concluded. “We should also look for things that are not disappearing at the rate we’d expect them to.”