Breast density is known to naturally decrease as a woman ages, and now a study suggests that the more time it takes for breast density to decline, the more likely it is that the woman could develop breast cancer.
Researchers have long known that women with dense breasts have a higher risk of breast cancer. But according to the study, published last week in the journal JAMA Oncology, the rate of breast density changes over time also appears to be associated with the risk of cancer being diagnosed in that breast.
“We know that invasive breast cancer is rarely diagnosed simultaneously in both breasts, thus it is not a surprise that we have observed a much slower decline in the breast that eventually developed breast cancer compared to the natural decline in density with age,” Shu Jiang, an associate professor of surgery at Washington University School of Medicine in St. Louis and first author of the new study, wrote in an email.
Breast density refers to the amount of fibrous and glandular tissue in a person’s breasts compared with the amount of fatty tissue in the breasts – and breast density can be seen on a mammogram.
“Because women have their mammograms taken annually or biennially, the change of breast density over time is naturally available,” Jiang said in the email. “We should make full use of this dynamic information to better inform risk stratification and guide more individualized screening and prevention approaches.”
The researchers, from Washington University School of Medicine in St. Louis and Brigham and Women’s Hospital in Boston, analyzed health data over the course of 10 years among 947 women in the St. Louis region who completed routine mammograms. A mammogram is an X-ray picture of the breast that doctors use to look for early signs of breast cancer.
The women in the study were recruited from November 2008 to April 2012, and they had gotten mammograms through October 2020. The average age of the participants was around 57.
Among the women, there were 289 cases of breast cancer diagnosed, and the researchers found that breast density was higher at the start of the study for the women who later developed breast cancer compared with those who remained cancer-free.
The researchers also found that there was a significant decrease in breast density among all the women over the course of 10 years, regardless of whether they later developed breast cancer, but the rate of density decreasing over time was significantly slower among breasts in which cancer was later diagnosed.
“This study found that evaluating longitudinal changes in breast density from digital mammograms may offer an additional tool for assessing risk of breast cancer and subsequent risk reduction strategies,” the researchers wrote.
Not only is breast density a known risk factor for breast cancer, dense breast tissue can make mammograms more difficult to read.
“There are two issues here. First, breast density can make it more difficult to fully ‘see through’ the breast on a mammogram, like looking through a frosted glass. Thus, it can be harder to detect a breast cancer,” Dr. Hal Burstein, clinical investigator in the Breast Oncology Center at Dana-Farber Cancer Institute, who was not involved in the new study, said in an email. “Secondly, breast density is often thought to reflect the estrogen exposure or estrogen levels in women, and the greater the estrogen exposure, the greater the risk of developing breast cancer.”
In March, the US Food and Drug Administration published updates to its mammography regulations, requiring mammography facilities to notify patients about the density of their breasts.
“Breast density can have a masking effect on mammography, where it can be more difficult to find a breast cancer within an area of dense breast tissue,” Jiang wrote in her email.
“Even when you take away the issue of finding it, breast density is an independent risk factor for developing breast cancer. Although there is lots of data that tell us dense breast tissue is a risk factor, the reason for this is not clear,” she said. “It may be that development of dense tissue and cancer are related to the same biological processes or hormonal influences.”
The findings of the new study demonstrate that breast density serves as a risk factor for breast cancer – but women should be aware of their other risk factors too, said Dr. Maxine Jochelson, chief of the breast imaging service at Memorial Sloan Kettering Cancer Center in New York, who was not involved in the study.
“It makes sense to some extent that the longer your breast stays dense, theoretically, the more likely it is to develop cancer. And so basically, it expands on the data that dense breasts are a risk,” Jochelson said, adding that women with dense breasts should ask for supplemental imaging when they get mammograms.
But other factors that can raise the risk of breast cancer include having a family history of cancer, drinking too much alcohol, having a high-risk lesion biopsied from the breast or having a certain genetic mutation.
For instance, women should know that “density may not affect their risk so much if they have the breast cancer BRCA 1 or 2 mutation because their risk is so high that it may not make it much higher,” Jochelson said.
Some ways to reduce the risk of breast cancer include keeping a healthy weight, being physically active, drinking alcohol in moderation or not at all and, for some people, taking medications such as tamoxifen and breastfeeding your children, if possible.
“Breast density is a modest risk factor. The ‘average’ woman in the US has a 1 in 8 lifetime chance of developing breast cancer. Women with dense breasts have a slightly greater risk, about 1 in 6, or 1 in 7. So the lifetime risk goes up from 12% to 15%. That still means that most women with dense breasts will not develop breast cancer,” Burstein said in his email.
“Sometimes radiologists will recommend additional breast imaging to women with dense breast tissue on mammograms,” he added.
The US Preventive Services Task Force – a group of independent medical experts whose recommendations help guide doctors’ decisions – recommends biennial screening for women starting at age 50. The task force says that a decision to start screening earlier, starting in their 40s, “should be an individual one.” Many medical groups, including the American Cancer Society and Mayo Clinic, emphasize that women have the option to start screening with a mammogram every year starting at age 40.
“It’s also very clear that breast density tends to be highest in younger women, premenopausal women, and for almost all women, it tends to go down with age. However, the risk of breast cancer goes up with age. So these two things are a little bit at odds with each other,” said Dr. Freya Schnabel, director of breast surgery at NYU Langone’s Perlmutter Cancer Center and professor of surgery at NYU Grossman School of Medicine in New York, who was not involved in the new study.
“So if you’re a 40-year-old woman and your breasts are dense, you could think about that as just being really kind of age-appropriate,” she said. “The take-home message that’s very, very practical and pragmatic right now is that if you have dense breasts, whatever your age is, even if you’re postmenopausal – maybe even specifically, if you are postmenopausal – and your breasts are not getting less dense the way the average woman’s does, that it really is a reason to seek out adjunctive imaging in addition to just mammography, to use additional diagnostic tools, like ultrasound or maybe even MRI, if there are other risk factors.”