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.”
Evaluating breast density
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.”