For years, experts have known that dense breast tissue is linked to an increased risk of breast cancer — but a study published Thursday in JAMA Oncology found that a slower rate of decline in one breast's density is often followed by a cancer diagnosis.

Study details and key findings

For the study, researchers at  Washington University conducted a nested case-control study of 947 patients who attended breast cancer screenings for up to 10 years. The study was sampled from the Joanna Knight Breast Health Cohort, which tracked 10,481 women from 2008 to 2020 who were cancer-free when they started. 

Every one to two years, participants underwent routine screening mammograms to measure breast density and screen for cancer. In total, 289 participants were diagnosed with breast cancer during the study period.

The researchers compared changes in the breast tissue of the participants diagnosed with cancer to the changes in the 658 participants who did not develop breast cancer.

Overall, the researchers observed a decrease in breast density in all participants, regardless of breast cancer development.

From the start, breast density was higher in the participants who developed breast cancer. When they measured each breast's density separately, the researchers found a significantly slower decline in density in breasts that developed cancer compared with the other breast in the same patient.

Commentary

According to Shu Jiang, the study's lead author and an associate professor of public health sciences at Washington University, the findings could provide an individualized, dynamic tool to help evaluate breast cancer risk. "I hope they can get this into clinical use as soon as possible — it will make a huge difference," she said.

"Right now, everybody only looks at density at one point in time," Jiang added. However, breast density is measured during routine mammograms. "So this information is actually already available, but it's not being utilized," she said. Now, a person's risk of developing breast cancer could "be updated every time she gets a new mammogram."

While experts have acknowledged breast density as a risk factor for breast cancer for years, this is the first study to track changes in density over time and identify a link to breast cancer.

Although larger studies are needed to confirm the findings, Karen Knudsen, CEO of the  American Cancer Society, said the study's data are "exciting."

"This is the first study I've seen that looks specifically across time at changes from breast to breast, instead of averaging the two breasts, where you might miss these changes," Knudsen said.

The study noted that while women are informed about breast density and the risks associated with it, that information could be more useful. "We need to know how to follow women with dense breasts, instead of just alerting them," Knudsen said.

According to Knudsen, a potential next step could be to examine breast density over time in individuals taking medication to prevent breast cancer to determine whether the density declines.

"There could be different risk stratification guidelines set up to monitor those who are having much slower decline in tissue density, versus those who are not," Jiang noted. (Rabin, New York Times, 4/28; Jiang et al., JAMA Oncology, 4/27)