In the U.S., breast density is of normal distribution among women. Normal distribution means that 10% are fatty and 10% are in that extremely dense category, while the rest, 80%, are scattered, or heterogeneously dense. If you take an average, women are somewhere in between the scattered and heterogeneously dense categories.
How is breast density determined?
Radiologists determine breast density by evaluating a mammogram, an X-ray image used to screen for early signs of breast cancer.
Average-risk women should begin screening for breast cancer starting at age 40, as supported by recommendations of experts in the field, including the American Cancer Society, American College of Radiology/Society of Breast Imaging, among others, and as per new recommendations from the U.S. Preventive Services Task Force.
Annual breast cancer screening has the greatest reduction in breast cancer-related death and saves the most lives.
Why is it important for people to know their breast density?
Dense breast tissue on a mammogram can mask breast cancer on a mammogram. Heterogeneously dense or extremely dense tissue appears white on a mammogram image and can mask or blend in with tumors, which also show as white. Fatty tissues appear black.
Independent of this, dense breast tissue also increases the risk of breast cancer. Multiple studies demonstrate at least a moderate association between mammographic breast density and breast cancer risk, with women who have higher mammographic breast density (considered the heterogeneously dense and extremely dense groups) carrying a greater lifetime risk of developing breast cancer compared to women who have mammographically fatty or scattered density.
Screening with supplemental imaging in addition to mammograms is therefore encouraged if someone has dense breast tissue. Additional methods to screen for breast cancer include ultrasounds and MRIs.
What factors influence breast density?
Breast density is specific to each person and is influenced by various factors, including genetics, age, having given birth, body mass index, menopausal status, and hormone replacement therapy, which increases the length or amount of exposure to female hormones, such as estrogen and progesterone.
For example, dense breasts can be associated with being younger than 50, and with a low body mass index. Breast density tends to decrease as women get older.
What did the study in JAMA Oncology find?
In the study, 10,481 women who were without symptoms and cancer-free when the study began were observed from November 2008 to October 2020. During the 12-year period, they had routine mammograms every one to two years, where changes to their breast density were monitored over time.
There were 289 women who developed breast cancer. At the start of the study, breast density was higher in the participants who ended up being diagnosed with breast cancer, and over time, their density declined.
But when researchers analyzed mammogram images separately for each breast, they found a new element to breast density: it declined significantly slower in the breast where participants later developed breast cancer.
Why are these findings important?
These findings are important because participants were followed for over 10 years and suggested that breast density is not a static risk factor — it is dynamic and can change over time.
Decreasing breast density lowers breast cancer risk, but the rate of decrease appears to provide additional, more nuanced, and dynamic information on the potential risk of developing breast cancer.
The information we gather from a mammogram, such as your breast density category and now potentially the rate of breast density changes, as observed in mammograms over time, is and should be taken into consideration when assessing breast cancer risk over a woman’s lifetime. This can help inform the most appropriate and personalized screening strategies for each woman.