The benefit of mammography can be confusing when considering the physical differences among women.
Roughly half of all women have dense breast tissue, and women with high mammographic density are more likely to develop breast cancer during their lives.
However, dense breast tissue does not seem to increase the risk of death among breast cancer patients, according to a study led by the National Cancer Institute (NCI), part of the National Institutes of Health, and conducted in collaboration with the NCI-sponsored Breast Cancer Surveillance Consortium (BCSC).
In the study of over 9,000 women with a confirmed diagnosis of breast cancer, researchers found that breast density was not associated with the risk of death from breast cancer or death from all causes combined.
Dense breast tissue can make it difficult for physicians to detect cancer in a mammogram, because that tissue appears as extensive white areas on an X-ray.
That’s because glandular and connective tissue, together known as fibroglandular tissue, block the passage of X-rays to a greater extent than fatty tissue, allowing some tumors to hide. In this situation breast ultrasound and breast MRI are used in conjunction with the mammogram to further evaluate dense breast tissue.
For most women, mammographic breast density decreases as they get older, reflecting gradual replacement of fibroglandular tissue by fatty tissue during the normal aging process.
Patients included in the NCI study were 30 years or older at breast cancer diagnosis, which occurred primarily between January 1996 and December 2005.
The patients were followed, on average, for 6.6 years, at which time 1,795 deaths were reported, including 889 from breast cancer and 810 from other causes.
To analyze breast density, scientists used the Breast Imaging-Reporting and Data System score, which is given by a radiologist based on visual examination of a mammogram. Scientists also analyzed data on tumor characteristics and other personal and health factors.
The analysis showed that breast cancer patients with high-density breasts did not have a higher risk of death from breast cancer than patients with lower density breasts, after adjusting for other health factors and tumor characteristics.
The lack of an association between breast density and breast cancer death is consistent with findings from an earlier, smaller study that examined their association.
In fact, this study found an increase in risk of breast cancer death associated with lower breast density among specific subgroups, particularly breast cancer patients who are obese.
One possible explanation for the finding is that breasts with a higher percentage of fat may provide a microenvironment that facilitates cancer growth and progression.
These findings, where researchers identify subsets of women whose low breast density is associated with their poor prognoses, underscore the need for an improved understanding of the biological components that are responsible for breast density.