Although mammographic density is associated with increased risk of developing breast cancer, a new National Cancer Institute–sponsored study suggests that dense breasts do not increase the risk of death in women who are already diagnosed with breast cancer (Gierach GL, et al. J Natl Cancer Inst. 2012;104:1218-1227). In a large population of women with breast cancer, high density on mammogram was not associated with increased risk of breast cancer–specific death or all-cause mortality.
Breasts with a greater proportion of fibroglandular tissue block the passage of x-rays to a greater extent than fatty tissue. These breasts are said to be mammographically dense. Typically, mammographic breast density decreases with age, which is thought to be due to a gradual replacement of fibroglandular tissue with fatty tissue as part of normal aging.
The study included more than 9000 women aged 30 years or older who received a confirmed diagnosis of breast cancer between January 1996 and December 2005. Patients were followed for a mean of 6.6 years; at follow-up, 1795 deaths were reported, 889 from breast cancer and 810 due to other causes.
For the analysis of breast density, the Breast Imaging, Reporting & Data System score was used by radiologists who reviewed individual mammograms. The investigators also analyzed data on tumor characteristics and personal and health factors.
Patients with high-density breasts were not at higher risk of breast cancer death, compared with those with low-density breasts in an analysis adjusted for health-related and tumor-related characteristics. This finding is consistent with that of a previous smaller study that analyzed the association between breast density and mortality risk. The risk of death due to breast cancer was increased among women with lower breast density who were obese. The authors suggested that a tumor microenvironment with a high percentage of fat may precipitate cancer growth and proliferation.
Lead author Gretchen L. Gierach, PhD, National Cancer Institute, stated in a press release, “Overall, it was reassuring to find that high mammographic breast density, one of the strongest risk factors for breast cancer, was not related to risk of death from breast cancer or death from any cause among breast cancer patients. Given that we identified subsets of women with breast cancer for whom low density was associated with poor prognoses, our findings underscore the need for an improved understanding of the biological components that are responsible for breast density.”