Women with dense breasts are at higher risk of developing breast cancer than those with fatty ones, according to a new Canadian study.
The risk of breast cancer was about five times greater in women with extensive dense tissue in the breast compared to those with little or no dense tissue, according to the results of a paper that will be published Thursday in The New England Journal of Medicine.
"Increased risk of breast cancer, whether detected by screening or other means, persisted for at least 8 years after study entry and was greater in younger than in older women," say scientists at The Campbell Family Institute for Breast Cancer Research at Princess Margaret Hospital.
For women younger than the median age of 56 years, 26 per cent of all breast cancers and 50 per cent of cancers detected less than 12 months after a negative screening test were attributable to density in 50 per cent or more of the mammogram.
The researchers compared breast densities with the risk of breast cancer in more than 1,000 women with the disease and 1,000 who did not.
The study's findings show that in addition to increasing risk of breast cancer, dense tissue also makes the disease more difficult to see in a mammogram, which increases the risk of cancer development between tests.
"Risk may be underestimated if it is based solely on cancers found at screening, because cancers masked by dense tissue will be omitted," the study says.
The radiographic appearance of the breast varies among women because of differences in the composition of breast tissue.
Breast tissue consists of fat; supporting tissue -- or stroma; and epithelium, the tissue that forms ducts and lobules.
While fat appears dark on a mammogram, denser tissues such as the stroma and epithelium look light.
Density decreases with age and on average, women lose about one per cent of their breast density each year.
The study is significant, researchers say because it reinforces the fact that there is an increased risk of breast cancer for women with dense breasts.
"This study establishes that breast density is an extremely important risk factor for developing breast cancer," says the study's key investigator Dr. Norman Boyd.
"Depending on a woman's age, between 16-30 per cent of breast cancers can be attributed to extensive density. Other risk factors, including family history and the known genes, account for a much smaller proportion of the disease," said Boyd, a principal investigator in The Campbell Family Institute for Breast Cancer Research and a University of Toronto professor.
This research is supported by grants from the National Cancer Institute of Canada and the Canadian Breast Cancer Research Alliance.
An estimated 22,300 Canadians will be diagnosed with breast cancer this year, says the Canadian Cancer Society. About 5,300 will die of the disease.
With a report from CTV's Avis Favaro and Elizabeth St. Philip
