OTTAWA, Jan. 17 (Xinhua) -- 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 a new Canadian study available Wednesday.

    For women younger than the median age of 56 years, 26 percent of all breast cancers and 50 percent of cancers detected less than 12 months after a negative screening test were attributable to density in 50 percent or more of the mammogram, scientists at Campbell Family Institute for Breast Cancer Research at Princess Margaret Hospital have said.

    The researchers compared breast densities with the risk of breast cancer in more than 1,000 women with the disease and 1,000 who were without the disease.

    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.

    Breast tissue consists of fat; supporting tissue -- or stroma; and epithelium, the tissue that forms ducts and lobules. Density decreases with age and on average, women lose about one percent of their breast density each year.

    The study is significant, as "it establishes that breast density is an extremely important risk factor for developing breast cancer," says the study's key investigator Dr. Norman Boyd.