Dense breast tissue, a suspected risk factor for breast cancer, may run in families, according to researchers at the Ontario Cancer Institute in Toronto.

Women with extensive dense breast tissue have a risk of breast cancer that is 1.8 to 6.0 times that of women of the same age with little or no breast tissue density.

Researchers conducted two studies of twins to determine the genetic factors involved in breast density. A total of 353 pairs of identical twins and 246 pairs of fraternal twins were recruited form the Australian Twin Registry, and 218 pairs of identical twins and 134 pairs of fraternal twins were recruited in Canada and the United States. Each twin was given a mammogram.

Investigators found that genetic factors could account for 60 percent of the variation in breast density in the twins from Australia, 67 percent in the twins from North America and 63 percent in all twins participating in the study, according to the findings published in the New England Journal of Medicine.

"These results show that the population variation in the percentage of dense tissue on mammography at a given age has high heritability (genetic factors)," concluded the researchers. "Because mammographic density is associated with an increased risk of breast cancer, finding the genes responsible for this phenotype could be important for understanding the causes of the disease."

Other sources: New England Journal of Medicine