A major risk factor for breast cancer in middle-aged women - dense breast tissue - appears to run in families, research suggests.
A unique study of 400 mother-daughter pairs has found that mothers with dense breast tissue are more likely to have daughters with dense breast tissue.
The finding underscores the significance of understanding breast density, which is now established as an inheritable characteristic.
"The study provides further evidence of the importance of risk assessment and prevention of breast cancer early in life," said Norman Boyd, a scientist at the Campbell Family Institute for Breast Cancer Research at Toronto's Princess Margaret Hospital.
Breasts are made up of fat and fibroglandular tissue. Density, as seen on a mammogram, reflects the amount of fibroglandular tissue in the breast. Fat appears dark on a mammogram while denser tissues appear light. There is no way to self-examine for breast density, and it isn't related to the size of a woman's breasts.
Dr. Boyd was the scientist whose groundbreaking work in 2007 confirmed that middle-aged women with dense breasts are five times more likely to develop cancer.
His new study, published yesterday in The Lancet Oncology, is the first to measure breast density in young women (the daughters in the study were aged 15 to 30). He found there are many variables that affect it, including the height, weight and the density of the breast tissue of the person's mother.
Dr. Boyd didn't use mammograms to measure breast density in the young women because they are more sensitive to the negative health consequences of radiation, which has been linked to breast cancer later in life. Instead he used magnetic resonance imaging (MRI) and measured breast water concentration, which also shows the amount of fibroglandular tissue in the breast.
The study found that the amount of breast density in the daughters was significantly higher than in mothers, and decreased as their age and weight increased. As well, the taller the women, the more dense their breasts.
Higher levels of growth hormones in blood were also linked to higher breast density.
Further research needs to be done to understand how the breasts of young women change as they age. "Knowing the genes associated with variations in breast tissue would be major step forward as well," Dr. Boyd said.
Currently, breast cancer screening begins at the age of 50 in most provinces. Trying to detect cancer in teenagers would be futile, Dr. Boyd said, because the incidence of the disease at that age is almost nil.
"However, it may be good to assess risk for cancer earlier in life than we do now - even if more research needs to be done to understand what the determinants of risks are and how they could be modified," he added.
Susan McCracken, 54, and her 17-year-old daughter, Madeleine Bognar, were one of the mother-daughter pairs in Toronto recruited for the study. Ms. McCracken said she volunteered because she believes in the importance of this kind of research.
"Breast density is a key predictor. I was thinking about my daughter's health and her future," she said, noting that her mother-in-law died of breast cancer.
Ms. McCracken, a research associate in the Campbell Family Institute for Breast Cancer Research, suggested that young women with high breast density should be screened more frequently and watched more closely by their doctors.
