New Canadian research is causing doctors to rethink how they consider breast density when estimating a woman's risk of breast cancer.

Family doctors may not appreciate how much density increases a woman's risk of breast cancer, said Dr. Norman Boyd.
(CBC)

Cancer occurred five times more often in women with extremely dense breasts than in those with the fattiest tissue in a study of Ontario and B.C. women, the research team reports in Thursday's New England Journal of Medicine.

Dense breasts make tumours harder to detect because on mammograms, fat looks dark, while dense tissue, including tumours, looks light. The new findings are from a study that compared more than 1,100 women who developed breast cancer after their first X-ray with an equal number of women who did not get the disease.

The results put new emphasis on the difficulties associated with preventing breast cancer in women with dense breasts because density also seems to contribute to the cause of the cancer.

"Our hypothesis is that that greater number of cells poses a larger target to chemicals that might cause genetic damage that could give rise to cancer in the future," said the study's lead author, Dr. Norman Boyd of the Ontario Cancer Institute in Toronto.

Only age and hereditary mutations of the genes BRCA 1 and 2 created a higher risk than density, the researchers found. Despite the increased risk from density, the majority of women with the risk factor don't get the disease, Boyd said.

Cancer survivor and aquafit instructor Marlene Cairns said staying in shape is important to help prevent recurrence.
(CBC)

The average 50-year-old woman has a 2.5 per cent risk of developing breast cancer over 10 years. If she has extensive breast density, the risk doubles to five per cent.

The density depends on the proportion of connective duct-lining and milk-gland tissue to fat. A woman can't tell how dense her breasts are, so mammograms are used to evaluate density.

Breast density is primarily inherited, but hormone replacement therapy can increase density, while the breast cancer drug tamoxifen decreases it.

"Now we can also incorporate a woman's breast density to help inform the decision as to whether or not one should take tamoxifen," said Dr. John Mackey of the Alberta Cancer Board in Edmonton.

The study confirms that dense breast tissue can mask tumours on mammograms.

"I think the masking thing is important, and it does happen, but the most important thing is that this is an incredible risk factor," said Dr. Karla Kerlikowske, of the Veterans Affairs Medical Center in San Francisco, who wrote an accompanying editorial.

"This probably counts for a large percentage of the cancer that's occurring."

Study raises screening questions

The findings raise other questions that need further research, such as whether women with dense breasts should be screened more often or with something other than mammography, such as an ultrasound or MRI.

Breast cancers are the second-most deadly kind in women, after lung cancers.

Marlene Cairns of Charlottetown found a lump in her breast three years after a clean mammogram. Cairns, an aquafit instructor, knows the importance of staying in shape and eating well to prevent more tumours from forming, as well as how the dense tissue in her breast puts her at increased risk.

"I have to be very diligent and be very careful, in that if I feel anything at all, I have to get to my doctor," said Cairns, who is also taking tamoxifen to reduce the chances of a recurrence.

With files from the Associated Press