Over the years, doctors and many of their female patients have become grimly familiar with the risk factors for breast cancer. Menstruating before the age of 12, being a longtime user of estrogen-based birth control pills or not having children—all these things statistically increase a woman’s chances of getting breast cancer, a disease that will probably kill about 5,400 Canadian women in 1995. Some experts have long suspected that density—the amount of fibrous supportive tissue in a woman’s breasts—might also be a factor. And in a study published last week, Toronto-based researchers reported on a study that showed exactly that: among women over 40, the risk of cancer increased with the amount of breast density, and women with high breast density levels had a five times greater chance of being afflicted with breast cancer.
The findings, published in the Bethesda, Md.-based Journal of the National Cancer Institute, were hailed as pointing to an important direction for further research. “Now, we need to find out more about why some women have dense breasts,” said Dr. Michael Poliak, a
cancer researcher at Montreal’s Jewish General Hospital. “It may be something that can be modified by drugs or even lifestyle changes.” Dr. Norman Boyd, chief of epidemiology at the Ontario Cancer Institute, who headed the study, said that another study was already under way to determine whether diet may be a factor in determining breast density. Women with dense breasts should not be unduly alarmed by the study, added Boyd, “because the majority of women with dense breasts still don’t get cancer.”
But the study suggested that their chances are far higher than for women whose breasts contain less fibrous material and more fat and glandular material. For the study, Boyd and his team examined records from the 45,000-woman Canadian National Breast Cancer Screening Trial carried out during the 1980s, and selected 354 women between the ages of 40 and 64 who developed breast cancer. Using a digitized computer-based system, Martin Yaffe, a medical physicist at Toronto’s Sunnybrook Health Science Centre, enhanced the quality of the mammograms made during the earlier study. Radiologists then used a six-point scale to evaluate the percentage of dense tissue in breasts, which shows up as a white area on mammograms. The result: women with between 25and 50per-cent breast density were found to be almost 2V2 times more likely to develop cancer, and women with more than 75-per-cent breast density stood a five times greater risk of getting breast cancer than women with low density.
Boyd and Yaffe have already launched follow-up projects, including a study of female twins, to see whether breast density is genetic in origin. In another study, researchers are testing dense-breasted women on different diets to see whether the low-fat, high-fibre eating habits associated with low cancer rates in some parts of the world play a role in breast density. Meanwhile, Boyd advised women over 50 with dense breasts to follow the procedures recommended for all women in that age group: regular breast self-examination and annual mammograms. That recommendation takes on added urgency in the light of the cruel fact that tumors in high-density breasts are often the most difficult to detect.
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