In a review of studies looking at more than 2,200 Canadian women, those with denser breasts were three to five times more likely to be diagnosed with breast cancer during the study period.
The study appears in the January issue of The New England Journal of Medicine.
"The time has come to acknowledge breast density as a major risk factor for breast cancer," Karla Kerlikowske, M.D., of the University of California, San Francisco, writes in an editorial in the same issue. The researchers included Norman Boyd, M.D., of Canada's Ontario Cancer Institute and the Campbell Family Institute for Breast Cancer Research.
The association between dense breasts and breast cancer isn't new; it dates back to 1970, note Boyd's team and Kerlikowske.
The researchers suggest breast cancer risk assessment and screening techniques such as digital mammography, ultrasound, and magnetic resonance imaging (MRI) as possibly beneficial for women with denser breasts.
Boyd's team reviewed three studies on 1,112 Canadian women with breast cancer and an equal number of women without breast cancer. All of the women were 40 to 70 years old (average age: 56). Each had gotten a mammogram every year or two for the previous eight years. None was diagnosed with breast cancer based on the first of those mammograms.
Based on the mammograms, women with extensive breast density were three to five times more likely to be diagnosed with breast cancer during the study period. Those results held when the researchers considered other breast cancer risk factors.
It can be harder to spot tumors on mammograms of dense breasts. So it's possible the tumors were masked by breast density on the first mammogram but appeared on the later tests.
That's the most likely explanation, Boyd's team writes. If so, other imaging techniques, including ultrasound, MRI, and digital mammography might help spot such tumors early, note the researchers. But they also point out another possibility — that denser breasts might harbor faster-growing tumors.
Younger women tend to have denser breasts than older women. But it's not clear if the new findings apply to women under 40, since women in that age range weren't included in the studies reviewed by Boyd's group.
Many factors — including genetics and lifestyle — can affect breast cancer risk. Early detection often improves survival.
SOURCES: Boyd, N. The New England Journal of Medicine, Jan. 18, 2007; Vol. 356: pp. 227-236. Kerlikowske, K. The New England Journal of Medicine, Jan. 18, 2007; Vol. 356: pp. 297-300.
By Miranda Hitti
Reviewed by Louise Chang, M.D