Having dense breasts increases your breast cancer risk and makes tumours harder to find. Doctors explain the puzzle behind this common condition.

Just when you thought you were up to speed on breast cancer risk factors, a new game changer has physicians and clinicians talking: dense breasts. Misunderstood and often incorrectly associated with bumpy, hard or occasionally sore breasts, dense tissue is more worrisome than you think.

What are dense breasts?
Your breasts are comprised of three types of tissue: fat, epithelium (the glands and ducts that produce milk) and stroma (supporting tissue). Breasts with a higher ratio of epithelium and stroma to fat are considered dense. According to Dr. Norman Boyd, senior scientist at the Campbell Family Cancer Research Institute at the Princess Margaret Cancer Centre in Toronto, women with dense breasts have this tissue ratio for the majority of their adult lives. "The tissue develops in adolescence when the breasts form," he says. "Past the age of 40, it gradually decreases and is replaced by fat on average by about one percent every year." Once a woman reaches menopause, that natural decrease in density jumps to eight percent on average; however, some women still have dense breasts into their 60s and beyond.

Breast density is quite common. "It’s been estimated that approximately 50 percent of women have heterogeneously dense and/or extremely dense breast tissue," says Dr. Christine Wilson, medical director of the screening mammography program at the BC Cancer Agency. Risk factors that influence density include genetics (Dr. Boyd says that 60 percent of breast density variation can be ex­ plained by genes), delayed childbearing, combined estrogen/progesterone hor­mone replacement therapy (HRT) and a family history of breast cancer.

Dense breasts cannot be diagnosed by touch, appearance or symptoms of dis­ comfort. "Some women have really firm breasts that suggest that they may be dense, but we don’t know for sure until their breasts are viewed with mammography," says Dr. Ruth Heisey, chief of the department of family and communi­ty medicine at Women’s College Hospital and a GP oncologist specializing in breast diseases at Toronto’s Princess Margaret Hospital. A clinician who spots what appears to be excessive density during screening mammography may call you back for further examination if it is also accompanied by symptoms such as a lump or pain. "We don’t want to miss something that we can’t see on the mam­mogram," says Dr. Wilson. Each province and territory operates under different pro­tocols. Most centres may follow up with a breast ultrasound, while some will turn to magnetic resonance imaging (MRI) to rule out hidden cancers. If they’re follow­ing current mammography guidelines in Canada, many women won’t know that they have this problem until they go for their first mammogram at age 50.

Surprising health risks
If healthy breasts can be bumpy and hard, why does excessive density matter? There are several reasons. "Women with density of 75 percent or more of the breast have a risk of breast cancer that is four or five times higher than that of women of the same age who have little or no density," says Dr. Boyd. "And breast density is a much stronger risk factor than family history of breast cancer, which is twice that of women without a family history." The only factor that’s a larger risk for breast cancer is if you carry the BRCA1 or BRCA2 gene.

Not only does density increase your risk, but it makes tumours more difficult to spot. Mammography X-rays can easily pass through fat, but have difficulty penetrating dense epithelium tissue. To the trained clinician examining the mammogram, dense tissue and tumours both appear white on the X-ray, making it tricky to differentiate between healthy and cancerous breast tissue.

Decreasing density
There are currently two proven strategies to reduce density. "If somebody is taking combined HRT featuring both estrogen and progesterone, density will lessen slightly if she stops it," says Dr. Boyd. "The other strategy is the drug tamoxifen, which can reduce density, but it can increase the risk of blood clots going to the lungs, so it’s not something that everyone wants to take." Not all women with dense breasts can take advantage of these two strategies, so research is ongoing to uncover alternative solutions.

Dr. Caroline Diorio, an assistant professor in the department of social and preventative medicine at Université Laval in Quebec, is currently researching ways that lifestyle changes can alter breast density. Her latest study, published in June and funded by grants from the Canadian Breast Cancer Research Alliance, found an association between high-sugar diets and increased breast density in pre- and postmenopausal women. "I cannot say that [sugar] is causing an increase in breast density, but women who eat more sweet foods seem to have a higher density than women who eat less," she says. "I believe if we change our habits, we can reduce our density, but we need more studies to prove it."

Dr. Diorio also published a study in January on the consumption of omega-3 fatty acids. "My study suggests that post- menopausal women who consumed higher intake of long-chain omega-3 fatty acids, found mostly in fish, had less dense breasts." Her 2005 study also showed an association between an increased intake of vitamin D and calcium, and lower density readings in premenopausal women. While these findings are encouraging, more research is needed to confirm results and provide guidance on how to adopt these lifestyle changes.

Being proactive
Making proactive changes could reduce your risk of breast cancer "Maintaining a healthy weight, drinking no more than one alcoholic drink per day on average, taking 1,000 IU of vitamin D daily, and exercising regularly have been shown to be effective in reducing breast cancer risk," says Dr. Heisey.

While the chances of cancer are greater for women with dense breasts, it doesn’t guarantee you will develop the disease. "I view it like knowing that you have relatives with breast cancer," says Dr. Boyd. "There’s nothing you can do to change your relatives, but what does that knowledge do? It may increase your awareness, so if anything changes in the breast, you’re more likely to have it investigated, by medical experts at and it may encourage you to take steps to reduce your risk."