Having dense breasts can be the biggest risk factor of breast cancer in women – more so than all other known risk factors like obesity and family history, a recent University of California study reveals.

According to researchers, women’s whose breasts have more glandular tissue rather than fat are twice as likely to develop the cancer.

(The University of Leeds defines glandular tissue as a mix of two types of glands: endocrine and exocrine. Endocrine glands are ductless that secrete hormones into the blood. Exocrine glands have ducts and secrete hormones onto surfaces.)

Researchers assessed risk factors in over 200,000 women between the ages of 40 and 74 who were enrolled in the Breast Cancer Surveillance Consortium, a research initiative designed to monitor the quality of breast cancer screening.

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Of the 200,000 plus women, over 18,400 had breast cancer in various stages, while about 184,000 did not.

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Breast density was recorded for each woman according to the four categories outlined by the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS). Category A are breast made up mostly of fat; category B are breasts with scattered dense tissue but mostly fat; category C are breasts with moderately dense tissues and category D, which are breasts that are made up of at least 75 per cent dense tissue.

The results showed that breast density is the most prevalent risk factor for both pre-menopausal and post-menopausal women.

They also found that over 39 per cent of breast cancers in pre-menopausal women and over 26 per cent in post-menopausal women could have potentially been prevented if all women with higher breast density were shifted to lower-density BI-RADS category B.

But what surprised co-author Dr. Karla Kerlikowske the most was something the majority of women had in common.

“[I was surprised that] 90 per cent of women diagnosed with breast cancer have at least one of the risk factor we studied, which means if we use risk models with these factors, we should be able to predict who will get breast cancer,” she says. Tweet This

According to researchers, the risk is due to the high cellular content of dense tissue and interaction between cells that line the mammary ducts and surrounding tissue. And because both tissue and cancer show up as white on mammograms, it’s hard for radiologists to identify malignancy in dense tissue than in fat (which appears dark).

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Risk factors that are commonly known to be linked to breast cancer, like first full-term pregnancy over 30 years of age, only account for less than 10 per cent of the cases in the population, the study found.

Having high BMI

Women with high BMI typically have lower breast density, researchers note. However, age is a strong element of breast density as well.

“There is a high proportion of glandular and stroma tissue in pre-menopausal women and as women age it is replaced by fatty tissue,” Kerlikowske says. “However, the degree to which this occurs varies such that older women do have dense breasts.”

Researchers, however, are not saying that carrying extra weight (for the purpose of having fattier breasts) can protect you from the risk since obesity is a risk factor linked to the disease as well. Tweet This

In fact, researchers found that almost 23 per cent of women with breast cancers in this group could have been avoided if obese and overweight women maintained a body mass index of less than 25 (that translates to 155 lbs. for a woman who stands at 5’6” tall).

Family history

When it comes to those with a history of undergoing benign breast biopsies, only about seven per cent to nine per cent of both pre-menopausal and post-menopausal women with breast cancer accounted for cases of breast cancer diagnoses.

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Women with family history of breast cancer only accounted for about eight per cent of cases, and women who gave birth after 30 years of age made up between five and eight per cent of cases.

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“Family history is a strong risk factor but not as prevalent as dense tissue to family history,” Kerlikowske says. “Breast density is both a prevalent and strong risk factor which is why it contributes to the development of highest proportion of breast cancers in pre- and post-menopausal women.” Tweet This

For Kerlikowske, it’s important that individualized prevention measures are in place, and knowing this new information can help doctors and their patients do just that.

Breast cancer in Canada

According to the Canadian Breast Cancer Foundation, the disease will affect at least one in nine Canadian women.

The Canadian Cancer Society estimates that an average of 70 Canadian women is diagnosed with breast cancer every day; an average of 13 dies every day.

Breast cancer is the most common cancer among Canadian women and is the second leading cause of death from cancer in Canadian women.

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