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How breast density affects cancer risk

October 08, 2019 | UCI Health
uci health radiologist freddie combs reviewing breast scans

About 50% of women have dense breasts, which increases their risk of developing breast cancer and makes it more difficult to spot tumors with traditional mammograms.

“Women with dense breasts need to know about the risks and understand the limits of mammograms,” says Dr. Freddie J. Combs, a UCI Health radiologist and breast imaging director at Pacific Breast Care Center.

Since 2013, California has required that doctors inform women when dense breast tissue is revealed on mammograms. Now the U.S. Food and Drug Administration is proposing to make that the rule across the country.

Learning that you have dense breasts has confused and frightened some women. So, just what are dense breasts and how does it affect screening for cancer?

What is dense breast tissue?

“Breasts are made up of glandular and connective tissues as well as fat,” says Combs. “Breast density is a measure of the amount of glandular and connective tissues relative to fat. When the ratio is more than 50%, we consider that a dense breast.”

He says, for example, that about half the women screened at the Costa Mesa center have dense breasts.

Doctors classify breast tissue as "heterogeneously dense" — with large areas of dense fibrous and glandular tissue combined with some fatty tissue and occurs in about 40% of women — or "extremely dense," with very little fatty tissue and occurring in only 10% of women. The more dense the tissue, the harder the cancer can be to detect. 

Why do women have dense breasts? Breast size has nothing to do with it. The primary reason is genetics, Combs explains. Younger women and those with a smaller body mass index or who take hormone replacements are more likely to have dense breasts.

Why dense breasts can be a problem

There are two reasons that dense breasts can be problematic. It is harder to find cancer tissue with a mammogram. Second, people with dense breast tissue have a higher risk of developing cancer.

“When we do a mammogram, the X-ray beam doesn’t penetrate through the dense tissues as well as through fat,” explains Combs. “The dense tissues show up white, making it hard to read the mammogram.

“The more dense tissue in your breast,” he says, “the harder your cancer is to find.”

While it’s not known exactly why dense breast tissue is a risk factor for cancer, Combs says some postulate that it’s something like a “fertile field”: Because breast cancers occur in glandular tissue, the more glandular tissue you have, the more opportunities there are for cancer to develop.

“Dense breasts increase the risk of cancer, but it doesn’t make you high risk,” he says. “Neither does it affect the treatment for your cancer nor increase your risk of dying.”

What if a mammogram can’t find the cancer?

Pacific Breast Care Center performs only 3D mammograms, which have been shown to detect more cancers and reduce the recall rate for inconclusive screenings.

Combs says they can use a computer model to estimate a patient's lifetime risk of developing breast cancer. The computer model uses several factors, including family history, pregnancy history and menstrual history, to determine this risk.

Women with dense breasts may benefit from additional screening. Some may be offered an ultrasound, which can find additional cancers. One fact of ultrasounds is that they also find masses that aren't cancer, which may mean that women will undergo additional biopsies.

Ultrasound is an area of active research. While ultrasound does have a higher false-positive rate and may lead to more biopsies, it also increases the number of cancers found overall.

An MRI, or magnetic resonance imaging test, is another test that can be done in addition to a mammogram to evaluate dense breasts in high-risk women. MRI has a high sensitivity for detecting cancer.

“If you have dense breasts, it’s very important to talk with your physician about options,” says Combs. “We need a paradigm shift to educate patients about breast density and risk so that they’re empowered and have less uncertainty about screening tests and results.”

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