If you’ve ever heard, “you’ve got dense breasts” during a mammogram … were you also told how that might affect your risk of breast cancer?

Due to having been exposed to DES before birth, I began having mammograms early. Back then, mammograms were on film, and I routinely had to return for a follow up image. I was periodically subjected to having a cyst aspirated, in order to make sure it was indeed a cyst and not a tumor.

And yes, I got the “you have dense breasts” observation even after my hysterectomy and the advent of digital film.

But no one told me that dense breasts were a risk factor for breast cancer, even after I had two lumpectomy biopsies in 2011.

What is breast density?

Breasts are made up of fat, connective tissue, and glandular tissue (milk ducts and lobules). Dense breasts have a lot of glandular and connective tissue and not much fatty tissue.

Breast density is a measure of how the breasts look on a mammogram.

Radiologists describe the four levels of density thusly:

1. Least dense: almost entirely fatty breast (10%)
2. Scattered areas of fibroglandular density: the majority of the breast is fatty tissue but there is scattered dense breast tissue (20%)
3. Heterogeneously dense: most of the breast is dense tissue with some areas of less dense fatty tissue (20%)
4. Extremely dense: the breast is almost all dense glandular and fibrous tissue (10%)

About half of women over age 40 have dense breasts (the two most dense categories).

Here’s why it matters for early detection:

On a mammogram, fatty tissue appears dark and transparent; glandular and connective tissue appears as solid white areas. Because breast tumors also appear white on mammography, they can be hidden by or within dense breast tissue.

Other options for screening include breast tomosynthesis (3-D mammography), breast ultrasound, and breast MRI. 

What does the research say about the link to breast cancer?

From 2007, New England Journal of Medicine:

… after adjustment for other risk factors, extensive mammographic density was strongly and reproducibly associated with an increased risk of breast cancer, regardless of whether the cancer was detected by screening or other means.

The researchers said that masking, not rapid growth, seemed to be the causal factor. The risk of breast cancer associated with extensive mammographic density increased by a factor of 17.

From 2011, Journal of the National Cancer Institute, a study of post-menopausal women:

The risk of breast cancer increased progressively with increase in percent breast density… Women with higher breast density (>=50%) showed a 3.39-fold  increased risk of breast cancer compared with women with lower breast density (<10%).

Breast density seems to be associated with all breast cancer sub-types, according to a 2013 study in Spain reported in Breast Cancer Research.

A 2013 review of six studies (3,414 women with breast cancer) found:

Mammographic density (MD )is strongly associated with all breast cancer subtypes, but particularly tumors of large size and positive lymph nodes across all ages, and ER-negative status among women ages <55 years, suggesting high MD may play an important role in tumor aggressiveness, especially in younger women.

A 2017 study of 52,814 women in The Netherlands showed that all breast density measures were positively associated with breast cancer risk.

Breast density legislation

In 2009, Connecticut became the first state to pass breast density notification legislation.

This example of a patient notification is from Missouri:

If your mammogram demonstrates that you have dense breast tissue, which could hide abnormalities, and you have other risk factors for breast cancer that have been identified, you might benefit from supplemental screening tests that may be suggested by your ordering physician. Dense breast tissue, in and of itself, is a relatively common condition. Therefore, this information is not provided to cause undue concern, but rather to raise your awareness and to promote discussion with your physician regarding the presence of other risk factors, in addition to dense breast tissue. A report of your mammography results will be sent to you and your physician. You should contact your physician if you have any questions or concerns regarding this report.

No comment about the prose.

As of June, 30 states had passed notification laws:

Despite notification laws, researchers and doctors have not developed screening guidelines for women with dense breasts.

More fodder for the mythology surrounding “early detection.”

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