Understanding breast density

When you receive your mammogram report, you may note that at the end of the report it states that your breasts are dense and that this may limit the accuracy of the mammogram. This can be frightening to a patient: going through the procedure just to be told that it may miss a breast cancer because of dense tissue. What does that really mean?
The breasts are composed of glands and fat. The glands are composed of the lobules that produce milk and the ducts that bring the milk to the nipple. Fat surrounds the breast tissue. Most breast cancers arise from the ducts.  Lobular cancers are less common.
We know that very dense breast tissue can increase the risk of breast cancer 4-6 times.
The reason for this is that the tissue that is dense is more actively growing and dividing which allows mutations that can lead to cancer. Also, the glandular tissue may be more hormonally sensitive.
According to the American College of Radiology, breast density is divided into four categories:

1. Almost entirely fatty- the fatty tissue on mammography is black, and there may be small areas of glandular tissue that are  gray or white
2. Scattered fibroglandular density: scattered areas of gray or white tissue in a largely fatty breast
3. Heterogeneously dense breasts: large areas of the breast are white or gray, and these areas can hide small breast cancers
4. Extremely dense: most of the tissue appears white or gray on mammography and it may be difficult to identify a mass that could be hidden by these densities

If your breasts are extremely dense or heterogeneously dense, they would be considered dense breasts. About 40% of women over the age of 40 will fit into this category.
Most women will fit into category 2 or 3.
So what does a mammogram look for?
 A mammogram is looking for a mass which would often appear as an irregular white density, with irregular borders, sometimes containing calcifications. Non-invasive breast cancer or DCIS is  microscopic disease, and often there is no mass present. Calcification that are clustered, fine, and irregular are often seen in breast cancer masses.

If your breasts are considered dense, what type of screening should you have? Is yearly mammography enough?

The answer to this depends on your age and other risk factors. Your physician should take into account your family history, BRCA gene status, a history of breast irradiation prior to the age of 30, and a history of previous breast biopsies. There are several models that can predict the risk of breast cancer taking into account multiple factors, but these models don’t take into account breast density as a risk factor.

The Gail Model risks takes into account your age, age at first menstrual cycle, age at first live birth, number of first degree relatives with breast cancer, how many breast biopsies you have had, and if any demonstrated abnormal cells.

The Tyrer-Cusick  Model looks at age, weight, height, age of first menses and menopause, whether you have taken HRT and how long, your BRCA status if known, and and extensive family history of breast and ovarian cancer.
If either determines that your lifetime risk is greater than 20%, you would be considered at higher risk, and supplemental screening is optional. Remember that average lifetime risk of breast cancer is about 12%.

We know that mammography saves lives, because it can find invasive breast cancers before it spreads to lymph nodes. Whether additional screening with ultrasound of MRI actually saves lives has never been proven with studies. Most of the breast cancers found on sonogram or MRI that are not detected by mammogram are small and are lymph node negative. It is possible that some of these cancers are very slow growing and would have never been life-threatening.

Options for additional screening include 3D tomosynthesis, ultrasonography, or a breast MRI. Additional screening is recommended for women who carry the BRCA gene, or women who have had radiation to the chest (usually for Hodgkin’s disease before age 30). Insurance coverage for additional screening depends on the insurance carrier, and sometimes the state in which the study is done. An MRI can cost several thousand dollars, so insurance coverage criteria should be confirmed prior to administering the test.

Despite breast density, mammography, especially 3D mammography should always be the first screening tool used. A 2D mammogram will pick up 2-7 breast cancers per 1000 women screened. A 3D mammogram will pick up 3-9 breast cancers per 1000 women screened.
Adding sonography to a 2D mammogram will pick up 4-11 breast cancers per 1000. A 2D mammogram plus a contrast-enhanced MRI will pick up 12-17 breast cancers per 1000.

The decision to perform additional screening for breast cancer beyond a 2D or 3D mammogram is, in many cases, is a complex process,  taking into account many factors, and breast density should be considered in that equation.  By being better informed, and with the advice of your physician, an appropriate decision can be made for each individual woman.


Marilyn Jerome, MD
Foxhall Ob-Gyn Associates


References:


OBG Management, Volume 27, Number 10, October 2015. Get smart about dense breasts.
Berg, et al.