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Understanding Breast Density

Release Date: September 27, 2017
Breast cancer is the most common cancer in women in the United States. It is the second leading cause of cancer-related deaths in women.

Understanding Breast Density

Breast cancer is the most common cancer in women in the United States. It is the second leading cause of cancer-related deaths in women. The American Cancer Society's (ACS) estimates for breast cancer in the United States for 2017 are:

  • About 252,710 new cases of invasive breast cancer will be diagnosed in women.
  • About 63,410 new cases of carcinoma in situ (CIS) will be diagnosed (CIS is non-invasive and is the earliest form of breast cancer).
  • About 40,610 women will die from breast cancer.

The good news is that the death rates have been steadily going down since 1989. This is thought mainly to be due to earlier detection and improved treatments.

The female breast is made up of mainly lobules (milk producing glands), ducts (tiny tubes for milk to flow through), stroma (fatty and connective tissues that surround the ducts and lobules), and blood and lymphatic vessels. According to the ACS, most cancers start in the cells that line the ducts. However, some start in the cells that line the lobules.


At this time, mammograms (x-rays of the breast) are the best way to find breast cancer early, when it is easier to treat and before it is big enough to feel or cause symptoms. Having regular mammograms can lower the risk of dying from breast cancer.

When a mammogram is performed, the breast is placed between two plates and pictures are taken. The plates flatten the breast, holding it still while the X-ray is taken. The rays penetrate the breast and, in addition to visualizing any lumps or cysts, the thickness of the breast is also determined. This thickness is known as breast density.

The radiologist reading the mammogram assigns one of four categories of density to the mammogram:

  • Category A (Level 1): Almost entirely fatty and the easiest tissue for the x-rays to penetrate.
  • Category B (Level 2): Scattered areas of fibrous and glandular tissue.
  • Category C (Level 3): Heterogeneously dense. This level can make it hard to see small masses.
  • Category D (Level 4): Extremely dense and very difficult to see through on mammogram.

The following chart shows the different levels of density seen on a mammogram:

Breast Density Mammogram Chart

Determining breast density is important because dense breasts make it more difficult for physicians to find cancer on mammograms. Depending on the level of thickness, Levels 3 or 4 have the potential to camouflage early cancers. Approximately 50% of women in the United States have dense breasts and fall into Levels 3 and 4.

Breast Density Chart
Images used with permission of American College of Radiology (ACR)

Breast density is not determined by shape or size of the breasts, nor how they feel. Clinical breast exams (physical exams) cannot define density. Density is only established by a mammogram. Having dense tissue may increase the risk of getting breast cancer.

The State of New Jersey passed a law, effective May 1, 2014, that mandates physicians provide notification to patients that have dense breasts as determined by the interpreting radiologist. As per the New Jersey Breast Density Law, the following is to be stated:

"Your mammogram may show that you have dense breast tissue as determined by the Breast Imaging Reporting and Data System established by the American College of Radiology. Dense breast tissue is very common and is not abnormal. However, in some cases, dense breast tissue can make it harder to find cancer on a mammogram and may also be associated with a risk factor for breast cancer. Discuss this and other risks for breast cancer that pertain to your personal medical history with your healthcare provider. A report of your results was sent to your healthcare provider. You may also find more information about breast density at the website of the American College of Radiology, arrow icon."

The ACS recommends screening for early stages of breast cancer starting when women reach age 40. A women aged 40 and older should have a CBE (clinical breast exam as part of a routine physical exam) and screening mammogram every year, and should continue to do so for as long as she is in good health. Mammograms for an older woman should be based on the individual, her health, and other serious illnesses she might have. Age alone should not be the reason to stop having regular mammograms, according to the ACS. Women in their 20s and 30s should have a CBE (clinical breast exam as part of a routine physical exam) by a health care provider every three years. For more information, please visit: arrow icon; arrow icon; or, arrow icon.