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Learn about Breast Density

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On October 1, 2013, a Maryland State law went into effect requiring the inclusion of general information regarding breast density in your screening mammography report.

The breast imaging radiologists in the Johns Hopkins Breast Imaging section will provide the required general information but will also advise you of your breast density based on their interpretation of your imaging study. As always, you should discuss breast health with your personal physician.

Please use the information below to gain a better understanding of the concept of breast density. Our breast imaging radiologists will also be your resource for learning more about this important topic. 

  • The breast is a gland that is composed primarily of two tissues: gland and fat. The gland tissue produces and delivers milk to the nursing infant. The fat tissue contributes to the breast size, otherwise has no real function.

    Every woman has a unique blend of fat and gland tissue in her breast. This blend may change during different stages of life. The younger woman typically has more gland tissue than the older woman, as the younger woman may be nursing a baby.

  • The gland tissue in the breast is active during the years before menopause, building up and breaking down the milk glands during the menstrual cycle. (This may be one reason for breast tenderness in the young woman) Unfortunately, body tissues that undergo this normal change and growth can also undergo abnormal growth. Abnormal growth that is out of control is cancer. In the breast, the gland tissue can become cancerous. The fat tissue does not become cancerous.

    On a mammogram, the gland tissue appears “dense” or white. The fat appears dark. The denser gland tissue stands out sharply against the darker fat. Thus, it is easier to detect an abnormal change in the dense gland tissue if it surrounded by darker fat, rather than if it is surrounded by white gland tissue. (think of a polar bear in a snowstorm.)

    A woman with dense breasts may have breast cancer that cannot be detected on a mammogram, just as a polar bear may not be seen in a snowstorm. The term for breasts that are composed predominantly of gland tissue, with little fat, is “dense”. Cancers developing in the dense breast may not be detected as early as the cancers developing in a fatty breast.

    It is also now known that dense breasts have a greater risk of developing cancer, compared to fatty breasts.

  • We feel that it is important to inform you about your breast density, so that you and your doctor may make informed decisions about breast cancer screening.

    The mammogram should be used to annually screen every woman over 40.

    Ultrasound may be added to screen dense breasts. MRI is reserved for women at increased risk, not in normal risk women.

  • The only way to know is the mammogram. Density refers only to the mammographic appearance, and is not used to describe whether the breast is lumpy on clinical exam. When the medical provider or the woman feels that her breast is lumpy on exam, the mammogram may be dense or may be fatty.

  • Your breast density is not a choice. It is a feature of your physical self, as is height or hair color. However, the breast density usually decreases with age, as the woman becomes menopausal. It also may decrease with weight gain, as more fat is deposited in the breast.


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