Overview of Mammography
Mammography is a test that uses x-rays to create images of breast tissue. The test is performed to detect and evaluate abnormalities such as tumors and cysts. There are two basic forms of mammography: screening and diagnostic.
A screening mammogram is used to look for cancer in women with no symptoms and no history of breast surgery. Mammograms are used for comparative study, that is, the left breast is compared to the right breast, and current films are compared to older films. The goal of regular screening is to detect small cancers in breast tissue because they are easier to treat and present a better prognosis for the patient.
How often women under age 50 should have a screening mammogram continues to be controversial. Most prominent health organizations (e.g., World Health Organization, National Cancer Institute, American Cancer Society) agree that women between the ages of 50 and 69 can significantly reduce the risk for dying of breast cancer by having a mammogram every 1 or 2 years. They also agree, however, that there is insufficient evidence supporting the same benefit for women between age 40 and 49.
The National Cancer Institute and the American Cancer Society recommend that women 40 and older have a mammogram every 1 to 2 years, and that women who have significant risk factors discuss whether they should begin screening earlier, and how often, with their physicians.
Standard Mammography Studies
The breast is positioned between two panels and compressed. Two images of each breast are obtained for evaluation: a side (mediolateral oblique) view and a view from above (cranial-caudal). X-rays penetrate the breast and record the images on film.
Different tissues in the breast absorb different amounts of x-rays, producing different shades of black, gray, and white on the film:
- Fatty tissue absorbs a small amount of x-rays and appears black or dark gray.
- Normal fibrous and glandular tissues (milk glands, lymph nodes) contain water fluid and absorb a moderate amount of x-rays, and appear light gray.
- Fibrous and glandular tissues may contain calcium and appear nearly white or white.
Cancerous tissue contains watery fluid and sometimes calcium, making small cancers difficult to distinguish from normal breast tissue. As the cancer grows, it appears whiter than breast tissue on film.
Comparison with prior mammograms helps the radiologist recognize changes in the patterns of light and dark areas on the film that may indicate the presence of small breast cancers. Women are encouraged to have a mammogram at age 35 to provide a baseline study for comparison with mammograms taken when they are 40 and older.
Because small cancers are difficult to distinguish from normal breast tissue on a mammogram and may not be palpable during a breast exam, some go undetected. Cases that elude diagnosis are referred to as false negatives. These cancers are usually found after they have grown to a size that can be seen or felt.
Other conditions in the breasts may look similar to cancer, such as calcifications or cysts. Usually, the radiologist will ask for additional studies that focus on the area of the breast where the suspicious lesion is located. If additional studies are inconclusive, a biopsy is recommended. If the biopsy proves to be noncancerous, the finding is called a false positive.
Calcifications commonly develop in women's breasts because the breasts produce milk, which contains calcium. Because many breast cancers contain calcifications, it is important to determine whether or not they are within cancerous tissue. Most calcifications can be evaluated a mammogram. Those that are difficult to evaluate require additional studies.
A mass, or lump, found on a mammogram may be a lymph node, cyst, or fibroadenoma (fibrous milk gland). These types of masses usually are easy to identify and do not require additional studies. A new mass or a mass that has grown since the last mammogram was taken is usually evaluated with ultrasound.
Several masses can be seen in women who have fibrocystic disease. If there has been a change in the size or the shape of the edges of a mass, or if a suspicious calcification within a mass is seen, the radiologist may order additional studies.
An area in one breast that has a distinctly different appearance than the same area in the other breast is referred to as
asymmetric density. This finding usually requires additional studies with mammography or ultrasound.
Dense breast tissue can make mammogram evaluation difficult because the tissue can obscure small cancers.