Mammography tests are not perfect. Only the part of the breast that extends from the body can be imaged. This makes it easier to get an accurate picture of a larger breast than a smaller breast. In addition, the periphery of the breast does not appear on the image at all. It also may be difficult to detect abnormalities that exist in the breast tissue adjacent to the chest wall.

Breast cancers are most likely to develop in dense tissue, where they are most difficult to detect. Mammograms can result in false negative readings (particularly in younger women, who have dense breast tissue) or false positive readings. In a false negative reading, dense breast tissue may obscure a tumor and the mammogram may fail to detect its presence. In a false positive, the mammogram appears abnormal when, in fact, no cancer is present.
 
Other factors that can significantly impact the accuracy of the test include:

• Quality of film used.
  
  
• Experience and skill of the technician and radiologist. The appearance of breast tissue on a mammogram varies significantly from woman to woman. A skilled technician may be able to obtain more accurate films. The skill and experience of the radiologist can make a difference in how accurately the mammogram is analyzed.
  
  
• Breast density. Breast tissue becomes fattier and has fewer glands as women age, which results in mammograms that are easier to interpret. Breast tissue that is denser, as in younger women, makes it more difficult to detect abnormalities.

In addition, mammography does not detect all cancers. In some cases, such as the armpit or chest wall, the area of the body is not easily viewable. However, a lump may be felt in a breast exam. In other cases, the cancer may be too small to be revealed by this test. Up to 20 percent of breast cancers are missed by mammograms, according to the National Cancer Institute. For this reason, mammograms and physical exams are considered to be complementary techniques.

Mammograms of women with breast implants can be very difficult to interpret. The x-rays used in mammography do not easily penetrate silicone or saline implants, making it harder to image overlying and underlying breast tissue. 

Women with implants are likely to have four additional images taken. Known as implant displacement (ID) views, they involve pushing the implant against the chest wall while the breast is pulled over it. This provides better imaging of the front part of the breast. This technique is not as effective in women who have experienced the formation of scar tissue around the implants. ID images are most successful in women whose implants are placed underneath the chest muscle.

Women who have had breast-conserving surgery, or lumpectomy, will need to continue to schedule regular mammograms, while women who have had their breast removed will no longer need mammograms of that area. Patients should consult with their physician about how breast cancer surgery might impact their need for future mammograms.