Mammograms and Other Breast Imaging Tests

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What are the limitations of mammograms?

As is the case with most medical tests, mammography has limitations.

Even though mammograms can detect breast cancers too small to be felt, treating a small tumor does not always mean it can be cured. A fast-growing or aggressive cancer may have already spread before it’s found.

The value of a screening mammogram also depends on a woman’s overall health status. Detecting breast cancer early may not help prolong the life of a woman who has other kinds of serious or life-threatening health problems such as congestive heart failure, end-stage renal disease, or chronic obstructive pulmonary (lung) disease. The American Cancer Society screening guidelines emphasize that women with serious health problems or short life expectancies should discuss with their doctors whether to continue having mammograms. Our guidelines also stress that age alone should not be the reason to stop having regular mammograms.

False-negative results

A false-negative mammogram appears normal even though breast cancer is present. Overall, screening mammograms do not find about 1 in 5 breast cancers.

False negatives occur more often among women with dense breasts. Breasts often become somewhat less dense as women age, so false negatives are more common among younger women than among older women. False-negative results can delay treatment and promote a false sense of security for the woman.

False-positive results

A false-positive mammogram looks abnormal but no cancer is actually present. Abnormal mammograms require extra testing (diagnostic mammograms, ultrasound, and sometimes MRI or even biopsy) to find out if cancer is present.

False-positive results are more common in women who are younger, have dense breasts, have had breast biopsies, have breast cancer in the family, or are taking estrogen. About half the women getting annual mammograms over a 10-year period will have a false-positive finding. The odds of a false-positive finding are highest for the first mammogram, and are lower on subsequent mammograms. Women who have past mammograms available for comparison reduce their odds of a false-positive finding by 50%.

False-positive mammograms can cause anxiety. The extra tests needed to be sure cancer isn’t there cost time and money and also cause physical discomfort.

Over-diagnosis and over-treatment

While screening mammograms can find invasive breast cancer and ductal carcinoma in situ (DCIS, cancer cells in the lining of breast ducts) that need to be treated, it’s also possible that some invasive cancers and DCIS detected on mammography will not keep growing. This means that some tumors are not life-threatening, and never would have been detected if a woman had not gotten a mammogram. The problem is that doctors can’t tell these cancers from those that will grow and spread.

Our only hint that over-diagnosis may exist is through statistical analysis that compares the number of cancers found by mammography over long periods of time with the numbers of cancers that would have been expected without screening. Over-diagnosis is a concern because an over-diagnosed cancer will still be treated. This means that some women are treated unnecessarily because we don’t know which women fall into this group at the time the cancer is diagnosed. These cases would be considered over-treatment, which exposed the women unnecessarily to the adverse effects of cancer therapy. Because doctors often cannot be sure which cancers and cases of DCIS will become life-threatening, they are all treated. Although there is a wide range of estimates of the percentage of breast cancers that might be over-diagnosed by mammography, the most credible estimates range from 0-10%.

Last Medical Review: 12/08/2014
Last Revised: 10/20/2015