Mammograms and Other Breast Imaging Tests

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What are the limitations of mammograms?

Mammograms are the best breast cancer screening tests we have at this time. But mammography has limitations. A false-negative mammogram looks normal even though breast cancer is present. A false-positive mammogram looks abnormal but there’s no cancer in the breast.

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

It’s important to know that even though mammograms can show breast cancers that are too small to be felt, treating a small tumor does not always mean it can be cured. A fast-growing or aggressive cancer might have already spread.

False-negative results

  • A false-negative mammogram looks normal even though breast cancer is present. Overall, screening mammograms do not find about 1 in 5 breast cancers.
  • Women with dense breasts have more false-negative results. Breasts often become less dense as women age, so false negatives are more common in younger women.

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 the change is cancer.
  • 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 of 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. Women who have past mammograms available for comparison reduce their odds of a false-positive finding by about 50%.
  • False-positive mammograms can cause anxiety. The extra tests needed to be sure cancer isn’t there cost time and money and maybe even physical discomfort.

Over-diagnosis and over-treatment

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. But it’s possible that some of the invasive cancers and DCIS found would not grow or spread. (Finding and treating cancers that would never cause problems is called over-diagnosis.) This means that some cancers are not life-threatening, and never would have been found or treated if the woman had not gotten a mammogram. The problem is that doctors can’t tell these cancers from those that will grow and spread.

Over diagnosis leads to some women getting treatment that’s not really needed. We don’t know which women fall into this group when the cancer is found because we can’t tell which cancers will be life-threatening and which won’t ever cause problems. Treating women with cancers that would never cause problems would be considered over-treatment.

Because doctors often can’t be sure which cancers and cases of DCIS will become life-threatening, all cases are treated. It exposes these women to the adverse effects of cancer treatment that’s really not needed.

Still, over-diagnosis is not that common. There’s a wide range of estimates of the percentage of breast cancers that might be over-diagnosed by mammography, but the most credible estimates range from 0-10%.

Last Medical Review: 12/08/2014
Last Revised: 04/25/2016