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Mammograms are the best breast cancer screening tests we have at this time. But mammograms have their limits. For example, they aren’t 100% accurate in showing if a woman has breast cancer. They can miss some cancers, and sometimes they find things that turn out not to be cancer (but that still need further testing to be sure).

False-negative results

A false-negative mammogram looks normal even though breast cancer is present. Overall, screening mammograms miss about 1 in 8 breast cancers.

• Women with dense breasts are more likely to get false-negative results. 
• False-negative mammograms can give women a false sense of security, thinking that they don’t have breast cancer when in fact they do.
• It’s important to see your doctor if you have new breast symptoms, even if you’ve had a normal screening mammogram recently. Additional tests such as a diagnostic mammogram and/or a breast ultrasound may be needed to look more closely at the area where you're having symptoms.

False-positive results

A false-positive mammogram looks abnormal even though there is no cancer in the breast. Abnormal mammograms often require extra testing (diagnostic mammograms, ultrasound, and sometimes MRI or even a breast 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 at some point.
• The odds of a false-positive finding are higher for the first mammogram (or if previous mammograms aren't available for comparison). Women who have past mammograms available for comparison reduce their odds of a false-positive finding by about half.
• False-positive mammograms can cause anxiety. They can also lead to extra tests to be sure cancer isn’t there, which cost time and money and maybe even physical discomfort.

Mammograms might not be helpful for all women

The value of a screening mammogram depends on a woman’s overall health. Finding breast cancer early may not help her live longer if she has other serious or life-threatening health problems, such as serious heart, kidney, liver, or lung disease. The American Cancer Society breast cancer 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 often find 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, even if the tumor in the breast is still small.

Overdiagnosis and overtreatment

Screening mammograms can often 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 on mammograms would never grow or spread. (Finding cancers that would never cause problems is called overdiagnosis.) These 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.

Overdiagnosis leads to some women getting treatment that’s not really needed (overtreatment), because the cancer never would have caused any problems. Doctors can’t always tell which cancers could be life-threatening and which would never cause problems. Because of this, they advise treating all breast cancers. This exposes some women to the side effects of cancer treatment, even though it wasn't really needed.

Still, overdiagnosis isn't thought to happen very often. There’s a wide range of estimates of the percentage of breast cancers that might be overdiagnosed by mammography, but the most credible estimates range from 1% to 10%.

Radiation exposure

Because mammograms are x-ray tests, they expose the breasts to radiation. The amount of radiation from each mammogram is low, but it can still add up over time. For more on this, see Mammogram Basics.