Are Mammograms Accurate?

Mammograms aren’t perfect, but they’re the gold standard for catching breast cancer early. People who get regular mammograms lower their risk of dying from breast cancer by 65% compared to those who don’t get mammograms. Mammograms are safe and accurately detect up to 93% of breast cancers.

In rare cases, mammograms can miss breast tumors. This is called a “false negative” result. Sometimes mammograms may find a breast abnormality when the breast is actually healthy, called a “false positive.”

Mammograms will never be 100% accurate, but experts expect they will continue to improve over time. “With ongoing improvement of breast imaging technology as well as adding supplemental imaging when appropriate based on dense breasts, we will be able to show sustained improvement in this area,” says Amy Patel, MD, the medical director of the Breast Care Center at Liberty Hospital. 

Mammograms use X-ray images to build a picture of the breasts, which radiologists then analyze for breast cancer and other breast conditions. Traditional 2D mammograms capture two images of each breast. 2D mammograms have an 87% sensitivity rate, meaning they pick up 87% of breast cancers. 

A 3D mammogram, also known as digital breast tomosynthesis (DBT), uses a collection of 2D images to create a 3D picture of the breasts. Research shows that 3D mammograms catch more breast cancer than 2D mammograms and have a lower rate of false positives. 3D mammograms have a 93% sensitivity rate, one estimate found, meaning they pick up 93% of breast cancers.

This means that both types of mammograms can be wrong sometimes, either by showing an abnormality that looks like cancer but isn’t (false positive) or they miss cancer that’s there (false negative).

False positives

One risk of regular mammograms is getting an incorrect — or “false” — positive test result. When a mammogram appears abnormal, you’ll usually need to have more testing. This could mean an additional mammogram, ultrasound, or biopsy. If these tests find that you don’t have breast cancer, which they typically do when there’s an initial false positive result, you won’t receive unneeded treatment.

Still, false positives can cause unnecessary anxiety. False positives may lower a person’s confidence in mammograms, and in turn lower the chance of them coming back for future mammograms. 

“It can understandably be nerve-wracking if you're not sure what your final result is going to be,” says Kathryn Lowry, MD, an associate professor of radiology at the University of Washington School of Medicine and physician at the Fred Hutch Cancer Center. Coming in for follow-up screening also takes time and money. “It’s not trivial,” she says.

False positives are more common with your first mammogram than with later ones. When experts look at mammogram results, they compare new images to old images to find differences that suggest breast cancer is developing. With a first mammogram, they don’t have another mammogram to compare results to, so there may be more errors.

About 16% of first mammograms and 10% of following mammograms will give a false-positive result. Over the course of your life, your chance of having at least one false positive is 61% if you get a mammogram every year, or 42% if you get one every other year.

You may be at greater risk of a false-positive result if you are younger than age 50, have dense breasts, have had a breast biopsy, have a family history of breast cancer, and/or take birth control pills or estrogen and progestin hormone therapy after menopause.

False negatives

It’s also possible that mammograms miss cases of breast cancer. If a result says you don’t have cancer when you actually do, it’s called a false negative. This can lead to delayed treatment. Mammograms miss about 20% of breast cancers that are present at the time of screening. Breast cancer can grow at different rates, taking anywhere from weeks to years for a tumor to double in size. The seriousness of a false negative depends on how aggressive the cancer is.

People with dense breasts are more likely to have a false-negative result, regardless of the type of mammogram (2D or 3D) they get. That’s because it’s harder to spot cancer in dense breast tissue using a routine mammogram. “The sensitivity of mammograms decreases as your breast density increases,” Patel says. But with supplemental screenings — like fast breast MRI, contrast enhanced mammography, and whole breast ultrasounds — false negatives have a higher chance of being caught.

One major concern with mammograms is that they can detect cancer that wouldn’t have been terminal or even caused any symptoms. This is called overdiagnosis. But it’s not always clear which cancers are safe to leave alone. “We don’t have a crystal ball to tell the future and know the difference between a lethal cancer and one that’s unlikely to affect you during your lifetime,” wrote Marisa Weiss, MD, director of breast radiation oncology at Lankenau Medical Center and founder of Breastcancer.org, in an editorial. 

Ultimately, people who want to have cancer treated if detected should get annual mammograms. Patel says that most people want to have their cancer removed, even if it’s caught early or is slow growing, no matter their age. 

Plus, doctors use multiple tools now to decide on treatments, not just mammograms. They rely on follow-up biopsies, genetic markers, and risk assessments to find clues about how likely a tumor is to grow and spread, which can all help cut back on unnecessary treatments. A mammogram is just a starting point, and treatment decisions aren’t made solely based on it. 

Emerging research suggests that using radiologists and artificial intelligence (AI) systems to read mammograms may lead to fewer false positives and false negatives. In one study of mammograms from 106,000 women,  support from AI increased the percentage of breast cancers correctly detected.

 “We are seeing great promise from larger studies,” says Patel, but more research is needed, and it may be a while before AI detection is used widely. She thinks AI for mammograms and other breast imaging modalities will “be the standard down the road and play a crucial role in how we detect malignancies and avoid unnecessary biopsies.” 

Despite some of the limitations of mammograms, “if you want the benefit of early detection and you are prepared to be treated for breast cancer, if one is found,” says Weiss, “then you should continue having regular mammograms, no matter how old you are.”