A new study finds that ultrasound is comparable to mammography in detecting breast cancer, but doctors caution that ultrasound shouldn’t necessarily replace mammography.

Two tests may be better than one.

That’s the conclusion of researchers in a new study that looked at the reliability of both ultrasounds and mammograms.

Breast cancer is the most common cancer in women worldwide. It’s the second most common cancer overall.

The disease caused 522,000 deaths worldwide in 2012, according to the U.S. Centers for Disease Control and Prevention.

Mammograms are the most common way to screen for breast cancer. In the United States, the procedure is encouraged and available to most women, and is the case in many developed nations.

In less developed countries, it’s not as easy to get a mammogram. Even where they exist, they may not be affordable or accessible.

That’s why researchers decided to look at ultrasounds as an alternative.

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Their study was published in the Journal of the National Cancer Institute and included 2,809 participants across the United States, Canada, and Argentina.

Of those, 2,662 had three annual screenings. These included ultrasound and mammography. They each had a 12-month follow-up or a biopsy.

Ultrasound turned out to be just as good at detecting breast cancer as mammography. Ultrasound also found a greater number of invasive and node-negative cancers than the mammograms did.

On the downside, there were more false positives with ultrasound than with mammograms.

Ultrasound is cheaper than mammography. It’s also more portable. The study authors suggest that in countries where breast cancer screening is lacking ultrasound is an effective way to assess breast lumps.

There may be another benefit, as well.

“Where mammography is available, ultrasound should be seen as a supplemental test for women with dense breasts who do not meet high-risk criteria for screening MRI and for high-risk women with dense breasts who are unable to tolerate MRI,” Dr. Wendie A. Berg, Ph.D., the study’s lead author said in a press release.

Another study, published in the Journal of the American Medical Association in 2012, also concluded that for women at increased risk of breast cancer, adding ultrasound or an MRI to a mammogram found more cancers. That study also showed a higher rate of false positives from ultrasound.

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Sharon L. Koehler, D.O., F.A.C.S., is assistant professor of breast surgical oncology in the Department of Clinical Specialties at the New York Institute of Technology College of Osteopathic Medicine. She shared some of the pros and cons of each test.

Koehler believes mammography is the best screening test for breast cancer. She said there is data to prove its efficacy. Also, the images show masses, architectural distortions, calcifications, and asymmetries.

“When performed well, it is generally not operator dependent. There may be variability depending on the technician doing the test,” Koehler told Healthline.

But mammograms expose women to small doses of radiation. Also, mammography may miss masses in dense breasts.

“3-D mammography (tomosynthesis) and ultrasonography help to eliminate this occurrence,” she said.

Breast ultrasound has its advantages, too. The technician may look for lesions hidden within dense breast tissue (parenchyma), Koehler added. There’s no radiation involved.

How effective an ultrasound exam is depends on the skill of the person performing it. Human error can lead to overlooked lesions or misinterpreted results. But unlike mammography, ultrasound can’t make out architectural distortions, calcifications, or asymmetries.

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“As long as we are aware of ultrasound’s limitations, in countries where mammogram is not available, ultrasound is a good option,” said Dr. Lusi Tumyan, City of Hope assistant clinical professor and section chief of breast imaging in the Department of Radiology.

What does that mean for women in the United States and other developed nations?

Dr. Melanie Royce, an oncologist specializing in breast cancer, said it’s important to remember these tools assess different things. Royce is director of the breast cancer multidisciplinary team at the University of New Mexico Comprehensive Cancer Center.

“One is not better than another,” she told Healthline. “They are complementary. They should be seen as such and used as such rather than one as a substitute for the other. At least this is the case where both are widely available.”

Tumyan cautions patients about the higher false-positive rate of ultrasound compared to mammography. False positives often lead to more tests, including biopsies. That can add to healthcare costs. For some women, this is cause for a lot more anxiety, Tumyan told Healthline.

“On the other hand, mammography is less sensitive in patients with dense breast parenchyma.” Tumyan said. “Ultrasound in this patient population is a great supplemental examination that is also affordable.”

There’s no single rule that covers everyone. Tumyan added that patients would benefit from individualized screening programs.

“This would entail a balanced discussion of patient risk factors, as well as pros and cons of each screening examination,” she said.

Understanding these factors will help patients to make informed decisions.