• Posted March 1, 2022

Fewer Breast Cancers May Be 'Overdiagnosed' by Mammograms Than Thought

Screening mammograms can lead to overdiagnosis of breast cancer, but a new study finds it happens less often than experts have thought.

Researchers estimated that about 15% of breast cancers caught through routine mammography screening are overdiagnoses -- meaning the tumors would never have caused harm if they had not been detected.

The figure suggests those diagnoses are about half as common as some previous, widely reported studies have estimated.

"The good news is, it's less common than we'd thought," said Dr. Katrina Armstrong, of Massachusetts General Hospital in Boston.

She's co-author of an editorial published with the study March 1 in the Annals of Internal Medicine.

The problem with overdiagnosed cancers, Armstrong explained, is that they lead to unnecessary treatment, and the side effects and emotional toll that go with it.

Still, the odds of that happening are low for any one woman undergoing breast cancer screening.

According to Armstrong, about 7 in 1,000 women are diagnosed with breast cancer via mammography screening. So based on the new estimate, roughly 1 in 1,000 women who undergo screening will be diagnosed with a cancer that would never have caused problems.

While it's good to have that information, Armstrong said, it is unlikely to sway women's decisions on mammography. The concept of "overdiagnosis" is a difficult one, she said, and with the low risk to any one patient, it's generally thought that the benefits are worth it.

But on the population level, Armstrong said, overdiagnosis is a significant issue. In the United States alone, around 280,000 women are diagnosed with breast cancer each year -- many through mammography screening.

If overdiagnosed cases could be eliminated, Armstrong said, that would spare roughly 25,000 women from unnecessary treatment.

Why does overdiagnosis happen?

No screening test is perfect, and there are always downsides, said senior study author Ruth Etzioni, a professor at Fred Hutchinson Cancer Research Center in Seattle.

By definition, screening means testing symptom-free people for early evidence of disease. There is always a risk of a false positive result that may lead to needless invasive testing, as well as the risk of overdiagnosis.

"Screening is complex, and we need to acknowledge that," Etzioni said.

Overdiagnosis can result from finding a tumor that would never have progressed to the point of causing harm; in other cases, the tumor is growing, but would not have advanced to "clinical disease" before the person died of another cause.

With breast cancer, there is a noninvasive form called ductal carcinoma in-situ (DCIS) -- where cancer cells are present in the milk ducts, but not the breast tissue.

Most DCIS tumors do not progress to invasive cancer, Armstrong said. But there is no way to predict with certainty which ones will or will not.

There are trials underway, Armstrong noted, looking at whether DCIS can be safely managed with monitoring instead of immediate treatment.

But right now, at least in the United States, women with DCIS usually receive treatment: surgery, often followed by radiation and possibly hormonal therapy.

The current study used data from nearly 36,000 U.S. women ages 50 to 74 in the Breast Cancer Surveillance Consortium. Altogether, they underwent close to 83,000 screening mammograms between 2000 and 2018 -- leading to 718 breast cancer diagnoses.

"This is the highest-quality mammography database in the U.S.," said lead researcher Marc Ryser, of Duke University Medical Center in Durham, N.C.

Because of that -- and because of "flawed" methodology in past studies -- Ryser said the findings provide a more reliable estimate of the scope of breast cancer overdiagnosis.

Etzioni did stress that the findings do not provide a woman with a "personal" estimate. Instead, they clarify the long-term risk of overdiagnosis among women who are screened every other year, between the ages of 50 and 74 -- as recommended by the U.S. Preventive Services Task Force.

In the real world, women may be screened more often. Some medical groups recommend that average-risk women begin screening at age 40 or 45, as often as once a year.

To Armstrong, the field has a "responsibility" to prevent overdiagnosis, and overtreatment, as much as possible.

It can be done, she said, through ongoing research. Studies are looking at ways to improve the accuracy of screening technologies, and hunting for better ways to predict which breast tumors will progress.

Armstrong also stressed the importance of modifiable risk factors for breast cancer. Regular exercise and drinking only in moderation, if at all, are two ways women can help lower their risk of the disease.

More information

The American Cancer Society has more on mammography screening.

SOURCES: Ruth Etzioni, PhD, professor, public health sciences division, Fred Hutchinson Cancer Research Center, Seattle; Marc Ryser, PhD, assistant professor, population health sciences, Duke University Medical Center, Durham, N.C.; Katrina Armstrong, MD, physician-in-chief, Department of Medicine, Massachusetts General Hospital, Boston; Annals of Internal Medicine, March 1, 2022, online