According to a new study published Monday in the Annals of Internal Medicine, one in seven women diagnosed with breast cancer are "overdiagnosed"—a finding that experts say sheds light on the vague information on overdiagnosis previously available.

Overdiagnosis refers to the diagnosis of tumors that would have caused less harm if they had remained hidden. Marc Ryser, lead author of the study and an assistant professor at Duke University, explained, "The real harm comes in this: every woman with breast cancer gets a lot of pretty harmful invasive treatments, and for a woman with overdiagnosed breast cancer, it's all for naught. She does not derive any benefit, because she would never have known about it, and she would have died with it, but not from it. She was made a cancer patient for no reason."

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Study details and key findings

For the study, Duke researchers analyzed data from the Breast Cancer Surveillance Consortium—a platform that tracks screen-detected breast cancer cases. They included data from 35,986 women between the ages of 50 and 74.

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Of the patients included in the study, 64% were white, 19% were Asian, 12% were Black, and 11% identified as Hispanic. 

For their analysis, researchers built a model that estimated the percentage of tumors that never progress, the amount that progress, and the average amount of time it would take for symptoms to emerge.

Among study participants, the overdiagnosis rate increased from 11.5% at the first screening at age 50, to 23.6% at the last screening at age 74. The median age for participants' first screening was 56. On average, participants received 2.3 screenings. In total, 718 breast cancers were diagnosed among participants, including 80% that were invasive, and nearly 20% that were found at the earliest stages.

In addition to predicting that overdiagnoses occurred in 15.4% of screen-detected breast cancer cases, researchers also found that around one-third of overdiagnoses came from indolent or non-progressive tumors. The other two-thirds of overdiagnoses came from progressive cancers generally found in older women in their model, but the patients typically died of other causes before the cancer became lethal.

Commentary

Previously, scientists were unable to agree on an estimated overdiagnosis rate. "The range was from zero to 54%, so a huge range and virtually no consensus as to what the true rate [of overdiagnosis] was," Ryser said.

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One of the most prominent estimates came from a 2012 New England Journal of Medicine paper that suggested an overdiagnosis rate of around 31%.

"[The Duke study] was to correct the [poor] information that has been out there about breast cancer overdiagnosis for decades," said Ruth Etzioni, a biostatistician and professor at the Fred Hutchinson Cancer Research Center and the senior author on the study.

In general, the study's findings underscore that the benefits of mammography outweigh the risks. And according to Etzioni, the study's newly estimated overdiagnosis rate is the first step in providing clinicians with information that will help patients make more informed decisions about their treatment.

"Honestly, [the study] is reassuring," said Michael Hassett, an oncologist at the Dana-Farber Cancer Institute who did not work on the study. "Most of the cases we're finding are not overdiagnosis cases and most are true cases. The problem we're left with is less about overdiagnosis and more about how do we tailor the intensity of treatment to intensity of cancer."

"The frontier of really dialing down treatment is where it's at right now," Etzioni said, adding that the study's estimate is an average, and does not account for the overall health of a woman, the type of tumor, or any other factors that may influence decision making. "There is no good predictive model for that progression."

Still, experts said the study's findings don't minimize the dangers associated with overdiagnosis. (Gillespie, Modern Healthcare, 2/28; Chen, STAT News, 2/28; Ryser et al., Annals of Internal Medicine, 3/1)