Up to a fourth of breast cancers found through mammograms are harmless and would not cause noticeable disease during the women's lifetimes, a new study from Norway suggests.
Such women are said to be overdiagnosed, because their cancers would not have caused symptoms or death. Overdiagnosis is a problem because women may receive tests and treatment they did not actually need.
However, researchers currently have no way of knowing which women have these cases; the study showed that it's statistically probable that such cases are occurring.
"These women are treated with all the harms of treatment, and without the benefit of it," said study researcher Mette Kalager, at the Telemark Hospital in Norway.
In the study, for every 2,500 women who underwent mammogram screening over a 10-year period, 20 cases of breast cancer were identified that would have caused significant disease if they went undetected, and one death was prevented. However, 6 to 10 cases of overdiagnosis occurred.
Women should be informed about the benefits and risks of breast cancer screening, including the risk of overdiagnosis, so they can decide if they want a mammogram.
"You have to weigh the benefits and the harms, and you have to make up your mind [about] what is most important for you," Kalager said.
Most guidelines recommend mammograms for older women, but there are differences about what age mammograms should start, and how frequently they should be done. The U.S. Preventive Services Task Force recommends women ages 50 to 74 receive a mammogram every two years, while the American Cancer Society recommends yearly mammograms beginning at age 40.
Breast cancer overdiagnosis
Any screening test that attempts to identify a disease early on will come with a risk of overdiagnosis. Previous studies have found that rates of overdiagnosis from mammograms vary, from 0 percent to 54 percent, Kalager said.
In the new study, Kalager and colleagues analyzed cases of invasive breast cancer that occurred in Norway between 1996 and 2005, a time period over which the country began a breast cancer screening program for women ages 50 to 69. Different regions of the country began the screening program at different times, and the researchers compared cases among women who had been offered screening with those not offered screening.
About 7,800 women were diagnosed with breast cancer during the 10-year study period. Of these, the researchers estimated between15 percent and 25 percent were overdiagnosed — in other words, between 1,169 and 1,948 women would never have faced illness due to their cancers.
Because screening is started earlier in the United States than in Norway, overdiagnosis probably occurs more often here, according to Dr. Joann Elmore, of the University of Washington School of Medicine in Seattle, and Dr. Suzanne Fletcher, of Harvard Medical School in Boston, who wrote an editorial accompanying the study.
Patients want control
But while a doctor might consider the diagnosis of a cancer that never causes symptoms or death to be overdiagnosis, from a patient's perspective, it might be exactly what they want "because they want control of their lives," said Dr. Leo Twiggs, a professor of obstetrics and gynecology at the University of Miami School of Medicine, who was not involved in the study.
A person who is diagnosed with pre-cancer might want to change their behavior, such as increasing the frequency of their mammograms, Twiggs said.
Overdiagnosis and overtreatment could be reduced if researchers had tools to distinguish between cancers that are likely to progress and those that are unlikely to cause any problems within a patient's lifetime, Kalager said. But currently, we don't have those tools, she said.
A "watch and wait" approach may also be a tough sell for anxious patients, or for radiologists who don't want to be sued if they miss signs of disease, Elmore and Fletcher said in the editorial.
"Nevertheless, unless serious efforts are made to reduce the frequency of overdiagnosis, the problem will probably increase," as new imaging techniques are introduced, they wrote.
The study and editorial are published in the April 3 issue of the Annals of Internal Medicine.
Pass it on: Between 15 and 25 percent of breast cancer cases diagnosed through mammography may be cases of overdiagnosis.