THURSDAY, March 2, 2006 (HealthDay News) -- Getting a mammogram to detect breast cancer can save lives, but it also carries with it a 10 percent chance of being diagnosed with a "nonprogressive" cancer that would never have needed treatment, Swedish researchers report.
Similar studies on "over-diagnosis" of these non-life-threatening growths have shown rates of up to 54 percent, while one study found a rate of only 1 percent. However, none of these earlier studies was based on direct observations, as this new study was, the researchers said.
"We found that one in 10 breast cancers detected by screening mammography may be over-diagnosed," said lead author Dr. Sophia Zackrisson, a radiologist from the Epidemiological Research Group, at Malmo University Hospital. "It means that in the absence of screening, the tumour would never have given any symptoms in its host."
This level of over-diagnosis is lower than some previous studies found, Zackrisson said. "Over-diagnosis has to be put into perspective together with the number of lives saved by screening and other costs and benefits," she added.
The report appears in the March 4 edition of the British Medical Journal.
In the study, Zackrisson and her colleagues collected data on 42,283 women between 45 and 69 years of age. The women were randomly assigned to groups that either did or did not receive regular mammograms.
During the trial, the researchers analyzed the rate of over-diagnosis of breast cancer. Over-diagnosis included cases that would never otherwise have come to "clinical attention" -- meaning they wouldn't have troubled the women.
Zackrisson's team tracked the women's health outcomes until December 2001, 15 years after the trial ended.
They found that the rate of over-diagnosis of breast cancer was 10 percent in women screened when they were 55 to 69 years old, compared with women who were not screened, the researchers reported.
Whether over-diagnosis is a problem is not clear, Zackrisson said. "It is necessary to know more about the biologic characteristics of these tumors, she said. "Today, it is not possible to know which tumors are over-diagnosed. Until we know more, we have to go on with the same treatment for all tumors."
Women going for screening should be aware that there is a risk of over-diagnosis, Zackrisson said. "But this is just one side of screening -- women should be informed about all positive and negative effects of screening."
One expert found much to criticize in the study, however.
"These findings are wrong," countered Robert Smith, the director of cancer screening at the American Cancer Society. Smith thinks the method used to determine the rate of over-diagnosis was faulty, because it mixed women who were screened with those who weren't.
"You are always going to see higher rates in a group undergoing screening simply because you are finding a lot of cancers you don't expect to appear with symptoms for many years," Smith said.
Smith believes women should definitely get regular mammograms. "They shouldn't worry that any of the cancers detected are ones they would have never known about if they hadn't undergone screening," he said.
Most breast cancers are life-threatening, Smith noted. "They will kill you," he said.
"The good news is that the majority of breast cancers -- if found before symptoms develop, and especially if found before they spread to the lymph nodes -- are curable," Smith said. "That's the advantage and logic behind getting regular mammograms."
Though screening can find non-progressive cancer, that is no reason not to have mammograms, Smith said. "A small risk of being diagnosed with a non-progressive cancer is trivial to being diagnosed with a progressive cancer," he said.
"The more serious problem is over-treatment, not detection," Smith said. "If women are over-treated for non-progressive cancers, that's not a problem of mammography, that's a problem you lay at the feet of therapy and decisions about therapy."
More information
For more on breast cancer, visit the American Cancer Society.
