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In a "best-guess estimate," they assumed that breast cancer incidence increased by 0.25% over the study period. "This approach suggests that the excess detection attributable to mammography ... involved more than 1.3 million women in the past 30 years."
The "extreme" model assumed that incidence increased by 0.5% each year, an estimate that minimized surplus diagnoses of early-stage disease and maximized the deficit of late-stage cancer. This model found that screening mammography detected an excess of 1.2 million cancers over the study period.
The "very extreme assumption" model assumed that incidence increased by 0.5% each year, and that the baseline incidence of late-stage disease was the highest ever observed (113 cases per 100,000 women, in 1985). Even with this model – the most favorable toward mammography – the authors estimated overdiagnosis of slightly more than 1 million women.
"Our analysis suggests that whatever the mortality benefit, breast-cancer screening involved substantial harm of excess detection of additional early-stage cancers that was not matched by a reduction in late-stage cancers. This imbalance indicates a considerable amount of overdiagnosis involving more than 1 million women in the past 3 decades – and, according to our best-guess estimate, more than 70,000 women in 2008."
SEER breast cancer mortality data help put the findings into perspective, the authors said. There has been a substantial reduction in mortality, which fell from 71/100,000 to 51/100,000 over the study period. "This reduction in mortality is probably due to some combination of the effects of screening mammography and better treatment," they said. But because the absolute reduction in deaths (20/100,000) was larger than the absolute reduction in late-stage cancer (8/100,000), screening mammography can’t be the main driver of change.
"Furthermore ... the small reduction in cases of late-stage cancer that has occurred has been confined to regional (largely node-positive) disease – a stage that can now often be treated successfully, with an expected 5-year survival rate of 85% among women 40 years of age or older. Unfortunately, however, the number of women in the United States who present with distant disease, only 25% of whom survive for 5 years, appears not to have been affected by screening."
Better therapy may even have altered the impact of screening, they added.
"Ironically, improvements in treatment tend to deteriorate the benefit of screening. As treatment of [disease detected by methods other than screening] improves, the benefit of screening diminishes. For example, since pneumonia can be treated successfully, no one would suggest that we screen for pneumonia."
The findings show that there are not absolutes for women considering whether or not to get a mammogram.
"Proponents of screening should provide women with data from a randomized screening trial that reflects improvements in current therapy and includes strategies to mitigate overdiagnosis in the intervention group. Women should recognize that our study does not answer the question, ‘Should I be screened for breast cancer?’ However, they can rest assured that the question has more than one right answer."
Dr. Bleyer disclosed that he is a consultant and speaker for Sigma-Tau Pharmaceuticals. Dr. Welch disclosed that he speaks on the topic of overdiagnosis with universities and medical schools, as well as to the Pharmacy Benefit Management Institute; all honoraria were donated to charitable organizations.
