The introduction of widespread screening mammography in Norway was associated with an estimated 15%-25% "overdiagnosis" of breast cancer there, according to a report in the April 3 issue of the Annals of Internal Medicine.
This finding is consistent with those of previous studies in other countries, which estimated rates of overdiagnosis ranging from 0% to 54%, with randomized controlled trials tending to estimate it at approximately 30%, investigators said.
The result of this study in Norway adds to the evidence that "overdiagnosis and unnecessary treatment of nonfatal cancer creates a substantial ethical and clinical dilemma and may cast doubt on whether mammography screening programs should exist," said Dr. Mette Kalager of the department of epidemiology, Harvard School of Public Health, Boston, and her associates.
Screening mammography was uncommon in Norway until a national, state-funded program began in 1996. Screening mammography was then implemented gradually, in different geographic areas, over the course of 10 years. Since 2005, all women in Norway aged 50-69 years have been invited to participate in mammographic screening every 2 years, and approximately 77% of them do so.
"This staggered implementation allowed comparison of contemporaneous trends in breast cancer incidence in areas with and without mammography screening," as well as the comparison of current and historical trends in incidence. This in turn gave the researchers two analytic methods for calculating estimates of overdiagnosis.
They defined overdiagnosis as the percentage of cases of cancer that would not have become clinically apparent in a woman’s lifetime without screening. It refers to cases of breast cancer that would be diagnosed and treated without yielding any possible survival benefit – cases in which the tumor would never have progressed to a clinical stage or in which the woman would die from other causes before the breast cancer became evident.
The study population comprised 39,888 women diagnosed as having invasive breast cancer in 1996-2005, including 27,238 who were aged 50-79 years at diagnosis. A total of 7,793 women were diagnosed after the introduction of routine screening.
In fully adjusted analyses using the first method of estimating, 15%-20% of the cases of breast cancer found on screening mammography – that is, between 1,169 and 1,948 women – were overdiagnosed. In an analysis using the second method of estimating, 18%-25% of cases were overdiagnosed.
Thus, the overall rate of overdiagnosis in this study was 15%-25%, Dr. Kalager and her colleagues said (Ann. Intern. Med. 2012;156:491-9).
Moreover, the proportion of advanced-stage breast cancers decreased over time to the same degree among screened and unscreened women, while the proportion of stage I breast cancers markedly increased only among screened women. This indicates that the cancers being found on screening mammography were almost entirely early-stage, low-risk tumors.
"Our findings suggest that enhanced awareness is probably the reason for the reduction of late-stage cancer, not screening," the investigators noted.
Extrapolating their findings, they added that "after 10 years of biennial mammography screening, for every 2,500 women invited, 6-10 women have been overdiagnosed, 20 women are diagnosed with breast cancer ... and one death from breast cancer has been prevented.
"To put it differently, if 2,500 women are invited to undergo mammography screening over 10 years, 2,470-2,474 women will not be diagnosed with breast cancer, 2,499 will not die of breast cancer, but 6-10 women will be overdiagnosed," they said.
This study was supported by the Research Council of Norway and Frontier Science. Dr. Kalager and her coauthors had no relevant disclosures.