Norwegian Study Estimates Overdiagnosis of Breast Cancer from Screening
Overdiagnosis refers to the detection of tumors that, if left alone, would not cause any symptoms of disease or death. Because doctors cannot reliably distinguish these tumors from potentially fatal cancers, most screening-detected invasive breast cancers are treated, often with surgery and postoperative therapy that includes radiation plus hormone therapy, chemotherapy, or both systemic therapies.
The new findings, together with previous studies, suggest that a substantial proportion of screened women are diagnosed with invasive cancers unnecessarily and are exposed to the toxic effects of treatments they don't need.
The investigators analyzed data from the Norwegian Breast Cancer Screening Program, which began as a pilot program in 1996 in four of Norway's 19 counties and was later expanded to include the remaining counties over a 9-year period. Specifically, they compared the number of breast cancers diagnosed in counties that had screening programs in place with the number of cases diagnosed in counties that did not have screening programs during the same period.
The results suggest that between 15 and 25 percent of invasive breast cancers were overdiagnosed with the implementation of a mammography screening program. After 10 years of biennial screening, the study authors estimated that for every 2,500 women invited to be screened, 6 to 10 women were overdiagnosed, 20 were diagnosed with breast cancer that required treatment, and 1 death from the disease was prevented.
The analysis did not include noninvasive tumors known as ductal carcinoma in situ (DCIS), even though most of these lesions are only detectable by mammography. The authors said DCIS should be analyzed separately because different analytic methods would be required. Inclusion of DCIS would have further increased the estimates of overdiagnosis, since a large proportion of these tumors would never become life-threatening cancers.
The authors also acknowledged that additional factors beyond screening might have affected breast cancer rates in the counties.
Nonetheless, the "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," the authors wrote. Until doctors can reliably identify potentially fatal cancers that require early detection and treatment, "women eligible for screening need to be comprehensively informed about the risk for overdiagnosis," they concluded.
The authors of an accompanying editorial emphasized that women in the United States often start annual mammography screening at age 40, whereas Norwegian women start biennial screening at age 50. "Given more frequent screening over a longer time, overdiagnosis probably occurs more often in the United States than in Norway," they wrote.
Any amount of overdiagnosis is serious, however, and steps should be taken to address the issue, the editorialists continued. Most patient-education materials fail to mention overdiagnosis, and most women are unaware of its possibility. "We have an ethical responsibility to alert women to this phenomenon," they concluded.
"Overdiagnosis from cancer screening is one of the most pressing clinical issues in the field of cancer screening," said Dr. Barry Kramer, director of NCI's Division of Cancer Prevention and editor-in-chief of the NCI Physician Data Query (PDQ) Screening and Prevention Editorial Board.
"With increasingly sensitive screening tests for a variety of cancers, the problem is likely to increase," Dr. Kramer continued. "For that reason, NCI has identified studies to distinguish overdiagnosed cancers from life-threatening cancers as a high priority area of research."
—Edward R. Winstead