Overdiagnosis with screening mammography may be more likely than detection of large tumors

Researchers used data from 1975 through 2012 to examine tumor size and size-specific breast cancer incidence in women ages 40 and older.

Women ages 40 and older who undergo screening mammography are more likely to be overdiagnosed with breast cancer than to have a potentially large tumor be detected, according to a recent study.

Researchers used data from the Surveillance, Epidemiology, and End Results (SEER) program from 1975 through 2012 to examine tumor size and size-specific breast cancer incidence in women ages 40 and older. Size-specific cancer case fatality rate was then calculated for a baseline period when widespread screening mammography was not common (1975-1979) and for a period when screening mammography was widespread with 10 years of follow-up data available (2000-2012). Small tumors were defined as those measuring less than 2 cm or as in situ carcinoma; large tumors were defined as those measuring at least 2 cm. The study results were published in the Oct. 13 New England Journal of Medicine.

After screening mammography began, the proportion of detected breast tumors that were small increased from 36% to 68% while the proportion that were large decreased from 64% to 32%. The authors determined that this trend was due mainly to a large increase in the detection of small tumors (162 more were observed per 100,000 women after introduction of screening mammography vs. before) rather than to an increase in incidence of large tumors (30 fewer were observed per 100,000 women after introduction of screening mammography vs. before).

“Assuming that the underlying disease burden was stable, only 30 of the 162 additional small tumors per 100,000 women that were diagnosed were expected to progress to become large, which implied that the remaining 132 cases of cancer per 100,000 women were overdiagnosed (i.e., cases of cancer were detected on screening that never would have led to clinical symptoms),” the authors wrote. They noted that decreasing incidence of larger tumors is linked to screening mammography's ability to decrease breast cancer mortality but that for large tumors in their study, the decrease in the size-specific case-fatality rate suggested that at least two-thirds of the reduction in breast cancer mortality was due to improved treatment.

The authors emphasized that their findings are limited because the underlying true incidence of breast cancer could not be observed, among other factors, and said that their estimates of the extent of overdiagnosis and the reduction in breast cancer mortality related to screening mammography are not precise. “The data regarding size-specific incidence, however, make clear that the magnitude of overdiagnosis is larger than is generally recognized,” the authors wrote. “Furthermore, the data regarding size-specific case fatality rate clarify that decreasing breast-cancer mortality largely reflects improved cancer treatment.”

The author of an accompanying editorial agreed with the study's limitations but said that “Rather than focusing on statistical issues and study design, we should move forward by agreeing that overdiagnosis does occur, even though the exact percentage of overdiagnosed cases remains unknown.” The editorialist noted that any solutions to the problem must involve both population-level and patient-level approaches and said that incentives and feedback systems must also be modified. “The threat of medical malpractice litigation coupled with financial incentives to do more can conflict with our goal of helping patients,” the editorialist wrote. “The mantras, ‘All cancers are life-threatening’ and ‘When in doubt, cut it out,’ require revision.”

In addition, the editorialist noted, clinicians “need better tools to evaluate medical data and classify findings on the basis of clinical judgment,” better ways to tell which tumors are self-limiting and which will progress, and better methods of communicating about overdiagnosis at both population and patient levels. “Building trust in science and medicine starts by taking ownership of all aspects of the screening cascade, including the collateral damage of our well-intentioned efforts,” the editorialist wrote.