Mammograms Contribute to Over-Diagnosis of Breast Cancer

National Cancer Institute (NCI, Rockville, MD, USA) used data from the Surveillance, Epidemiology, and End Results (SEER) program from 1975 to 2012 to calculate the tumor-size distribution and size-specific incidence of breast cancer among women 40 years of age or older.

The researchers then calculated size-specific cancer case fatality rate for two time periods: a baseline period before the implementation of widespread screening mammography (1975-1979), and a period encompassing the most recent years, for which 10 years of follow-up data were available (2000-2002). The data was used to track how many cancers were found when small (under two centimeters), versus large, when they are presumably more life-threatening, assuming that if the true number of breast cancer cases is stable, an increase would indicate over-diagnosis.

The results showed that after the advent of screening mammography, detected breast tumors that were small increased from 36% to 68%, while detected tumors that were large decreased from 64% to 32%. The researchers concluded that if 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, implying that the remaining 132 cases were over-diagnosed, and would never have led to clinical symptoms. The study was published on October 13, 2016, in the New England Journal of Medicine (NEJM).

“The magnitude of the imbalance indicates that women were considerably more likely to have tumors that were over-diagnosed than to have earlier detection of a tumor that was destined to become large,” concluded lead author H. Gilbert Welch, MD, MPH, of DTI, and colleagues. “However, with respect to only these large tumors, the decline in the size-specific case fatality rate suggests that improved treatment was responsible for at least two thirds of the reduction in breast cancer mortality.”

“We get credit for curing disease that would never have harmed the patient. We receive positive feedback from patients thanking us for ‘saving my life,’ alarming feedback from patients with ‘missed diagnoses,’ and no feedback at all from patients whose cancer was over-diagnosed,” wrote Professor Joann G. Elmore, MD, MPH, in an accompanying editorial. “The mantras ‘all cancers are life-threatening’ and ‘when in doubt, cut it out’ require revision.”

Related Links:
Dartmouth Institute for Health Policy and Clinical Practice
Geisel School of Medicine
U.S. National Cancer Institute