Overdiagnosis of breast cancer was more likely to occur than earlier detection of a tumor that would become large after implementation of widespread screening mammography, according to a study published in the New England Journal of Medicine.

The rate of detection of large tumors decreased after the introduction of screening mammography, but this more favorable tumor-size distribution was principally the result of increased detection of small tumors. According to these results, decreased mortality from breast cancer since screening mammography is primarily the result of improved systemic therapy.

Screening mammography aims to detect small malignancies before they are sufficiently large to be symptomatic. Therefore, effective screening would result in detecting a greater number of small tumors and, over time, fewer large tumors.

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Researchers used data from the Surveillance, Epidemiology, and End Results (SEER) program from 1975 to 2012. They calculated tumor-size distribution and size-specific incidence of breast cancer in women aged 40 years or older.

This study then assessed the rate of size-specific cancer case fatality from the years prior to the broad implementation of screening mammography, 1975 to 1979, and from a time period containing the most recent years in which 10 years of follow-up data were available, 2000 to 2002.

After the introduction of screening mammography, the total proportion of small tumors (invasive and less than 2 cm or in situ carcinomas) increased from 36% to 68%. Similarly, the proportion of large tumors (invasive tumors 2 cm or larger) fell from 64% to 32%.

These changes, however, were due primarily to a significant increase in the detection of small tumors, with 162 more cases of cancer observed per 100,000 women.

When researchers applied the assumption of a stable disease burden over time, only 30 of the additional 162 small tumors were expected to become large tumors. This implied 132 cases per 100,000 women were over diagnosed. In other words, these were cases of detection of tumors that were unlikely to ever result in clinical symptoms.

Although decreased detection of large tumors indicates the potential of screening to reduce the mortality of breast cancer, the reduced rate of size-specific case fatality indicates treatment improvements as responsible for at least two-thirds of the improvement.


1. Welch HG, Prorok PC, O’Malley AJ, Kramer BS. Breast-cancer tumor size, overdiagnosis, and mammography screening effectiveness. N Engl J Med. 2016;375(15):1438-1447.