County-Based Analysis Suggests Widespread Overdiagnosis of Breast Cancer in Mammography Screening


Key Points

  • Across counties in the United States, each 10% increase in screening was associated with a 16% increase in diagnosis but no change in mortality.
  • Each 10% increase in screening was associated with a 25% increase in detection of small tumors but no reduction in detection of larger tumors.

In a study reported in JAMA Internal Medicine, Harding et al found that on the county level in the United States, mammography screening for breast cancer is associated with an increase in detection of small breast cancers but no reduction in breast cancer mortality.

Study Details

This ecologic study involved data from approximately 16 million women aged ≥ 40 years residing in 547 counties reporting to the Surveillance, Epidemiology, and End Results (SEER) cancer registries during the year 2000. Of these women, 53,207 were diagnosed with breast cancer in 2000 and were followed for the next 10 years. The study covered the period January 1, 2000, to December 31, 2010, and the analysis was performed between April 2013 and March 2015. The extent of screening in each county was assessed as the percentage of women who had received a screening mammogram in the prior 2 years. The incidence of and mortality from breast cancer were calculated for each county and age adjusted to the U.S. population.

During follow-up, fewer deaths were attributed to breast cancer (7,729, 42.4%) than to other causes (10,511, 57.6%). The median percentage of women across counties who had a mammogram in the past 2 years as of 2000 was 62.2%, with a range of 39.1% to 77.8%.

Increased Diagnosis, No Change in Mortality

Across counties, the extent of mammography screening was positively correlated with breast cancer incidence (weighted r = 0.54, P < .001) but not with breast cancer mortality (weighted r = 0.00, P = .98). Each absolute increase of 10% in the extent of screening yielded an increase of 16% in breast cancer diagnoses (relative rate [RR] = 1.16, 95% confidence interval [CI] = 1.13–1.19; absolute difference of 35–49 cases per 100,000 population) but no significant change in breast cancer deaths (RR = 1.01, 95% CI = 0.96–1.06; absolute difference of −2 to +3 deaths per 100,000).

Analysis stratified by tumor size showed that each 10% increase in screening was associated with a 25% increase in the incidence of small (≤ 2 cm) breast cancers (RR = 1.25, 95% CI = 1.18–1.32) and a 7% increase in the incidence of larger (> 2 cm) breast cancers (RR = 1.07, 95% CI = 1.02–1.12).

Each 10% increase in screening was associated with an increased incidence of early disease (stage 0–II; RR = 1.22, 95% CI = 1.17–1.28) and no change in the incidence of locally advanced or metastatic disease (stage III–IV; RR = 1.02, 95% CI = 0.97–1.07) and with greater frequency of breast-conserving surgical procedures (RR = 1.24, 95% CI = 1.15–1.34) but no reduction in non–breast-conserving surgical procedures (eg, total and radical mastectomy).

The investigators concluded: “When analyzed at the county level, the clearest result of mammography screening is the diagnosis of additional small cancers. Furthermore, there is no concomitant decline in the detection of larger cancers, which might explain the absence of any significant difference in the overall rate of death from the disease. Together, these findings suggest widespread overdiagnosis.”

Richard Wilson, DPhil, of Harvard University, is the corresponding author of the JAMA Internal Medicine article.

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