Widespread overdiagnosis of breast cancer at screening mammography indicated in new study

The debate over the association of breast cancer screening and disease-related mortality continues today in light of the publication of new study carried out by investigators at Harvard University (MA, USA). This ecological study of 16 million women concluded that there is a significant correlation between screening mammography and increased diagnosis of small breast cancers, but no significant impact on breast cancer deaths or incidence of larger breast cancers. The authors suggest that their data, published online yesterday in JAMA Internal Medicine, indicate widespread overdiagnosis in the screening population.

The ultimate goal of screening mammography remains reducing breast cancer deaths by enabling earlier detection of the disease when it is at a more treatable stage. However, concern remains that mammography screening is causing overdiagnosis of indolent tumors that may not have otherwise become clinically apparent. Harnessing the considerable variation in mammography rates between locations across the United States, lead author Richard Wilson (Harvard University) and colleagues aimed to further investigate the association between mammography and breast cancer overdiagnosis.

The ecological study involved analysis of 16 million women aged 40 years and older living in 547 counties, all of whom reported to Surveillance, Epidemiology and End Results cancer registries in the year 2000. Of this study group, 53,207 individuals were diagnosed with breast cancer that year underwent 10-year follow-up. Within this group, the authors studied the rates of screening in each county, measuring incidence and incidence-based breast cancer deaths during the subsequent 10 years.

At the county level, a positive correlation was demonstrated between the scope of screening and breast cancer incidence (p < 0.001). No significant correlation was identified between the extent of screening and breast cancer mortality (p = 0.98). An increase in screening by 10 percent points was associated with a 16% increase in breast cancer diagnoses, but no alteration in the rate of breast cancer deaths. The same increase in screening by 10 percent points was also linked to a 25% increase in the incidence of smaller breast tumors (≤2 cm).

The authors reported that: “Across US counties, the data show that the extent of screening mammography is indeed associated with an increased incidence of small cancers but not with decreased incidence of larger cancers or significant differences in mortality. … What explains the observed data? The simplest explanation is widespread overdiagnosis, which increases the incidence of small cancers without changing mortality, and therefore matches every feature of the observed data.”

The authors do, however, highlight the need to be wary of such ecological studies owing to ecological fallacy. They concluded by commenting: “As is the case with screening in general, the balance of benefits and harms is likely to be most favorable when screening is directed to those at high risk, provided neither too frequently nor too rarely, and sometimes followed by watchful waiting instead of immediate active treatment.”

Sources: Harding C, Pompei F, Burmistrov D, Welch HG, Abebe R, Wilson R. Breast Cancer Screening, Incidence, and Mortality Across US Counties. JAMA Intern Med. doi: 10.1001/jamainternmed.2015.3043 (2015) (Epub ahead of print); The JAMA Network press release