Screening mammography may result in overdiagnosis of breast cancer

1. Screening mammography practices resulted in an increase in breast cancer incidence.

2. Despite an increased incidence of small breast cancers, there was no associated decrease in incidence of larger breast cancers.

Evidence Rating Level: 2 (Good)

Study Rundown: Screening mammography is intended to decrease breast cancer mortality by detecting smaller cancers at earlier, potentially more treatable stages. However, if the diagnosis of larger breast cancers does not concomitantly decrease, screening mammography may actually be leading to the overdiagnosis of smaller tumors that may not become clinically apparent. The possibility of overdiagnosis has not been thoroughly studied. This study examined the association between rates of screening mammography and the incidence and mortality of breast cancer.

In this study, there was a significant increase in breast cancer incidence, but no difference in breast cancer mortality. There was an increase in detection of small breast cancers. However, no complementary decrease in large breast cancer diagnoses was noted, perhaps indicating that screening mammographies may result in overdiagnosis. Strengths of this study include being a large study with over 16 million women as a study population. However, all ecological studies are subjected to ecological fallacy.

Click to read the study, published today in JAMA Internal Medicine

Click to read an invited commentary in JAMA Internal Medicine

Relevant Reading: Quantifying and monitoring overdiagnosis in cancer screening: a systematic review of methods

In-Depth [ecological study]: This study used data from the Surveillance, Epidemiology and End Results (SEER) cancer registries from January 1 to December 31, 2000, which reports from 547 counties in the United States. The authors identified women who were diagnosed with breast cancer, as well as the screening mammography practices from each county. Women that were diagnosed with cancer were followed for the next 10 years for outcomes, including mortality. The exposure of interest was the percentage of women in each county who had a mammogram in the past 2 years as of 2000. The outcomes of interest were the incidence of breast cancer, mortality and tumour size.

There was a positive association between the extent of screening mammography and breast cancer incidence (weighted r 0.54; p < 0.001). However there was no association between breast cancer mortality and the degree of screening mammography (weighted r 0.00; p = 0.98). An increase of 10% in the degree of screening resulted in a 16% increase in breast cancer incidence (RR 1.16, 95%CI 1.13-1.19) with no significant difference in breast cancer mortality (RR 1.01, 95%CI 0.96-1.06). When comparing degree of screening and tumour size, there was a strong association with incidence of small breast cancers (≤2 cm) but no accompanying decrease in the incidence of large breast cancers (>2 cm). Specifically, with a 10% increase in the extent of screening, there was a 25% increase in small breast cancer incidence (RR 1.25, 95%CI 1.18-1.32) and a 7% increase in large breast cancer incidence (RR 1.07; 95%CI 1.02-1.12).

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