1. After advent of mammography, detection rate of large tumors decreases while detection of smaller tumors increases.
2. The decreased rate of large tumor detection was a direct result of increased detection of smaller tumors and not due to a decreased incidence rate.
Evidence Rating Level: 2 (Good)
Study Rundown: This retrospective cohort study aimed to evaluate the effectiveness of mammography screening in clinical practice – namely, to assess the mortality benefit vs. the pitfall of potential harm through overdiagnosis. The researchers used data between 1975-2012 from the Surveillance, Epidemiology and End Results (SEER), a population-based registry of cancer incidence in the United States. Their study demonstrated that after screening mammography became widely introduced between the years of 1982-1990, there was an increase in the detection of small invasive tumors compared to large carcinomas. In assessing the relative risk of 10-year mortality in breast cancers by size, those that were >2cm had comparatively higher risks. The authors concluded that the rate of detection of large tumors likely decreased after the advent of screening mammography due the increased detection of smaller tumors, translating into a presumed reduction of mortality. However, it was felt that the increase in detection of small tumors, assuming a stable underlying disease burden, may have also represented an increase in overdiagnosis.
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In-Depth [retrospective cohort]: Researchers performed analyses on SEER data for women ≥40 years of age. They classified tumor size and calculated the 10-year risk of death based on tumor size stratification. After screening mammography was introduced, proportion of detected small invasive tumors (<2 cm or in situ carcinomas) increased from 36% to 65% while the proportion of detected large carcinomas decreased from 64% to 32% between 1975-2012. Prior to mammography, the incidence of large tumors predominated. There was an overall 1.8x fold increase in the detection of small invasive tumors (<2cm, or in situ carcinomas), compared to a 2x fold decrease in the detection rates of large carcinomas. The incidence of large tumors decreased by 30 cases/100,000 women (from 145 to 115) and the incidence of small tumors increased by 162 cases/100,000 women (from 82 to 244). Under the assumption that the burden of clinically significant breast cancer remained unchanged throughout this time, the authors inferred that these 30 cases/100,000 women were likely to become large tumors, while the remaining 132 represented overdiagnoses. In terms of mortality, reduction in mortality due to mammography was approximately 12 deaths/100,000 women. In terms of 10-year risk of death from breast cancer in large tumors, the relative risk for tumors ≥5.0cm, tumors between 3.0-4.9cm and 2.0-29cm was 0.79 (95%CI 0.74-0.84), 0.70 (95%CI 0.65-0.75) and 0.58 (95%CI 0.53-0.62), respectively. For small tumors, the relative risk for tumors <1.0cm and tumors between 1.0-1.9cm was 0.26 (95%CI 0.21-0.32) and 0.38 (95%CI 0.34-0.42).
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