What is the effect of screening mammography on breast cancer incidence?

Objective: To determine if screening mammography produced the expected increase in the incidence of early-stage breast cancer and reduced the expected incidence of late-stage breast cancer in women 40 years of age or older. Design: Population-based observation study using a before and after cohort time series design. Data Sources: 1) National Health Interview Survey; 2) Surveillance, Epidemiology and End Results (SEER) data; 3) United States Census. Results: The introduction of screening mammography in the United States has been associated with a doubling in the number of cases of early-stage breast cancer detected each year from 112 to 234 cases per 100 000 women — an absolute increase of 122 cases per 100 000 women. Concomitantly, the rate at which women present with late-stage cancer has decreased from 102 to 94 cases per 100 000 women — an absolute decrease of 8 cases per 100 000 women. With the assumption of a constant underlying disease burden, it was estimated that only 8 of the 122 additional early-stage cancers diagnosed would progress to advanced disease. After excluding the transient excess incidence associated with hormone replacement therapy and adjusting for trends in the incidence of breast cancer among women younger than 40 years old, it was estimated that breast cancer was overdiagnosed (i.e., tumours were detected on screening that would never have led to clinical symptoms) in 70 000 women in 2008 and 1.3 million women in the past 30 years. Conclusion: Despite substantial increases in the number of early-stage breast cancer detected, screening mammography has only marginally reduced the rate at which women present with late-stage cancer.