Breast cancer is the most frequently diagnosed cause of death from cancer in women worldwide and the second leading cause of death from cancer in women in developed countries. Screening for breast cancer aims to reduce mortality from this cancer. Despite widespread screening programs, incidences of metastatic cancers (or advanced-stage cancers that have already spread and are therefore more deadly) have remained stable since 1975.
Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study and published in the British Medical Journal concluded that:
"Annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available."
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Data from the Surveillance, Epidemiology, and End Results (SEER) program, 1975 through 2012 and published in the New England Journal of Medicine in October, 2016 concluded that:
"Although the rate of detection of large tumours fell after the introduction of screening mammography, the more favourable size distribution was primarily the result of the additional detection of small tumours. Women were more likely to have breast cancer that was overdiagnosed than to have earlier detection of a tumour that was destined to become large. The reduction in breast cancer mortality after the implementation of screening mammography was predominantly the result of improved systemic therapy."
A Cochrane review published in June 2013 generated the data for the graphic below, essentially showing mammography screening reduced the number of deaths through breast cancer from 5 to 4 in 1,000 women: