The influence of breast screening on breast cancer incidence in England: observational study based on cancer registries and bulletins of the NHS Breast Screening Programme.

Background: To assess the amount of breast cancer overdiagnosis associated with the National Health Service Breast Screening Programme (NHSBSP) that started in 1988 in England.

Methods: First, numbers of breast cancers in women eligible for breast screening not attending screening were estimated for the period 1995-2019, which were extrapolated to all women. A second method was based on ratios of incidence rates of breast cancers in women aged 50-69 to women aged 70 years or more in 1971-1985. The ratio was used for estimating expected numbers of cancers in 1988-2019, and 1995-2019.

Results: From 1995 to 2019, 506,607 non-invasive and invasive breast cancers were diagnosed among women aged 50-64 years (1995-2001) and 50-70 years (2002-2019). A first method estimated that 95,297 cancers were in excess to the number of cancers that would be expected had the NHSBSP not existed. 42,567 screen-detected non-invasive and micro-invasive cancers represented 45.8% of the total excess cancer. 18.8% of all cancers diagnosed among women invited to screening, 25.1% of cancers found in women attending screening, and 35.1% of cancers detected by screening would represent overdiagnosis. A second method estimated that, 18.0% of all cancers diagnosed in 1988-2019, and 18.2% of all cancers diagnosed in 1995-2019 among women invited to screening would represent overdiagnosis.

Conclusion: The two independent methods obtained similar estimates of overdiagnosis. The NHS Breast Screening Programme in England is associated with substantial amount of overdiagnosis.