Long-term effectiveness and overdiagnosis in the NHS breast screening programme: a cohort study with up to 33 years of follow-up

BACKGROUND: Population-based mammographic screening programmes have been operating for over 30 years and whilst many studies have assessed the short- to medium-term effectiveness of screening, few have assessed their long-term effectiveness. The aim of this study was to assess the long-term effectiveness of the NHS Breast Screening Programme in England and Wales. METHODS: 2,509,384 women aged 49-64 years and uninvited to screening upon study entry were studied, with screening status changing on invitation and attendance. Women were followed for breast cancer diagnosis and death for a maximum of 33 years. Poisson regression was used to assess the effect of screening on breast cancer incidence and mortality compared to an uninvited comparison group, and overdiagnosis. RESULTS: After adjustment for confounders, invitation to screening was associated with a 28% reduction in breast cancer mortality (RR: 0.72; 95% CI: 0.71-0.74) and attendance at screening was associated with a 33% reduction (RR: 0.67; 95% CI: 0.65-0.69). There was a 4% excess of breast cancer diagnoses associated with being screened (RR: 1.04; 95% CI: 1.02-1.07) with at least 5 years follow-up post-screening. CONCLUSION: These findings indicate that screening continues to be effective in reducing breast cancer mortality many years after cessation of screening and that the level of overdiagnosis associated with long-term follow-up is low.