Long-term effects of once-only flexible sigmoidoscopy screening on colorectal cancer incidence and mortality: 21-year follow-up of the UK Flexible Sigmoidoscopy Screening randomised controlled trial

BACKGROUND: Flexible sigmoidoscopy screening reduces colorectal cancer incidence and mortality; however, uncertainty exists about the duration of protection and differences by sex and age. We assessed effects of once-only flexible sigmoidoscopy screening after 21 years' follow-up. METHODS: The UK Flexible Sigmoidoscopy Screening Trial is a multicentre randomised controlled trial that recruited men and women aged 55-64 years from general practices serving 14 hospitals. Among participants indicating that they would attend flexible sigmoidoscopy screening if invited, randomisation (2:1) to the control (no further contact) or intervention (invited to once-only flexible sigmoidoscopy screening) group was performed centrally in blocks of 12, stratified by centre, general practice, and household type. Masking of intervention was infeasible. Primary outcomes were colorectal cancer incidence and mortality. The Kaplan-Meier method estimated cumulative incidence. Primary analyses estimated intention-to-treat hazard ratios (HRs) and risk differences, overall and stratified by subsite, sex, and age. The trial is registered with ISRCTN, number 28352761. FINDINGS: =0·0007) but there was no difference by age. INTERPRETATION: We show that once-only flexible sigmoidoscopy screening reduces colorectal cancer incidence and mortality for two decades and provide important data to inform colorectal cancer screening guidelines. FUNDING: National Institute for Health and Care Research Health Technology Assessment Programme and the Medical Research Council.