PETER: protocol for a randomised controlled trial of the effects of postal and digital invitations and reminders on participation in colorectal cancer screening among young adults.

INTRODUCTION: Colorectal cancer (CRC) screening programmes have reduced CRC incidence and mortality in individuals aged 50 and older in many western countries, whereas they continue to rise among younger adults where CRC screening is not routinely available. In Germany, some insurance plans have started offering screening below age 50, but adherence has remained low and evidence on the preventive potential of CRC screening below age 50 remains limited. The Effekte postalischer und digitaler Einladung und Erinnerung auf die Teilnahme an der Darmkrebsfrüherkennung bei jungen Erwachsenen: Eine randomisierte, kontrollierte Studie trial aims to assess the effect of various invitation schemes on the use of faecal immunochemical test (FIT) and the number of polypectomies and early-detected CRCs. METHODS: In a two-arm randomised controlled trial, 80 000 individuals aged 45-49 and 10 000 individuals aged 40-41 covered by the BARMER health insurance are randomised 1:1. The intervention is an invitation and reminder via paper-based mail, the control is an invitation and reminder via email. Routine healthcare data are used to track FIT use and resulting diagnostic colonoscopies, polypectomies and CRC diagnoses. The primary composite endpoint is the proportion of insured individuals aged 45-49 who underwent a polypectomy or received a CRC diagnosis within 12 months after receiving the initial invitation. Key secondary endpoints include the proportion of individuals who used the FIT, and the proportion of individuals with a positive FIT result. ETHICS AND DISSEMINATION: This study was approved by the ethics committee of the medical faculty of Heidelberg University (S-157/2025). Results will be disseminated through peer-reviewed publications. TRIAL REGISTRATION NUMBER: DRKS00037817.