Sequential invitations to FOBT screening and colorectal cancer incidence

The effect of different sequences of invitations to Faecal Occult Blood Test (FOBT) screening regarding colorectal cancer (CRC) incidence has never been evaluated. In 2008-2012, all residents in Stockholm-Gotland, Sweden, born 1938-1954, were randomly assigned by birth year to different calendar years of invitation to guaiac-based FOBT (g) or Faecal Immunochemical Test (f) screening at 60-69 years (1-5 rounds), or not (0). Linkage was made to the national Cancer- and Cause of Death Registers on CRC diagnosis and mortality 1958-2020, and the Swedish Colorectal Cancer Register regarding stage. Follow-up started age 60 and CRC incidence, calculated per 100,000 person-years, was assessed during screening (age 60-69) and post screening (age 70-73). Stage I-II and III-IV was assessed post screening. 364,668 individuals were included. During screening, incidence rate ratio was significantly higher in sequences (0, g, g, g, g) (RR 1.25, 95% CI 1.09-1.43), (g, g, g, g, f) (RR 1.17, 95% CI 1.01-1.35), and (g, g, f, f, f) (RR 1.14, 95% CI 1.01-1.29). Post screening, the largest decrease was seen in sequences (g, g, g, g, f) and (g, g, g, f, f), RR 0.65, 95%, CI 0.47-0.90, and RR 0.53, 95% CI 0.30-0.94, respectively. There was an overall decreasing trend along sequences from (0, 0, 0, 0, g) to (g, g, f, f, f) post screening and both stages I-II and III-IV (p < 0.001). We could demonstrate a decreased CRC incidence post screening proportional to the number of invitations with implications for future modeling studies and risk-based screening strategies.