Organized Colorectal Cancer Screening and Changes in Mortality and Incidence Trends: A Population-Based Study

Background: Organized colorectal cancer (CRC) screening programs have been widely implemented across Europe; however, robust population-level evaluations of their real-world effectiveness, particularly for programmes based exclusively on faecal immunochemical testing (FIT), remain limited. The Galician CRC screening programme was progressively implemented between 2013 and 2019. Methods: We conducted a population-based ecological time-series study using data from the Galician Tumour Registry (ICD-10 C18–C21) for 2015–2023. Age-standardized mortality (ASMR) and incidence (ASIR) rates were analyzed. Structural changes associated with program implementation were evaluated using interrupted time-series (ITS) models, estimating annual percent change (APC) before and after implementation and the net change in slope (ΔAPC). Absolute and relative changes in ASMR and ASIR were calculated by comparing 2015–2017 and 2019–2023. Analyses were performed for the overall population and for individuals aged 50–69 years. Results: Between 2015 and 2023, overall CRC mortality declined significantly (APC −3.00%; 95% CI −3.37 to −2.63). ITS analysis demonstrated a marked modification of mortality trajectories following program implementation. Mortality shifted from an increasing pre-implementation slope (APC +13.70%; 95% CI 10.12, 17.39) to a significant annual decline post-implementation (APC −3.62%; 95% CI −4.47, −2.76), yielding a ΔAPC of −17.32. In individuals aged 50–69 years, the structural change was more pronounced (ΔAPC −19.88), with post-implementation mortality decreasing by −8.08% annually (95% CI −10.43, −5.66). Incidence showed a comparable structural modification. Overall APC changed from +15.26% (95% CI 5.48, 25.95) before implementation to −2.48% (95% CI −5.29, 0.41) afterwards (ΔAPC −17.74). In the screening-eligible population, APC shifted from +21.32% (95% CI 4.60, 40.71) to −3.74% (95% CI −7.62, 0.30), corresponding to a ΔAPC of −25.06. Descriptively, ASMR declined from 41.92 to 35.91 per 100,000 (−14.33%), and ASIR from 98.37 to 85.16 per 100,000 (−13.42%) between 2015–2017 and 2019–2023. Relative reductions were larger in individuals aged 50–69 years and were more pronounced for colon cancer than for rectal cancer. Conclusions: Implementation of an organized FIT-based screening program was associated with a structural change in CRC mortality and incidence trends, particularly among individuals aged 50–69 years.