Impact of South Korea's national gastrointestinal cancer screening programme on incidence and mortality: a population-based difference-in-differences study.

[OBJECTIVES] The objectives of this study were threefold. First, to examine the long-term association between implementation of South Korea's National Cancer Screening Programme and trends in incidence and mortality for major gastrointestinal cancers at the population level. Second, to assess whether programme-associated effects differed across implementation phases and population subgroups defined by age, sex and region. Third, to evaluate the specificity of observed trends by comparing screened cancers with pancreatic cancer, which is not included in the national screening programme, using a quasi-experimental difference-in-differences approach.

[DESIGN] Population-based quasi-experimental study using a difference-in-differences approach.

[SETTING] Nationwide analysis using publicly available national cancer registry, screening and mortality data obtained from Statistics Korea (Korean Statistical Information Service) and related national databases. We applied a quasi-experimental difference-in-differences design comparing pre-intervention (1999-2002), early implementation (2003-2011) and long-term effect (2012-2021) periods, reflecting the expected latency between screening implementation and population-level outcome changes.

[PARTICIPANTS] Individuals eligible for NCSP screening based on age and programme criteria between 1999 and 2021, compared with contemporaneous populations not eligible for screening.

[PRIMARY OUTCOME MEASURES] Age-standardised and sex-standardised incidence and mortality rates for colorectal, gastric, liver and pancreatic cancers.

[RESULTS] After the implementation of the National Cancer Screening Programme, age-standardised and sex-standardised incidence and mortality declined substantially for screened cancers. Between 2012 and 2021, gastric cancer incidence and mortality decreased by 28.1% and 24.1%, respectively, while colorectal cancer incidence declined by 22.4% and mortality by 28.6%. Liver cancer mortality decreased by 38.2% over the same period. In contrast, no significant decline was observed for pancreatic cancer. Difference-in-differences and joinpoint analyses confirmed statistically significant trend changes for screened cancers (p<0.05), whereas no beneficial trend was detected for pancreatic cancer. Subgroup analyses indicated persistently lower screening participation and follow-up among rural and socioeconomically disadvantaged populations.

[CONCLUSIONS] South Korea's National Cancer Screening Programme was associated with long-term declines in incidence and mortality for gastrointestinal cancers targeted by organised screening. These findings suggest that sustained, population-based screening programmes may contribute to reductions in cancer burden, while underscoring the importance of improving equitable participation and follow-up care.