Spillover improved survival in non-invited patients of the colorectal cancer screening programme

Objectives In colorectal cancer screening, randomized clinical trials have shown a 16% mean reduction in colorectal cancer mortality, but the Finnish randomized health services study showed no effect. We quantified spillover (the total indirect effect caused by the programme on the non-invited) and corrected the effectiveness estimate of the Finnish programme. Methods We retrieved from the Finnish Cancer Registry data on all non-invited colorectal cancer patients diagnosed in 1999–2013 in municipalities that adopted screening ( n = 18,948). Patients were stratified by three 5-year diagnostic periods and two calendar periods of programme adoption in the municipality of residence. Follow-up ended on 31 December 2013. We measured the spillover effect in patient survival, based on differences of adjusted estimates of the colorectal cancer-related hazard of death between pairs of consecutive diagnostic periods. Results The spillover effect was estimated as 9 percentage points (95% confidence interval: −1 to 19 percentage points). It was 13 percentage points in men (−1 to 26 percentage points) and 5 percentage points in women (−9 to 20 percentage points). The corrected effect estimate of implementing screening in Finland was 5 percentage points. Conclusions The corrected Finnish effectiveness estimate was consistent with estimates from randomized trials. Indirect effects (spillover) bias the invitee–control contrast. In this case, spillover was an inherent benefit of the Finnish programme.