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.