Interpreting international comparisons of cancer survival: the effects of incomplete registration and the presence of death certificate only cases on survival estimates.

We have assessed the impact on survival estimates based on cancer registry data of incomplete ascertainment of cancer cases and the presence of cases registered purely from death certificate information (DCO cases). Using data from the Thames and Finnish Cancer Registries we obtained five-year relative survival estimates for 12 cancer sites, excluding DCOs as usual. We then made adjustments to allow for the effects of both the known proportion of DCOs and the estimated proportion of missing cases for each site. In general, adjusting for DCOs led to lower survival estimates, whilst adjusting for incompleteness had the opposite effect. The Finnish data were largely complete and had small proportions of DCOs, and hence the adjustments had little effect on estimated survival. The changes in the Thames estimates were more marked. When performing cohort survival analysis (based on diagnoses between 1990 and 1994), the increases in the survival estimates gained from adjusting for incompleteness were for the most part offset by the decrease produced when adjusting for DCOs. However, when performing period survival analysis based on the period 1997-2001 (when the DCO rate at Thames had fallen by around a half relative to the earlier period), the final estimates (adjusted for both effects) were generally higher than the unadjusted values--thus reducing the apparent difference between the two countries. It is important to take variations in DCO proportion and/or completeness into consideration when comparing survival estimates between different populations.