Implications of incomplete registration of deaths on long-term survival estimates from population-based cancer registries.

International comparison of population-based cancer survival is a key component of monitoring progress against cancer. Its validity depends to an unknown degree on completeness of ascertainment of deaths in the cancer registries involved which may vary according to legal and administrative circumstances. The aim of this study was to assess the impact of incomplete registration of deaths through various mechanisms on the validity of long-term absolute and relative survival estimates. For that purpose, we simulated underascertainment of deaths through linkage failure of registry data with death certificates with probabilities between 0.1 and 5%, and underascertainment of deaths by unregistered annual emigration with probabilities between 0.05 and 2%, using data from the Finnish Cancer Registry. The expected impact on estimates of 5-, 10- and 15-year absolute and relative survival was assessed. We demonstrate that even modest levels of under-registration of deaths may lead to severe overestimation of long-term survival estimates, ranging from 0 to 31 percent units in the scenarios assessed. In general, relative survival is much more affected than absolute survival, and potential problems are much larger for relative survival estimates in older compared with younger patients. Potential overestimation strongly increases with length of follow-up, and this increase is particularly pronounced for under-registration of deaths because of unrecorded emigration. Every effort should be made in cancer registry based survival analyses to ascertain deaths with close to 100% completeness. When such completeness cannot be achieved, long-term relative survival estimates and their comparison across populations must be interpreted with much caution.