Are "immortals" an issue for survival estimates derived from Canadian Cancer Registry data?

Background: The validity of survival estimates from cancer registry data depends, in part, on the identification of the deaths of deceased cancer patients. People whose deaths are missed seemingly live on forever and are informally referred to as "immortals." Their presence in registry data can result in inflated survival estimates. This study assesses the issue of immortals in the Canadian Cancer Registry (CCR) using a recently proposed method that compares the survival of long-term survivors of cancers for which "statistical" cure has been reported with that of similar people from the general population.

Data and methods: Data are from the population-based CCR record linked to the Canadian Vital Statistics - Death Database and tax data. Yearly interval-specific relative survival (IRS) estimates were derived up to 15 years after diagnosis for colon cancer cases, and for colon, rectal and melanoma cancer cases combined, diagnosed from 1992 to 2002.

Results: With increasing follow-up time since diagnosis, national colon cancer IRS estimates levelled off at 1.00, or slightly less, for each age group studied, indicating that survival did not exceed that of the general population. Similar results were obtained among males and females, and for colon, rectal and melanoma cancer cases combined. Provincial IRS point estimates for the three cancers combined also levelled off around 1.00, though with more variation in the estimates than at the national level.

Interpretation: Based on the results of this study, immortals do not appear to be an issue at either the national or the provincial level for survival estimates derived from CCR data.