Measuring the effect of including multiple cancers in survival analyses using data from the Canadian Cancer Registry

In survival analyses using cancer registry data, second and subsequent primary cancers diagnosed in individuals are typically excluded. However, this approach may lead to biased comparisons of survival between cancer registries, or over time within a single registry.

To examine the impact of including multiple primary cancers in the derivation of survival estimates using data from a population-based national cancer registry.

Five-year relative survival estimates for persons aged 15-99 years at diagnosis were derived using all eligible primary cases from the Canadian Cancer Registry (CCR)-a population-based registry containing information on cases diagnosed from 1992 onward-and then again using first primary cases only. Any pre-1992 cancer history of persons on the CCR was obtained by using auxiliary information.

The inclusion of multiple cancers resulted in lower estimates of 5-year relative survival for virtually all cancers studied. The effect was somewhat attenuated by age-standardization (e.g., from 1.3% to 1.0% for all cancers combined), and was greatest for bladder cancer (-2.4%) followed by oral cancer (-1.9%)-cancers that had the first and third lowest proportions of first cancers, respectively. For the majority of cancers the difference was less than 1.0%. Cancers for which there was virtually no difference (e.g., lung, pancreatic, ovarian and liver) tended to be those with a poor prognosis.

Inclusion of second and subsequent primary cancers in the analysis tended to lower estimates of relative survival, the extent of which varied by cancer and age and depended in part on the proportion of first primary cancers.