Use of a case-mix approach to study the trends in the incidence of second primary cancers

International audiencePURPOSE:To analyze trends in second primary cancer (SPC) incidence by using a case-mix approach to standardize on first cancer site distribution.METHODS:Cases registered by 13 French cancer registries between 1989 and 2010 and followed-up until June 2013 were included. The person-year approach was used to compute standardized incidence ratios (SIRs) of metachronous SPC. Usual SIRs and cancer site-specific weighted SIRs called "case-mix SIRs" (cmSIRs) were estimated by sex and calendar period of first cancer diagnosis. Calendar trends in SIRs and cmSIRs were compared.RESULTS:More than 2.9 million person-years at risk were included. Among males, SIRs dropped from 1.49 to 1.23 between 1989-1994 and 2005-2010, while cmSIRs decreased from 1.40 to 1.27. This difference seems mainly related to a stronger representation of prostate cancers (at lower risk of SPC) and a weaker contribution of bladder and head and neck cancers (at higher risk of SPC) in recent periods of diagnosis. Among females, both SIRs and cmSIRs have remained stable at around 1.22 and 1.21, respectively.CONCLUSIONS:The cmSIR is an indicator that is not influenced by changes in first cancer site distribution. Its use should be encouraged to assess second cancer incidence control