National cancer incidence is estimated using the incidence/mortality ratio in countries with local incidence data: is this estimation correct?

BACKGROUND: In countries with local cancer registration, the national cancer incidence is usually estimated by multiplying the national mortality by the incidence/mortality (I/M) ratio from pooled registries. This study aims at validating this I/M estimation in France, by a comparison with estimation obtained using the ratio of incidence over hospital discharge (I/HD) or the ratio of incidence over health insurance data (long-duration diseases, I/LDD). METHODS: This comparison was performed for 22 cancer sites over the period 2004-2006. In France, a longitudinal I/M approach was developed relying on incidence and mortality trend analyses; here, the corresponding estimations of national incidence were extracted for 2004-2006. The I/HD and I/LDD estimations were performed using a common cross-sectional methodology. RESULTS: The three estimations were found similar for most cancers. The relative differences in incidence rates (vs. I/M) were below 5% for numerous cancers and below 10% for all cancers but three. The highest differences were observed for thyroid cancer (up to +21% in women and +8% in men), skin melanoma (up to +13% in women and +8% in men), and Hodgkin disease in men (up to +15%). Differences were also observed in women aged over 60 for cervical cancer. Except for thyroid cancer, differences were mainly due to the smoothing performed in the I/M approach. CONCLUSION: Our results support the validity of I/M approaches for national estimations, except for thyroid cancer. The longitudinal version of this approach has, furthermore, the advantage of providing smoothed estimations and trend analyses, including useful birth-cohort indicators, and should thus be preferred.