[Estimating the incidence of colorectal cancer in France from a hospital discharge database, 1999-2003].

BACKGROUND: Cancer incidence in France is monitored by district-level cancer registries, which cover only 15% of the population. Incidence at the national and regional level are estimated from mortality data by extrapolating the ratio between incidence and mortality observed in the districts covered by a cancer registry. Using the incidence/mortality ratio might not be relevant at the district-level (département). This study aims to produce district-level estimations of colorectal cancer incidence, using the ratio between incident cases from cancer registries and surgery admissions for colorectal cancer identified in the national hospital discharge database.

METHODS: This ratio was studied for the period 1999-2003 in the 13 districts covered by a cancer registry. For each sex separately, the number of incident cases was analyzed according to the number of surgery admissions for resection of colorectal cancer using a Poisson model. Age was introduced in the model as a fixed effect and district as a random effect. The model's ability to predict incidence was tested through cross-validation. The model was then extrapolated in order to estimate incidence of colorectal cancer in all French districts.

RESULTS: In the districts covered by a cancer registry, cross-validation showed the model had a good predictive ability, except in men for one district where the difference between predicted and observed incidence reached 10%. Estimated incidence rates, age-standardized on the world population, ranged broadly from 29 to 44 per 100,000 in men and from 17 to 27 per 100,000 in women. Incidence did not show any clear geographical pattern.

CONCLUSION: Among districts covered by a cancer registry, cross-validation showed overall good accuracy of predicted incidence. Inclusion of several admissions per patient was certainly a minor source of error in these estimations. Indeed, our selection only included 2% of multiple admissions, without geographical variations, in 2002 and 2003, years for which patient identifiers were available in the hospital discharge database. Estimated incidence rates presented moderate geographical variations and their prediction intervals should be taken into account.