1. University of Toulouse III; Toulouse F-31073 France
2. INSERM; Toulouse U1027, F-31073 France
3. Epidemiology Unit; CHU Toulouse; Toulouse F-31073 France
4. Institut Universitaire du Cancer de Toulouse-Oncopole; Toulouse F-31059 France
5. School of Mathematics and Statistics; University of Glasgow; Glasglow U.K.
6. Toulouse Institute of Mathematics; Toulouse UMR C5583, F-31062 France