Fig 1 Schematic diagram showing how study members contributed to unexposed and exposed groups. All study members were classified as unexposed on entry to the study. Those who were exposed to a CT scan remained in the unexposed group for the duration of the lag period (one year in most analyses, but five or 10 years in some). They were then transferred to the exposed group, provided that their date of transfer was before their date of exit from the study. Study members who had no CT scan remained in the unexposed group for the duration of the study