Families at risk of colorectal cancer: who are they?

The first degree kinships of 305 index cases have been studied to determine whether an early age of onset or a particular site distribution characterizes familial aggregations of colorectal cancer. The probands comprised 100 patients aged 55-74 years and 205 patients under 55 years at diagnosis and were drawn from a large population database. Ascertainment and verification were complete for 2566 of 2657 first degree relatives. The history of cancer in 296 relatives was validated in 96% of cases from medical or other records. Among kinships ascertained through index cases under 55 years of age, less than 5% had three or more individuals affected by colorectal cancer. The comparable proportion of older probands' families was 3%. Probands with proximal disease were no more likely to have a positive family history of bowel cancer than those with disease distal to the splenic flexure. These findings are consistent with other population based studies of the epidemiology of familial colorectal cancer but contrast with reviews from referral centres and family cancer clinics.