Purpose: Estimates of familial colorectal cancer risks are useful in genetic counseling and as a guide to determining entry into screening programs and trials of chemoprevention. Furthermore, they provide an insight into the contribution of the known colorectal cancer genes to the familial risk of the disease. There is a paucity of data about the familial colorectal cancer risk associated with early-onset disease outside the recognized cancer predisposition syndromes.
Methods: This was a retrospective cohort study. The parents and siblings of 205 patients with colorectal cancer aged less than 55 years at diagnosis were studied for mortality and cancer incidence.
Results: The overall standardized mortality ratio of colorectal cancer compared with the Northern Irish population was 3.54 (95 percent confidence interval, 2.59-4.79). There was some evidence that a family history of colorectal cancer is associated with a greater risk of colon (4.16; 95 percent confidence interval, 2.83-5.91) rather than rectal cancer (2.62; 95 percent confidence interval, 1.43-4.40). Risks in parents (2.54; 95 percent confidence interval, 1.45-3.72) were lower than in siblings (6.15; 95 percent confidence interval, 3.90-9.23).
Conclusion: First-degree relatives of patients with early-onset disease are at a marked increase in risk. There is evidence that risks vary depending on the type of affected relative and by the site of colorectal cancer. This information should be considered in formulating screening strategies.