Familial risk of colon and rectal cancer in Iceland: Evidence for different etiologic factors?

The aim of this study was to characterize the familial risk of colon and rectal cancer using 2 population‐based registries in Iceland, the Icelandic Cancer Registry and a genealogy database. The standardized incidence ratio (SIR) was used to estimate the risk among relatives of colorectal cancer index cases diagnosed in Iceland over a 46‐year period (1955–2000). The 2,770 colorectal cancer patients had 23,272 first‐degree relatives. Among first‐degree relatives, there was an increased risk of both colon (SIR 1.47, 95% confidence interval (CI) 1.34–1.62) and rectal cancer (SIR 1.24, 95% CI 1.04–1.47). An increased risk of colon cancer was observed among siblings of colon cancer patients (SIR 2.03, 95% CI 1.76–2.33), whereas no such increase was observed for parents or offspring. Furthermore, the risk of rectal cancer was only increased among brothers (SIR 2.46 95% CI 1.46–3.89) of rectal cancer patients and not among their sisters (SIR 1.0 95% CI 0.40–2.06). The added risk of colon cancer among first‐degree relatives was independent of site of colon cancer in the proband. Our results confirm that family history of colorectal cancer is a risk factor for the disease. However, family history has a different association with colon cancer than with rectal cancer, suggesting that the 2 cancer types may have different etiologic factors. Our results have implications for colon and rectal cancer screening programs. © 2006 Wiley‐Liss, Inc.