Colorectal cancer rates among first-degree relatives of patients with inflammatory bowel disease:: a population-based cohort study

Background Inflammatory bowel disease (IBD) and colorectal cancer might share a common cause and, therefore, relatives of patients with IBD could be at increased risk of this malignant disease. We aimed to assess cancer rates among first-degree relatives of patients with IBD to try to determine whether an association between the two diseases exists. Methods In a population-based study, we identified 114 102 first-degree relatives by registry linkage and followed them up for cancer occurrence. We used standardised incidence ratio (SIR) of cancer as relative risk. Findings 560 colorectal cancers were identified among relatives. First-degree relatives of patients with Crohn's disease or ulcerative colitis were not at increased risk of cancer (SIR 0.90, 95% CI 0.82-0.97). The relative risk was 0.96 (0.87-1.06, n=379) for colon cancer and 0.78 (0.68-0.91, 181) for rectal cancer. The SIRs were not affected by age, relation of patient, or type or extent of IBD in the patient. Relatives of patients with both IBD and colorectal cancer had an 80% increased risk of colorectal cancer. Interpretation Our results do not endorse a common cause of IBD and colorectal cancer. The slightly decreased relative risk for colorectal cancer among relatives could indicate the proportion of all colorectal cancer cases attributable to IBD.