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 sha re a common cause and, therefore, relatives of patients with IBD could be a t increased risk of this malignant disease. We aimed to assess cancer rates among first-degree relatives of patients with IBD to try to determine whet her an association between the two diseases exists. Methods In a population-based study, we identified 114 102 first-degree rel atives by registry linkage and followed them up for cancer occurrence. We u sed standardised incidence ratio (SIR) of cancer as relative risk. Findings 560 colorectal cancers were identified among relatives. First-degr ee relatives of patients with Crohn's disease or ulcerative colitis were no t at increased risk of cancer (SIR 0.90, 95% CI 0.82-0.97). The relative ri sk was 0.96 (0.87-1.06, n=379) for colon cancer and 0.78 (0.68-0.91, 181) f or rectal cancer. The SIRs were not affected by age, relation of patient, o r 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 colorec tal cancer. The slightly decreased relative risk for colorectal cancer amon g relatives could indicate the proportion of all colorectal cancer cases at tributable to IBD.