Uncertainty prevails about the magnitude of excess risk of small bowel cancer in patients with inflammatory bowel disease (IBD).
To quantify the risk of small bowel adenocarcinoma and neuroendocrine tumors in patients with ulcerative colitis (UC) and Crohn’s disease (CD), we undertook a population-based cohort study of all IBD-patients diagnosed in Norway and Sweden from 1987 through 2016. Patients were followed through linkage to national registers. We calculated standardized incidence ratios (SIRs) with 95% confidence intervals (CIs) of small bowel adenocarcinomas and neuroendocrine tumors for patients with CD and UC. We excluded the first year of follow-up to reduce reverse causality.
Among 142.008 IBD-patients with a median follow-up of 10.0 years, we identified 66 adenocarcinomas and 57 neuroendocrine tumors in the small bowel. The SIR of small bowel adenocarcinoma was 8.3 (95% CI, 5.9-11.3) in CD and 2.0 (95% CI, 1.2-3.1) in UC. The incidence rates of adenocarcinomas were highest in CD with stricturing disease and extent limited to the small bowel, at 14.7 (95% CI, 8.2-24.2) and 15.8 (95% CI, 8.4-27.0) per 100,000 person-years, respectively. The SIR of neuroendocrine tumors was 2.5 (95% CI, 1.5-3.9) in CD and 2.0 (95% CI, 1.4-2.8) in UC.
Patients with CD experienced an 8-fold increased risk of small bowel adenocarcinomas, while both UC and CD patients experienced an about 2-fold increased risk of neuroendocrine tumors, and UC patients experienced 2-fold increased risk of small bowel adenocarcinoma. The small absolute excess cancer risk suggests that active surveillance to diagnose small intestinal cancer early is unlikely to be cost-effective.

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