Background: Venous thromboembolism (VTE) may be a harbinger of cancer. Inflammatory bowel disease (IBD) is known to increase the risk of VTE, but it is unclear whether VTE in IBD patients is also a marker for occult cancer. We assessed the risk of cancer after VTE in IBD patients.

Methods: We conducted a population-based cohort study by linking Danish medical registries during 1978-2008. We calculated standardized incidence ratios (SIRs) by comparing observed cancer incidence after VTE in IBD patients with that expected based on national cancer incidence in the Danish population.

Results: A total of 895 IBD patients with VTE were followed for a total of 5290 person-years. During the first year of follow-up, 28 (3.1%) patients were diagnosed with cancer, corresponding to an SIR of 3.2 (95% confidence interval [CI]: 2.2, 4.7). Patients with ulcerative colitis or Crohn's disease had similar relative risks (SIR = 3.1 [95% CI: 1.9, 4.9] and SIR = 3.5 [95% CI: 1.7, 6.3], respectively). In IBD patients <55 years the SIR was 5.7 (95% CI: 2.3, 11.8) and in patients ≥55 years the SIR was 2.8 (95% CI: 1.8, 4.3). During the second and subsequent years of follow-up 61 cancers were diagnosed (SIR = 1.2 [95% CI: 0.92, 1.6]).

Conclusions: In patients with IBD a VTE event is not only a consequence of their disease, but might also be a marker of occult cancer. We suggest that IBD patients with VTE should follow the same diagnostic work-up guidelines as non-IBD VTE patients.