A diagnosis of inflammatory bowel disease (IBD) in childhood more than doubles the relative risk of developing cancer but the absolute risk remains low, according to the findings of a Swedish nationwide cohort study.

Using several of Sweden’s comprehensive national registers 9,405 children with IBD were identified during the study period from 1964 to 2014. Of these 4,648 (49%) had ulcerative colitis, 3,768 Crohn’s disease, and the remaining 989 had unclassified colitis.

For every IBD case, up to ten reference individuals without IBD matched for gender, age and place of residence were randomly selected.

The median age at first diagnosis of IBD was 15 years, and the median age at end of follow-up was 27.

There were 497 first diagnoses of cancer in the IBD patients during 148,682 person-years of follow-up (3.3 per 1,000 person-years).

In the reference group, the equivalent figures were 2,256 cancers in 1,486,983 person-years of follow-up (1.5 per 1,000 person-years).

This equated to one extra cancer per 556 IBD patients followed for a year compared with the reference subjects.

Those with childhood IBD had more than double the relative risk (RR) of cancer; RR 2.2 (95% CI: 2.0-2.5).

The risk was higher in ulcerative colitis than Crohn’s disease RR 2.6 (95% CI: 2.3-3.0) vs RR 1.7 (95% CI: 1.5-2.1).

The risk of developing cancer was higher in those who had primary sclerosing cholangitis RR 6.6 (95% CI: 4.9-8.7), and among those with long-standing colitis (≥ 10 years) RR 3.9 (95% CI: 2.9-5.0), and those having a first-degree relative with cancer onset before the age of 50, RR 4.4 (95% CI: 3.2-5.8) than in those who did not.

The highest risks were for gastrointestinal cancers: colorectal cancer RR 19.5 (95% CI: 14.7-26.2); small intestine cancer RR 12.8 (95% CI: 3.4-51.8) and liver cancer RR 134 (95% CI: 59.6-382) compared with the reference groups. However, the absolute numbers were low.

As the authors point out, relative risks for gastrointestinal cancers were high, but absolute risks were low.

Overall the relative risk of gastrointestinal cancer in the IBD cohort compared with the reference group was 18.0 (95% CI: 14.4-22.7).

Neither a history of extra-intestinal manifestations of IBD nor having surgery was significantly associated with developing cancer. The study was underpowered with regard to drug exposures.

The study authors state: ‘We cannot rule out that thiopurines or TNF inhibitors increase the risk of cancer, as our study was not big enough to recognise whether drugs are a major risk factor for cancer development in children and young adults. Instead, we suggest that extent and duration of chronic inflammation might be the main driving mechanisms underlying the increased risk of cancer.’

Dr Chris Barclay