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The well-known cancer risk accompanying inflammatory bowel disease (IBD) applies even when symptoms begin in childhood or adolescence, a meta-analysis affirmed.

Cancer rates in adulthood were approximately doubled compared with the general population, both for patients with pediatric-onset Crohn's disease (pooled RR 2.03, 95% CI 1.67-2.46) and for those with ulcerative colitis diagnosed prior to age 18 (pooled RR 2.61, 95% CI 2.00-3.40), reported Tine Jess, MD, DMSci, of Aalborg University in Copenhagen, and colleagues .

Gastrointestinal malignancies largely drove the increased cancer rates, including tumors of the liver and the large and small intestine, with relative risk ratios ranging from about 20 to an astonishing 55 -- but rates of lymphoid cancers, melanomas, and non-melanoma skin cancers were also significantly higher than in the general population, with relative risks of 2-4.

The findings were based on five population-based studies conducted in Europe and North America with a total of nearly 20,000 patients with pediatric-onset IBD, with some 280,000 person-years of follow-up. Cancer was eventually diagnosed in 715 of these patients.

The studies were published from 2013 to 2020, covering time periods going back as far as 1964. The three largest were based on comprehensive national health records from Denmark, Finland, and Sweden; these comprised more than 90% of the included IBD population. Study quality, as rated on the , was fair for three of the studies and good for the other two.

Median age at IBD diagnosis was 14-16 in the five studies, while the median age at cancer diagnosis ranged from 24 to 37. Crude cancer incidence rates varied considerably among the studies: from 1.0 per 1,000 patient-years in the Finnish population to 3.3 per 1,000 patient-years in Sweden. Risk ratios relative to the general population ranged from 2.00 in the Canadian province of Manitoba to 3.60 in Finland.

There was considerable heterogeneity in the underlying data, Jess and colleagues noted, with I² values of 50%-60% for the risk ratio calculations.

But the researchers also emphasized that the absolute risks were small. "[D]espite markedly increased relative rate estimates for gastrointestinal cancers among patients with pediatric-onset IBD compared with general pediatric populations, this risk corresponds to a mean incidence rate of 0.3 cases of liver cancer, 0.6 cases of colorectal cancer, and 0.1 cases of small bowel cancer per 1,000 person-years in this population."

The analysis also sought to examine whether exposure to thiopurine drugs such as azathioprine (for many patients, the backbone of IBD therapy) contribute to cancer risk. Some previous analyses had indicated this could be the case. In the new meta-analysis, only two of the five included studies had data on thiopurine exposure: these did suggest a slightly increased risk (RR 2.09 relative to the general population, versus 1.82 for patients not treated with these agents). But overall, Jess and colleagues concluded, the question remains unsettled.

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