In IBD, count the affected relatives rather than checking their age

Design

nationwide register-based matched cohort study, Sweden, 1996-2023, median follow-up 11 years

Population

124,387 patients with inflammatory bowel disease and 1,213,641 general-population comparators matched on age, sex, parish and year

Primary outcome

colorectal cancer incidence rates and rate differences by number and age at diagnosis of affected first-degree relatives

Effect

incidence 1.17 (95% CI 1.12-1.23) vs 0.88 (0.86-0.89) per 1,000 person-years; 2 or more affected relatives added 2.69 (0.60-4.78) per 1,000 person-years, early-onset family history 0.42 (-0.47 to 1.31)

Swedish national registers followed 124,387 patients with inflammatory bowel disease and 1,213,641 matched general-population comparators between 1996 and 2023, a median of 11 years each, recording colorectal cancer against the number of first-degree relatives affected and their age at diagnosis. There were 1,882 cancers among the IBD patients, an incidence of 1.17 per 1,000 person-years (95% CI 1.12 to 1.23), against 0.88 (95% CI 0.86 to 0.89) in comparators.

The pattern within IBD is what challenges current practice. The largest increase came with two or more affected first-degree relatives: 2.69 additional cancers per 1,000 person-years (95% CI 0.60 to 4.78) compared with no family history. A family history of early-onset colorectal cancer, the feature guidelines single out, added only 0.42 per 1,000 person-years (95% CI -0.47 to 1.31), an interval that includes no effect at all. On the absolute scale, family history raised incidence by about the same amount in IBD as in the general population; its relative effect looked smaller in IBD only because the baseline risk was already higher.

That last point is worth stating carefully, because it is the kind of thing that gets misread in both directions. Family history matters just as much in IBD as in anyone else. What it does not do is multiply an already-raised risk, which is what a relative-risk framing implies and what guidelines built on relative risk tend to assume.

The practical consequence is where extra attention should go. Guidelines direct it toward patients whose relatives had early-onset disease; these data suggest the patient with two or more affected first-degree relatives is the one carrying the clearest excess. That is a question the authors raise rather than answer, and this is register data, not a trial of surveillance strategies, so nothing here justifies abandoning an existing protocol. It does justify making sure the number of affected relatives is recorded and looked at when the interval is set.

• Record the number of affected first-degree relatives for every patient with IBD, and revisit it at surveillance planning
• Give particular attention to the patient with two or more affected first-degree relatives
• Do not drop attention to early-onset family history, but recognise the absolute excess it carries here was small and its interval crossed zero
• Keep colitis-specific factors, duration, extent, inflammation burden and primary sclerosing cholangitis, as the backbone of surveillance timing
• Treat this as a reason to record better data, not as authority to change an established surveillance interval

The statistics, in plain English

These are incidence rate differences, which are more useful than relative risks here because they say how many extra cancers occur rather than by what factor risk multiplies. The excess with two or more affected relatives, 2.69 per 1,000 person-years, has an interval from 0.60 to 4.78: the effect is real but its size is imprecise, because few patients have two affected first-degree relatives. The early-onset figure of 0.42 with an interval from -0.47 to 1.31 crosses zero, meaning no effect could not be excluded. Register studies capture what was coded, so family history is only as good as the registers, and unmeasured differences in surveillance intensity between groups cannot be ruled out.

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