To Örebro University

Background: Ulcerative colitis (UC) is a risk factor for colorectal cancer (CRC). However, available studies reflect older treatment and surveillance paradigms, and most have assessed risks for incident CRC without taking surveillance and lead-time bias into account, such as by assessing CRC incidence by tumour stage, or stage-adjusted mortality from CRC. We aimed to compare both overall and country-specific risks of CRC mortality and incident CRC among patients with UC.

Methods: In this population-based cohort study of 96 447 patients with UC in Denmark (n=32 919) and Sweden (n=63 528), patients were followed up for CRC incidence and CRC mortality between Jan 1, 1969, and Dec 31, 2017, and compared with matched reference individuals from the general population (n=949 207). Patients with UC were selected from national registers and included in the analysis if they had two or more records with a relevant International Classification of Disease in the patient register (in the country in question) or one such record plus a colorectal biopsy report with a morphology code suggestive of inflammatory bowel disease. For every patient with UC, we selected matched reference individuals from the total population registers of Denmark and Sweden, who were matched for sex, age, birth year, and place of residence. We used Cox regression to compute hazard ratios (HRs) for incident CRC, and for CRC mortality, taking tumour stage into account.

Findings: During follow-up, we observed 1336 incident CRCs in the UC cohort (1.29 per 1000 person-years) and 9544 incident CRCs in reference individuals (0.82 per 1000 person-years; HR 1.66, 95% CI 1.57-1.76). In the UC cohort, 639 patients died from CRC (0.55 per 1000 person-years), compared with 4451 reference individuals (0.38 per 1000 person-years; HR 1.59, 95% CI 1.46-1.72) during the same time period. The CRC stage distribution in people with UC was less advanced (p<0.0001) than in matched reference individuals, but taking tumour stage into account, patients with UC and CRC remained at increased risk of CRC death (HR 1.54, 95% CI 1.33-1.78). The excess risks declined over calendar periods: during the last 5 years of follow-up (2013-17, Sweden only), the HR for incident CRC in people with UC was 1.38 (95% CI 1.20-1.60, or one additional case per 1058 patients with UC per 5 years) and the HR for death from CRC was 1.25 (95% CI 1.03-1.51, or one additional case per 3041 patients with UC per 5 years).

Interpretation: Compared with those without UC, individuals with UC are at increased risk of developing CRC, are diagnosed with less advanced CRC, and are at increased risk of dying from CRC, although these excess risks have declined substantially over time. There still seems to be room for improvement in international surveillance guidelines.