LAUSR

OBJECTIVES Paediatric-onset inflammatory bowel disease (pIBD) increases the risk of developing several different cancer forms. In this case-control study, we aimed to assess the impact of medical treatment and disease activity on the risk of developing disease-associated cancer (DAC) and treatment-associated cancer (TAC). METHODS In a previous study, we identified 27 cases of DAC (colorectal cancer, small bowel cancer and cholangiocarcinoma) and 28 TAC (lymphoma and skin cancer) in 6,689 patients with pIBD in Denmark and Finland during the period 1992-2015. In this study, the patient charts were reviewed manually. Cancer-free patients from another population-based pIBD cohort were included as controls. We recorded data on phenotype, medical treatment, surgery, and relapses. Logistic regression was used to estimate adjusted odds ratios (aOR) with 95% confidence intervals (95%CI) to estimate the relative risk. RESULTS We included 16 cases with DAC, 21 with TAC and 331 controls. For DAC, lower frequencies of IBD-relapses were associated with an increased risk of cancer (OR 0.2 [95%CI:0.04-0.8]). For TAC, we found an increased risk in patients receiving thiopurines at any point during the follow-up period (aOR: 11.7 [95%CI: 2.1-116.2]) and an association with proportion of follow-up time being exposed to thiopurines (aOR 5.6 [95CI: 1.1-31.5]). CONCLUSIONS In this nation-wide study, covering all pIBD patients from Denmark and Finland, we found that pIBD patients treated with thiopurines had an increased risk of TAC.