Little is known about the risk of colorectal cancer among patients with irritable bowel syndrome (IBS).
We conducted a nationwide cohort study using data from the Danish National Registry of Patients and the Danish Cancer Registry from 1977 to 2008. We included patients with a first-time hospital contact for IBS and followed them for colorectal cancer. We estimated the expected number of cancers by applying national rates and we computed standardised incidence ratios (SIRs) by comparing the observed number of colorectal cancers with the expected number. We stratified the SIRs according to age, gender, and time of follow-up.
Among 57,851 IBS patients, we identified 407 cases of colon cancer during a combined follow-up of 506,930 years (SIR, 1.14 (95% confidence interval (CI): 1.03-1.25) and 115 cases of rectal cancer, corresponding to a SIR of 0.67 (95% CI: 0.52-0.85). In the first 3 months after an IBS diagnosis, the SIR was 8.42 (95% CI: 6.48-10.75) for colon cancer and 4.81 (95% CI: 2.85-7.60) for rectal cancer. Thereafter, the SIRs declined and 4-10 years after an IBS diagnosis, the SIRs for both colon and rectal cancer remained below 0.95.
We found a decreased risk of colorectal cancer in the period 1-10 years after an IBS diagnosis. However, in the first 3 months after an IBS diagnosis, the risk of colon cancer was more than eight-fold increased and the risk of rectal cancer was five-fold increased. These increased risks are likely to be explained by diagnostic confusion because of overlapping symptomatology.
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