Colorectal cancer after open-access colonoscopy: a community and case survey

Objective: To evaluate whether colonoscopy protects against subsequent colorectal cancer (CRC). Design: Case and population survey. Setting: Townsville region in northern Queensland, between mid 1985 and January 1998. Subjects: All 8430 patients who underwent 11 148 colonoscopies performed by the author at the Mater Private Hospital (a community-based open-access colonoscopy service) between July 1985 and December 1996; those who were subsequently diagnosed with CRC, to January 1998; and ail 476 residents diagnosed with colorectal cancer between 1994 and 1997. Main outcome measures: Age-standardised CRC incidence for 1994-1997 for patients who had had a previous colonoscopy and for the remaining community; Dukes' grade of CRCs. Results: For people aged 50 years and over, the incidence of CRC was significantly lower among those who had had a previous colonoscopy than in the remaining community (1.14 versus 2.31 per 1000 patient-years; P=0.0046). For people aged 35-49 years, the incidence was 0.35 Versus 0.31 per 1000 patient-years (P=0.86). Thirty-one CRCs developed in 29 people who had had previous colonoscopy; only five of these CRCs were graded Dukes C, with none graded Dukes D. In contrast, almost half the CRCs in the rest of the community were graded Dukes C or D (P<0.001). All but one of those diagnosed with CRC on repeat colonoscopy had risk factors (personal or family history), and 23 of the CRCs were subclinical, with 20 being diagnosed during surveillance colonoscopy. Conclusions: Patients aged over 50 years who had previously undergone a colonoscopy and ensuing treatment were less likely to be diagnosed with CRC than otherwise expected. Surveillance colonoscopy led to diagnosis of CRCs with lower Dukes grades.