Objective: To determine whether the risk of colorectal cancer (CRC) decreases after colonoscopy compared with sigmoidoscopy or no lower endoscopy. Patients and Methods: Patients 67 to 80 years old in the 5%random Medicare sample of the Surveillance, Epidemiology and End Results and Medicareelinked database were grouped into those who underwent colonoscopy or flexible sigmoidoscopy from January 1, 1998, through December 31, 2002, and those who did not undergo lower endoscopy. We excluded patients with inflammatory bowel disease, history of colon polyps, or family history of CRC. All patients were followed up until the diagnosis of CRC or carcinoma in situ, death, or December 31, 2005. The risk of CRC after colonoscopy was compared with the risk after sigmoidoscopy or no lower endoscopy. The multivariate Cox proportional hazards model was used in statistical analysis. Results: In the colonoscopy group (n=12,266), 58 CRCs (0.5%) were diagnosed during follow-up compared with 66 CRCs (1.0%) in the sigmoidoscopy group (n=6402) and 634 (1.5%) in the control group (n=41,410) (all P<.001). In the sigmoidoscopy group, 771 patients (12.0%) underwent colonoscopy within the next 12 months. In multivariate Cox regressions, colonoscopy was associated with a decreased risk of distal CRC (hazard ratio [HR], 0.266; 95%CI, 0.161-0.437) and proximal CRC (HR, 0.451; 95%CI, 0.305-0.666); sigmoidoscopy was associated with a decreased risk of distal CRC (HR, 0.409; 95%CI, 0.207-0.809) but not proximal CRC. Conclusion: Among older patients, the risk of distal CRC decreased after both colonoscopy and sigmoidoscopy; the risk of proximal CRC decreased after colonoscopy but not sigmoidoscopy.
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