Protective association of colonoscopy against proximal and distal colon cancer and patterns in interval cancer.

BACKGROUND: The protective effect of colonoscopy against proximal colorectal cancer is variable and depends on the detection and complete removal of precancerous polyps. OBJECTIVE: To estimate the efficacy of colonoscopy in a medical center with open-access screening colonoscopy since 1998. DESIGN: Nested case-control study with incidence density sampling. SETTING: University-affiliated Veterans Affairs Medical Center. PATIENTS: Colorectal cancer (CRC) cases and control subjects selected from screening age patients matched by age, gender, and date of first primary care visit. MAIN OUTCOME MEASUREMENT: Colonoscopy preceding the CRC diagnosis date. RESULTS: A total of 20.2% of CRC cases had a colonoscopy in the preceding 10 years compared with 49.0% of control subjects (adjusted odds ratio [aOR], 0.20; 95% confidence interval [CI], 0.11-0.34). Colonoscopy was strongly associated with decreased odds of both distal CRC (aOR, 0.16; 95% CI, 0.07-0.34) and proximal CRC (aOR, 0.26; 95% CI, 0.11-0.58). The fraction of cases attributed to interval cancers was 10.5%. Missed lesions predominantly localized to the cecum and rectum, and recurrent lesions clustered in the hepatic flexure. Cecal intubation rate was 93% (98% in adequately prepped patients), and the adenoma detection rate was 45.2% in the control group. LIMITATIONS: Single-center, retrospective case-control design. CONCLUSION: In an open access colonoscopy program characterized by a high cecal intubation rate and adenoma detection rate, colonoscopy was strongly associated with reduced odds of both distal and proximal CRC. Among interval cancers, missed lesions clustered in the cecum and rectum and recurrent lesions in the hepatic flexure.