Background & Aims: Colonoscopy is used widely for colorectal cancer (CRC) screening; however, its long-term impact on the incidence and mortality of CRC is not known. Methods: We assessed CRC incidence and mortality in a group of asymptomatic average-risk patients who underwent screening colonoscopy between 1989 and 1993 at a university hospital. By using standardized incidence ratios and standardized mortality ratios, we compared our observed CRC rates with expected rates from the Surveillance, Epidemiology, and End Results (SEER) data. Results: The cohort comprised 715 patients (mean age, 61 ± 6.5 y; 59% male; 95% Caucasian) with 10,492 patient-years of follow-up. There were 12 cases of CRC: 5 found at baseline and 7 found after a median follow-up period of 8 years (range, 3-16 y). When the first 2 years of follow-up were excluded, there were 7 incident cases of CRC (95% confidence interval [CI], 2-13) over 9075 person-years of follow-up. The expected number based on SEER data was 21. The incidence rate was 0.77 cases per 1000 person-years, and the standardized incidence ratio was 0.33 (95% CI, 0.10-0.62), consistent with a relative risk reduction in CRC incidence of 67%. Three patients died from CRC (95% CI, 0-9). The expected number of deaths based on SEER data was 9. The mortality rate was 0.29 per 1000 person-years, and the standardized mortality ratio was 0.35 (95% CI, 0.0-1.06), consistent with a relative reduction in CRC death of 65%. Conclusions: In this average-risk cohort, CRC incidence and mortality were reduced after screening colonoscopy. These results provide additional evidence for the effectiveness of colonoscopy as a primary CRC screening modality.
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