• Division of Cancer Prevention, National Cancer Institute, Bethesda, Maryland.
• Division of Gastroenterology, Hepatology and Nutrition, Departments of Medicine and Epidemiology, University of Pittsburgh, Pittsburgh, Pennsylvania. Electronic address: rschoen@pitt.edu.

BACKGROUND & AIMS: The contribution of surveillance colonoscopy, as opposed to that of initial colonoscopy examination, to prevention of colorectal cancer (CRC) is uncertain. We estimated the preventive effect of surveillance colonoscopy by applying the recently developed metric of adenoma dwell time avoided needed to prevent 1 CRC case (DTA).

METHODS: We followed subjects in the prostate, lung, colorectal and ovarian (PLCO) cancer screening trial who underwent colonoscopies following positive findings from sigmoidoscopies (colonoscopy cohort, n=15,935) for CRC incidence for 10 years. The number and timing of adenomas removed during surveillance were measured in a subset (n=3492) of patients and extrapolated to the entire cohort to estimate the total avoided adenoma dwell time. A previously determined DTA value of 612 dwell years was applied to estimate the number of CRC cases prevented by surveillance. Proportional reduction in CRC was computed as C_P/(C_O+C_P), where C_O and C_P are observed and estimated prevented cases, respectively.

RESULTS: In the colonoscopy cohort of the PLCO, 2882 subjects had advanced adenomas (AAs), 572 had 3 or more non-advanced adenomas (NAA₃₊), 4496 had 1-2 non-advanced adenomas (NAA_1-2), and 7985 had no adenoma (NA). The mean number of subsequent colonoscopy examinations over 10 years were 1.80 for subjects with AAs, 1.63 for subjects with NAA₃₊, and 1.46 for subjects with NAA_1-2. Average years of avoided adenoma dwell time per subject were 4.0 for subjects with AAs, 5.5 for subjects with NAA₃₊, and 2.4 for subjects with NAA_1-2. There were 56 cases of CRC in subjects with AAs, 4 cases of CRC in subjects with NAA₃₊, and 33 cases of CRC in subjects with NAA_1-2. Estimated proportional reductions in CRC incidence were 25.0% in subjects with AAs (95% CI, 16%-34%), 34.4% in subjects with NAA_1-2 (95% CI, 25%-40%), and 30.4% overall (in subjects with AAs, NAA₃₊, or NAA_1-2; 95% CI, 25%-40%). Absolute CRC incidence reductions were 7.1 per 10,000 PY in subjects with AAs and 4.1 per 10,000 PY in subjects with NAA_1-2.

CONCLUSIONS: Using the recently developed metric of DTA, we estimated that surveillance colonoscopy during 10 years of follow up (in the PLCO colonoscopy cohort) prevented 30% of CRC cases. Because the methodology for estimation is indirect, the true effect is uncertain.