Colorectal cancers detected after colonoscopy frequently result from missed lesions

Background & aims: Colorectal cancers (CRCs) that are detected in patients who have received colonoscopies (interval cancers) arise from missed lesions, incomplete resections of adenomas, or de novo. We estimated rates of interval cancer from missed lesions.

Methods: Adenoma miss rates, cancer prevalence among patients with adenoma (based on size), and rates of adenoma-to-cancer transitions were estimated from the literature. We used a model to apply these risk estimates to a hypothetical average-risk population that received screening colonoscopies. We calculated the proportion of individuals with tumors missed at the baseline colonoscopy and tumors that arose from missed adenomas during a 5-year follow-up period. Sensitivity analyses were performed to assess robustness.

Results: We found that 0.7 per 1000 persons undergoing a screening colonoscopy had a cancer that was missed at the baseline colonoscopy and an additional 1.1 per 1000 subsequently developed cancer from a missed adenoma. Therefore, the expected rate of individuals with CRC, based on missed adenomas, was 1.8 per 1000 persons within 5 years. By using the most conservative assumptions-a low miss rate and low prevalence of cancer in adenomas-0.5 per 1000 persons would have a detectable CRC within 5 years after a screening colonoscopy. In contrast, using the highest reported miss rates and cancer prevalence, CRCs from missed lesions would occur in 3.5 per 1000 screened persons.

Conclusions: A significant number of patients undergoing a screening colonoscopy that did not detect cancer actually have a malignant lesion or adenoma that could progress in a short interval. Most interval cancers might reflect missed rather than new lesions. Improving adenoma detection could reduce the rate of interval cancers.