Background: Interval colorectal cancer (CRC) occasionally is detected in patients who have recently undergone colonoscopy. Systematic evaluation of CRC detected after colonoscopy could identify ways to improve the quality and the outcome of colonoscopy.
Methods: This study examined cancer diagnoses in the course of the dietary Polyp Prevention Trial, a randomized study of a dietary intervention on recurrence of adenomatous polyps. An algorithm was developed to classify each cancer into one of 4 etiologies: (1) incomplete removal (cancer at the site of previous adenoma), (2) failed biopsy detection (cancer in an area of suspected neoplasia with negative biopsy specimens), (3) missed cancer (large, advanced stage cancer found at a short interval after colonoscopy), or (4) new cancer (small, early stage cancer after a longer time interval).
Results: Of 2079 patients, 13 had cancer detected over 5810 person years of observation (PYO) (2.2 cases/1000 PYO); 7/13 or 53.8% of patients had either a potentially "avoidable" cancer or one detectable at an earlier time interval because of incomplete removal (4/13) or missed cancer (3/13).
Conclusions: Interval cancer occurs despite colonoscopy. Improved quality of colonoscopy may have reduced cancer prevalence or resulted in earlier cancer detection in over 50% of prevalent cancers in the dietary Polyp Prevention Trial.