Objective The quality of colonoscopy is key for ensuring protection against colorectal cancer (CRC). We therefore aimed to elucidate the aetiology of postcolonoscopy CRCs (PCCRCs), and especially to identify preventable factors.

Methods We conducted a population-based study of all patients diagnosed with CRC in South-Limburg from 2001 to 2010 using colonoscopy and histopathology records and data from the Netherlands Cancer Registry. PCCRCs were defined as cancers diagnosed within 5 years after an index colonoscopy. According to location, CRCs were categorised into proximal or distal from the splenic flexure and, according to macroscopic aspect, into flat or protruded. Aetiological factors for PCCRCs were subdivided into procedure-related (missed lesions, inadequate examination/surveillance, incomplete resection) and biology-related (new cancers).

Results We included a total of 5107 patients with CRC, of whom 147 (2.9% of all patients, mean age 72.8 years, 55.1% men) had PCCRCs diagnosed on average 26 months after an index colonoscopy. Logistic regression analysis, adjusted for age and gender, showed that PCCRCs were significantly more often proximally located (OR 3.92, 95% CI 2.71 to 5.69), smaller in size (OR 0.78, 95% CI 0.70 to 0.87) and more often flat (OR 1.70, 95% CI 1.18 to 2.43) than prevalent CRCs. Of the PCCRCs, 57.8% were attributed to missed lesions, 19.8% to inadequate examination/surveillance and 8.8% to incomplete resection, while 13.6% were newly developed cancers.

Conclusions In our experience, 86.4% of all PCCRCs could be explained by procedural factors, especially missed lesions. Quality improvements in performance of colonoscopy, with special attention to the detection and resection of proximally located flat precursors, have the potential to prevent PCCRCs.