An evaluation of computed tomographic colonography in the English national bowel cancer screening programme

Colorectal cancer (CRC) is a common, important disease with a mortality approaching 50%. Outcomes can be improved by screening because early CRC has an excellent prognosis and there is a long, easily treatable premalignant phase. England has a national bowel cancer screening programme (BCSP) based on faecal occult blood testing (FOBt), followed by colonoscopy for those testing positive. However, not all subjects can undergo colonoscopy due to frailty or technical failure. The preferred alternative is computed tomographic colonography (CTC), an X-ray scan of the cleansed, gas-distended colon. Little is known regarding performance characteristics and other attributes of CTC in the English BCSP, the subject of this thesis. The thesis is structured as follows: Section A provides an overview of CRC screening and the BCSP, followed by a systematic review and meta-analysis of the diagnostic performance of CTC for FOBt-positive subjects. Section B describes current usage of CTC in the BCSP via a national survey of current practice and an analysis of neoplasia detection rates in comparison to colonoscopy, including sub-analyses of the effects of (a) variable FOBt positivity and (b) terminal digit preference bias. Section C evaluates patient experience via comparative analysis of national questionnaires for CTC and colonoscopy. Reasons for non-attendance for colonic testing (despite a positive FOBt result) are investigated via an interview study, and a discrete choice experiment tests the influence of extracolonic abnormalities on the acceptability of CTC (for both patients and healthcare professionals). Section D evaluates methods to improve diagnostic accuracy, using eye-tracking to investigate why radiologists miss polyps, and the effect of increased viewing speeds. The different spectrum of disease encountered in a screening population is explored via comparison of the CTC appearances of screen-detected and symptomatic colorectal cancers. Section E concludes and summarises the thesis, and makes recommendations for future research