The advent of CT scanners with larger bore sizes have offered the potential for radiotherapy departments to undertake the majority or all treatment planning simulation via CT. Wide-bore scanners can present some unique issues when being commissioned for clinical use. During the process of converting a radiotherapy department to full CT simulation, several issues regarding images produced by a wide-bore scanner were identified. These were investigated by using electron density and image resolution phantoms. It was found that the reconstruction algorithm used by the scanner of interest for extended field of view (FOV) imaging, combined with the extended X-ray source-to-detector distance, influenced the resolution and quality of images. The reconstruction technique influenced the relationship between electron density and CT number with distance from the scanner axis, leading to image artefacts. A variation of 400 CT units is seen for cortical bone across the scanner FOV, with smaller variations for water and breast tissue. It is anticipated that this variation will impact on tissue delineation, and subsequent dose calculation would become questionable should beams pass through large areas of artefact. Image resolutions of 0.5 and 0.3 line-pairs per millimetre (lp/mm) were achievable in the primary and extended FOV regions respectively. Several aspects of image production with a wide-bore scanner that can influence imaging for radiotherapy treatment planning have been highlighted. Departments should be mindful of these issues when using a GE Lightspeed wide-bore scanner and should consider how scanner settings should be optimised for the use of images in treatment planning.