Simplified "on-couch" daily quality assurance procedure for CT simulators.

For most computed tomography (CT) simulators, radiation therapists must remove the flat couch top to perform daily CT quality assurance (QA), and then use separate tools to perform localization-laser QA. This process wastes time and effort and creates the opportunity for accidents to occur. In this study, we tested a simple yet comprehensive daily QA program and phantom that we designed for CT simulators used in radiation oncology that would enable us to use only one tool to perform laser and imaging QA on a flat couch. To construct a modified QA phantom, we attached three adjustable legs and fastened two metric scales (one vertically and one horizontally) to a commercial CT QA phantom. The adjustable legs helped to position the phantom conveniently in the needed position. The two metric scales were used for localization-laser QA, while the phantom body was used for CT imaging QA. We used five different scanners with their designated couches from two manufacturers to evaluate this phantom. Since the couch was scanned with the phantom, we evaluated the couch's effect on image quality. We found that the presence of the couch top changed the uniformity of water's CT number but did not change the visual image resolution; it also produced different, yet reproducible, effects on image quality. The effects were greatest in the section of the phantom closest to the couch top. For a commercial carbon fiber couch top, the variation was within 3 Hounsfield Unit (HU). The effect was couch- and scanner-specific and could be incorporated into the QA acceptability criteria for each CT scanner. By using the proposed program and phantom, we have been able to implement a more thorough QA program while decreasing the amount of effort and time the simulation therapists spend performing laser and imaging QA.