Anne T Davis, Antony L Palmer, Andrew Nisbet

This paper reviews publications related to the use of CT scans for radiotherapy treatment planning, specifically the impact of scan protocol changes on CT number and treatment planning dosimetry and on CT image quality. A search on PubMed and EMBASE and a subsequent review of references yielded 53 relevant papers. CT scan parameters significantly affect image quality. Some will also affect Hounsfield unit (HU) values though this is not comprehensively reported on. Changes in tube kilovoltage and, on some scanners, field of view and reconstruction algorithms have been found to produce notable HU changes. The degree of HU change which can be tolerated without changing planning dose by more than 1% depends on the body region and size, the planning algorithms, the treatment beam energy and type of plan. A change in soft tissue HU value has a greater impact than changes in HU for bone and air. The use of anthropomorphic phantoms is recommended when assessing HU changes. There is limited published work on CT scan protocol optimisation in radiotherapy. Publications suggest that HU tolerances of +/- 20 HU for soft tissue and +/- 50 HU for lung and bone would restrict dose changes in the treatment plan to less than 1%. Advances in knowledge: Literature related to the use of CT images in radiotherapy planning has been reviewed to establish the acceptable level of HU change and the impact on image quality of scan protocol adjustment. Conclusions have been presented and further work identified.