Scanning protocol influence on relative electron Density-CT number calibrations and radiotherapy dose calculation for a Halcyon Linac

Tissue Relative Electron Density (RED), typically determined via computation of CT-based Hounsfield unit (HU) values, has an accuracy which is affected by the choice of CT scanning parameters. The influence of these on the RED-CT number calibration curves is of importance, not least concerning variations in CT x-ray tube voltage and current. The purpose of this study was to examine the effect of variations in CT x-ray tube voltage and current on the calibration curves of RED - CT number conversion, with the aim of improving the accuracy of dose calculation in radiotherapy treatment planning. An electron density phantom CIRS have been conducted at 200 mAs, for tube voltages of 80, 110, and 130 kVp, corresponding images being calibrated in terms of RED-CT number conversion curves. Subsequent treatment plans have been generated, identified in terms of the applied tube voltages, the 130 kVp plan being designated the reference plan. The remaining plans have been compared to the reference plan via analysis of isodose distribution and dose. The influence of tube voltage were being found to be greatest for high-density substances such as bone in comparison to low-density materials. Conversely, the CT number of a titanium rod has been consistent, regardless of the choice of tube voltage or current. The effect of tube current variation on the calibration curves has been minor at higher tube voltages but more pronounced using lower voltages and current. The presence in the phantom of plugs of high-density materials has resulted in beam hardening artefacts, impacting on the RED values for other plugs. In achieving accurate dose calculation for radiotherapy treatment plans the work underscores the need to account for effects of CT scanning parameters on the RED-CT number conversion curves. Further emphasized is the importance of careful selection of imaging protocols in seeking to minimize beam hardening artefacts and to ensure accurate determination of RED. The study demonstrates that the accuracy of RED-CT number conversion curves is substantially affected by the presence of high-density materials in the phantom, which causes beam hardening artefacts that affect the precision of RED values for other plugs.