Abstract
CT simulation is the first chain in radiotherapy treatment, the patient image is acquired by CT scanner to determine the exact location and densities of patient organs in specific coordinate called Pixel, each pixel has a unit called CT number or Hounsfield unit (HU) which representing the linear attenuation coefficient value of X-ray beam that interacts with each different tissue density. on the other hand, Electron density (ED) is the number of electrons contained by certain tissue of specific density, the ratio between each tissue electron density and water electron density is called relative electron density (RED), the conversion curve between CT number and RED should be measured and transferred to the treatment planning system (TPS) which are then used in the dose calculation algorithms. This study aims to evaluate how variations in tube voltage and current affect CT number-RED conversion curves. Utilizing a GE Lightspeed-RT CT scanner and CIRS (062M) phantom with inserts of diverse tissue densities, images were obtained at tube voltages of 80, 100, 120, and 140kV, at constant settings of tube currents ranging from 50 to 380mA, and vice versa. CT number mean and standard deviation were recorded for all inserts at different settings using Micro Dicom viewer software. Tube voltage variation at constant current affected high RED materials (dense bone), while tube current variation at constant voltage minimally impacted the CT-RED curve, except at low tube voltage (80kV). Considering evidence from this study and other studies on CT number's influence on dose calculation, it is advisable to account for scanner settings and specific CT-ED look-up tables, particularly in scenarios involving high-density materials, for accurate radiotherapy treatment planning