The CT number corresponds to electron density and its influence on dose calculation was studied. Five kinds of CT scanners were used to obtain images of electron density calibration phantom (Gammex RMI 467). Then the differences between CT numbers for each scanners were ±2% in homogeneous medium and 9.5% in high density medium. In order to investigate the influence of CT number to dose calculation, patients' thoracic CT images were analyzed. The maximum dose difference was 0.48% for each organ. It acquired the phantom images inserted high density material in the water phantom. Comparing the doses calculated with CT images from each CT scanner, the maximum dose difference was 2.1% in 20 cm in depth. The exact density to CT number conversion according to CT scanner is required to minimize the uncertainty of dose depends on CT number. Especially the each hospital with various CT scanners has to discriminate CT numbers for each CT scanner. Moreover a periodic quality assurance is required for reproducibility of CT number.

KeywordsElectron density, CT number, Dose calculation