is very important to accurately determine the electron density distribution of human tissues for treatment planning in heavy-ion radiotherapy to calculate the patient dose distribution. Currently, to estimate the electron density of body tissues, one -to-one correspondence from CT number to electron density is performed using calibration phantom of known electron densities beforehand. However, because the CT number includes the information of both the effective atomic number and the electron

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... sity, electron density is not determined uniquely. The purpose of this study is to examine a direct estimate of the electron density by computer simulation using the method of decomposition to photoelectric data and Compton data from Dual Energy CT in the projection domain. In addition, we report the underlying examination about the cause of the estimated error of the electron density.