The electron density is indispensable to the treatment planning for heavy-ion radiotherapy, so that the precise measurement of that is important for advancing radiotherapy. Usually the electron density for the radiotherapy is derived from an image based on CT number by conventional CT. CT number include ambiguilty due to the beam hardening. In a method of dual-energy x-ray CT, the information on an electron density can be obtained directly using monochromatic x-rays. The precision of the electron density was experimentally proved to be about 1%.The 8th Conference on Biology and Synchrotron Radiatio
