  Purpose: Electron density is the most important tissue property influencing
photon and ion dose distributions in radiotherapy patients. Dual-energy
computed tomography (DECT) enables the determination of electron density by
combining the information on photon attenuation obtained at two different tube
voltages. Most algorithms suggested so far use the CT numbers provided after
image reconstruction as input parameters, i.e. are imaged-based. To explore the
accuracy that can be achieved with these approaches, we quantify the intrinsic
methodological and calibration uncertainty of the seemingly simplest approach.
