Dual- and multi-energy CT for particle stopping-power estimation: current state, challenges and potential.

Range uncertainty has been a key factor preventing particle radiotherapy from reaching its full physical potential. One of the main contributing sources is the uncertainty in estimating particle stopping power (ρs) within patients. Currently, the ρs distribution in a patient is derived from a single- energy CT (SECT) scan acquired for treatment planning by converting CT number (μHU) of each voxel to ρs using a Hounsfield look-up table (HLUT), also known as the CT calibration curve. μHU and ρs share a linear relationship with electron density but fundamentally differ in additional physical properties, such as photon attenuation cross section or effective atomic number and stopping number or mean excitation energy, respectively. Due to this degeneracy, the HLUT approach is particularly sensitive to differences in elemental composition between real human tissues and tissue surrogates as well as tissue variations within and among individual patients. The use of dual-energy CT (DECT) for ρs prediction has been shown to be effective in reducing the uncertainty in ρs estimation compared to SECT. The acquisition of CT data over different x-ray spectra yields additional information on the material elemental composition. Recently, multi-energy CT (MECT) has been explored to deduct material-specific information with higher dimensionality, which has the potential to further improve the accuracy of ρs estimation. Even though various DECT and MECT methods have been proposed and evaluated over the years, these approaches are still only scarcely implemented in routine clinical practice. In this topical review, we aim at accelerating this translation process by providing: 1) a comprehensive review of the existing DECT/MECT methods for ρs estimation with their respective strengths and weaknesses; 2) a general review of uncertainties associated with DECT/MECT methods; 3) a general review of different aspects related to clinical implementation of DECT/MECT methods; 4) other potential advanced DECT/MECT applications beyond ρs estimation.