In radiation oncology, CT imaging is of crucial importance for consistent and accurate delineation of target and organ-at-risk structures as well as for highly precise treatment planning and prediction of the deposited dose. Even though the potential benefit of dual-energy CT for those purposes was identified early, its implementation in clinical practice is still in an early stage. Here, we want to give an overview of current and potential future applications of dual-energy CT in the field of radiation oncology. Since the next generation of X-ray computed tomography with a photon-counting detector technology is rising at the horizon, we also want to give an outlook on radiotherapeutic applications that will benefit or even become possible for the first time with this technological evolution.