Functional imaging with positron emission tomography (PET) is interesting to optimize radiotherapy planning, and probably to perform dose redistribution in many cancers. However, in 2015, fluorodeoxyglucose (FDG)-PET is validated only for therapy planning for lung cancer. The interest of dose painting, PET in non-lung cancers, the interest of other tracers (even PET hypoxia tracers), DW-MRI, and PET-MR for radiotherapy needs to be demonstrated in prospective multicentric phase III studies on large series of patients.