Functional imaging has the potentialto provide a paradigm shift in radiation treatment planning and tosubstantially impact clinical outcomes. With its ability to not only considerthe pre-treatment conditions of patients but also their intrinsicresponsiveness to the treatment adaptive dose painting based on PET imagedbiomarkers provides an important step towards biologically guided radiationtherapy.Due to the variety of possible biomarkers and the differences in theirrespective PET tracers finding the most suitable combination of tracers andproposing a quantitative radiobiological model to simultaneously account forthem is and will be one of the main objectives of current and future studies.An at least equally challenging task is the further development of profound andclinically feasible dose painting calculation models to adequately process thedata acquired by functional imaging. To do so further research and clinicalstudies are indispensable.