Multimodality imaging plays a key role in the management of malignant tumours by radiation oncologists. The tumour extension at diagnosis identifies the patients in whom curative-intent radiotherapy is indicated. The target volumes and organs at risk are delineated on the relevant image modality (CT, MRI, PET), co-registered on a CT scan in treatment position for dose calculation purpose. The treatment machines are nowadays able to achieve heterogeneous dose distributions, possibly modulated within the target volume according to variations in the risk of failure. For instance, areas with high initial FDG uptake, low oxygenation, or insufficient response during radiotherapy could become the targets for selective dose increase. The scientific challenge includes registration and iterative segmentation of poorly contrasted images acquired on living bodies (variation in acquisition position, physiological movements and hollow organs filling, signal alteration induced by treatment…). The clinical validation of these innovative approaches and their diffusion in daily routine require a very close collaboration between many disciplines, based on a common language and understanding of radiotherapy target volumes, as illustrated in the present paper with FDG-PET imaging.