If you can't see it, you can miss it: the role of biomedical imaging in radiation oncology.

During the last three decades, 3D imaging as X-ray computerised tomography and magnetic resonance imaging was introduced to characterise tumour morphology for an improved delineation of the gross target volume and the clinical target volume. At present, the time has come to also start the assessment and correction of the temporal changes of the target volume. This is the basis of 'image-guided radiotherapy' (IGRT), which is characterised by the integration of 2D and 3D imaging modalities into the radiotherapy workflow. The vision is to detect deformations and motion between radiotherapy fractions (inter-fractional IGRT) and during beam delivery (intra-fractional IGRT). Another challenge in radiotherapy is to develop concepts to include and integrate biological imaging into radiotherapy, first by extending the morphological towards a biological planning target volume and second by delivering appropriate inhomogeneous dose distributions, e.g. with the new tools of photon- and particle- Intensity Modulated Radiotherapy (IMRT) techniques ('dose painting').