The POPCORN project is a HealthHolland/TKI supported project and stands for Physiologically Optimised Plausible CoRegistratioN for external beam therapy. Image registration is a key step in adaptive radiotherapy, it allows the tracking of anatomical changes over time and the simultaneous use of information from multiple imaging modalities. By registering images delineations of relevant structures can be propagated but also dose can be accumulated over time. The goal of POPCORN is to exploit knowledge of the underlying tissue properties for optimizing image registration and for quality assessment of the registrations. Elekta is teh industrial collaborator on this project.