Figure 4. Evaluation of deformable registration as applied in adaptive radiotherapy. The CT dataset (a) was warped to the CBCT dataset (b) using a monomodality diffeomorphic demons algorithm (c), (left column in results section) and a multimodality B‐Spline algorithm (d), (right column in results section). In (c), HU calibrations and various artifacts present in the CBCT dataset violated the monomodality assumption of the first algorithm, leading to unrealistic warping. In (d), the solution provided by algorithm 2 interpolates artifacts but is not able to model small local organ changes. The vortex map easily catches these characteristics of the displacement fields (e) and (f). When warping the dose distribution from the planning to the CBCT dataset, the distribution warped by algorithm 1 looks unnatural and unrealistic (g), while the dose distribution warped by algorithm 2 looks natural (h).