Visualization of image transformation

There is provided a method comprising: obtaining first and second datasets representative of first and second images of an object at different times, respectively; obtaining a deformation field, in particular a three dimensional vector field, representative of changes between the first and second data sets, wherein the deformation field is generated by performing a rigid or non-rigid registration; generating one or more masks and/or segmentations for selecting elements of the first image; selecting elements of the first image, in particular contours, points, and/or dosimetric image portions; transforming the first dataset using the deformation field to project the selected elements onto the second image; visualizing the deformation field or previously specified portions thereof; processing the deformation field or previously specified portions thereof to obtain data representative of different predetermined types of deformation, in particular coarse and fine deformations; and visualizing the deformation field or one or more selected portions thereof, thereby to visualise the predetermined types of deformations separately and to enable a differentiation between changes in the patient's body and changes, in particular errors in the patient's position.