1. A computer program fixed in a non-transitory computer readable medium executable on a computer to implement the steps of: (a) receiving a set of images of a treatment region of a patient for at least three different image times; (b) determining inverse consistent deformation maps each describing a deformation in the treatment region between pairs of images; (c) creating a transitive deformation map between at least one pair of the images, the pair comprising a first and second image, the transitive deformation maps combining the inverse consistent deformation maps between the first and second image and the inverse consistent deformation maps between the first and second images via third images of the set other than the first and second images; and (d) applying the transitive deformation map to image-type data registered to the first image to register the image-type data with the second image.