Methods and apparatus for comparing two image data sets from medical imaging data of a subject are disclosed. A first, three-dimensional image data set of the subject and a second, two-dimensional image data set of the subject are obtained. The first data set is registered with the second data set. Data from the first, three-dimensional image data set is then processed to determine a voxel in the first data set which corresponds to a given pixel in the second, two-dimensional image data set. The registration process may involve generating a two dimensional image from the three dimensional image and said projection image may be a virtual planar projection. The invention may be applicable to dosimetry analysis in which case a segment from the three dimensional image is propagated to corresponding pixels in the two dimensional image.