A computer-implemented tumor position determining model is trained, based on a plurality of sets of image data, to determine a subsequent position of a tumor in a subject based on a subsequent 2D or 3D representation of a surface of the subject, an initial image of the tumor in the subject and an initial 2D or 3D representation of a surface of the subject. Each set of image data comprises an initial training image of a tumor in a subject, an initial training 2D or 3D representation of a surface of the subject, a subsequent training image of the tumor in the subject and a subsequent training 2D or 3D representation of a surface of the subject. The subsequent training image and the subsequent training 2D or 3D representation are taken at a subsequent point in time than the initial training image and the initial training 2D or 3D representation and the plurality of sets of image data are from a plurality of different subjects.