1. A method in the field of medicine for tracking image sections which represent indicator body parts of a body in a process sequence of images in order to determine the position of another body part referred to as a treatment body part, wherein changes in the position of the indicator body parts are used as an indicator for changes in the position of the treatment body part of the body which is to be treated using a radiation treatment beam, said method comprising the following steps performed by a computer: providing advance image data generated before the treatment, the advance image data describing an advance sequence of a plurality of images representing a result of an advance analysis of the body; providing process image data which describe the process sequence of images, wherein the process sequence of images represents a result of a process analysis of the body which is performed after the advance analysis and during the treatment; providing correlation data which describe the correlation between a cyclic trajectory of the indicator body part and a trajectory of the treatment body part; determining by means of a computer, on the basis of the advance image data, a set of a plurality of sub-images in the respective images of the advance sequence, the plurality of sub-images undergoing similar changes in position in the advance sequence and undergoing a cyclic trajectory in the advance sequence; extracting by means of the computer during treatment, on the basis of the process image data and the determined set of sub-images, a set of image sections from the images of the process sequence which undergo similar changes in position and which have an image content which corresponds to the image content of the determined sub-images; tracking the set of extracted image sections in the process sequence as a group by means of a computer; and, determining a position of a treatment body part during the treatment on the basis of the position of the tracked representation of the indicator body parts and on the basis of the correlation data by means of the computer.