The present invention relates to a determination method that enables the position of a body part subject to vital movement to be determined, wherein the body part is to be irradiated. A region of an analytical image is determined, wherein the region represents a body part subject to vital movement, in particular a tumor in an anatomical body. A change in position from one analytical image to the next is performed that reflects the vital movement, wherein the body parts subject to vital movement are parts of the body that are to be irradiated or are not to be irradiated and that move as a result of vital functions such as respiration and/or heartbeat even when the anatomical body is otherwise kept deliberately at rest.