Method for identification of possible changes in range of a planned radiation field before irradiating a patient with charged particles, having the following steps: carrying out computer tomography over at least a part of the patient to create a three-dimensional computer tomography data set; providing an X-ray system from two high-resolution X-ray detectors which include an angle of 60° to 120°, preferably 75° to 105°, with one another, and two X-ray sources which are opposite the X-ray detectors, wherein each one pair of X-ray source and X-ray detector defines one projection direction; producing two actual X-ray images of at least one part of the patient in the two projection directions with the two X-ray detectors; calculating two digitally reconstructed ideal X-ray images from the computer tomography data set corresponding to the two projection directions of the X-ray system; undertaking a differential analysis to determine differences between the actual X-ray image and the associated ideal X-ray image for the two projection directions; examining the relevance of the established differences with regard to spatial expansion and with regard to localisation in relation to the planned radiation field and determining relevant two-dimensional variation fields; establishing a three-dimensional variation body through back projection of the two-dimensional variation fields into the three-dimensional computer tomography data set; calculating a mean electron density (p) of the three-dimensional variation body on the basis of the differences detected between the actual X-ray image and the associated ideal X-ray image for the two projection directions; identifying individual rays of the planned radiation field which pass through the three-dimensional variation body; calculating the modified ranges to be expected of the identified individual rays of the planned radiation field taking into account the modified electron density value (p) in the three-dimensional variation body; and Illustrating or forwarding information (114, 116, 118) about the calculated modified ranges. carrying out computer tomography over at least a part of the patient to create a three-dimensional computer tomography data set; providing an X-ray system from two high-resolution X-ray detectors which include an angle of 60° to 120°, preferably 75° to 105°, with one another, and two X-ray sources which are opposite the X-ray detectors, wherein each one pair of X-ray source and X-ray detector defines one projection direction; producing two actual X-ray images of at least one part of the patient in the two projection directions with the two X-ray detectors; calculating two digitally reconstructed ideal X-ray images from the computer tomography data set corresponding to the two projection directions of the X-ray system; undertaking a differential analysis to determine differences between the actual X-ray image and the associated ideal X-ray image for the two projection directions; examining the relevance of the established differences with regard to spatial expansion and with regard to localisation in relation to the planned radiation field and determining relevant two-dimensional variation fields; establishing a three-dimensional variation body through back projection of the two-dimensional variation fields into the three-dimensional computer tomography data set; calculating a mean electron density (p) of the three-dimensional variation body on the basis of the differences detected between the actual X-ray image and the associated ideal X-ray image for the two projection directions; identifying individual rays of the planned radiation field which pass through the three-dimensional variation body; calculating the modified ranges to be expected of the identified individual rays of the planned radiation field taking into account the modified electron density value (p) in the three-dimensional variation body; and Illustrating or forwarding information (114, 116, 118) about the calculated modified ranges.