A device for determining a deviation between an isocentre of an irradiation device displayed by a laser arrangement and the actual isocentre of the irradiation device comprising - a laser arrangement with a plurality of lasers (14, 16, 18, 20) and a laser control apparatus which is designed to control lasers (14, 16, 18, 20) of the laser arrangement such that they display the isocentre, - a laser detector apparatus that is designed to detect laser radiation emitted by the lasers (14, 16, 18, 20), and wherein the laser detector apparatus is furthermore designed to determine the position and/or alignment of laser radiation fields (40, 42, 65, 66) emitted by the lasers (14, 16, 18, 20), wherein the laser detector apparatus comprises at least one one-dimensional or two-dimensional laser detector that comprises at least one one-dimensional or two dimensional sensor array, - a device for determining the isocentre of an irradiation device for medically irradiating a patient with high-energy radiation comprising a beam detection apparatus with a detector disc, wherein on at least one disc surface of the detector disc, there is disposed a plurality of beam detectors for detecting high-energy radiation emitted by the irradiation device, wherein the beam detectors are disposed distributed along an S-shape on the disk surface, and wherein a rotary drive is provided by means of which the detector disc is rotatable about its central axis, and comprising an evaluation device which is designed to determine the isocentre of the irradiation device from the measured values detected by the beam detectors - an evaluation apparatus which is designed to determine a deviation between the isocentre displayed by the laser arrangement and the actual isocentre from the measured data recorded by the laser detector apparatus and the beam detector apparatus. - a laser arrangement with a plurality of lasers (14, 16, 18, 20) and a laser control apparatus which is designed to control lasers (14, 16, 18, 20) of the laser arrangement such that they display the isocentre, - a laser detector apparatus that is designed to detect laser radiation emitted by the lasers (14, 16, 18, 20), and wherein the laser detector apparatus is furthermore designed to determine the position and/or alignment of laser radiation fields (40, 42, 65, 66) emitted by the lasers (14, 16, 18, 20), wherein the laser detector apparatus comprises at least one one-dimensional or two-dimensional laser detector that comprises at least one one-dimensional or two dimensional sensor array, - a device for determining the isocentre of an irradiation device for medically irradiating a patient with high-energy radiation comprising a beam detection apparatus with a detector disc, wherein on at least one disc surface of the detector disc, there is disposed a plurality of beam detectors for detecting high-energy radiation emitted by the irradiation device, wherein the beam detectors are disposed distributed along an S-shape on the disk surface, and wherein a rotary drive is provided by means of which the detector disc is rotatable about its central axis, and comprising an evaluation device which is designed to determine the isocentre of the irradiation device from the measured values detected by the beam detectors - an evaluation apparatus which is designed to determine a deviation between the isocentre displayed by the laser arrangement and the actual isocentre from the measured data recorded by the laser detector apparatus and the beam detector apparatus.