The invention relates to a method for EPID-based verification, correction and minimisation of the isocentre of a radiotherapy device, in which method, according to the following steps: a) a measurement body is positioned, by means of the projection device, at the projection position in the current radiological isocentre of the radiotherapy device, b) an irradiation field, limited by the collimator, is applied for at least one pre-defined angular adjustment of the support arm, the patient couch and the collimator and c) at least one common dose image of the measurement body and the irradiation field is captured using the EPID, d) a dose profile for each direction within an EPID co-ordinate system is created on the basis of the captured dose image and e) in the plot of the dose profile an inflection point between a local dose minimum and a local dose maximum, and between a local dose maximum and a local dose minimum is determined at each of both expected bodily limits of the measuring body in the X-direction of the EPID-co-ordinate system and at each of the two expected bodily limits of the measuring body in the Y-direction of the EPID-co-ordinate system, and f) the determined positions of the inflection points are linked to the bodily limits of the measurement body in the X-direction and in the Y-direction, g) the position of the centre point of the measurement body relative to the EPID-centre is determined in the dose image on the basis of the linked bodily limits of the measurement body, the steps d) to g) being carried out in the same way for the field limits and the field centre point of the irradiation field, and h) a differential vector is determined from a deviation in position of the centre point of the measurement body from the EPID-centre and from a deviation in position of the field centre point of the irradiation field from the EPID-centre, and i) the vector components of the differential vector are used to correct the current radiological isocentre.
