Method for the EPID-based review, correction and minimisation of the isocenter of a radiotherapy machine that comprises at least a patient support table, which is rotatable about a table axis, a supporting arm, which is rotatable about a supporting-arm axis, a radiation-emitter head, arranged on the supporting arm, for generating a therapy beam, a rotatable collimator, a device for projecting a radiological isocenter and a digital recording system (EPID) for capturing dose images by means of the therapy beam, wherein, according to the following steps: a) by means of the projection device, a measurement element is positioned at the projection position in the current radiological isocenter of the radiotherapy machine, b) a radiation field defined by the collimator is applied for at least one predetermined angular position of the supporting arm, of the patient support table and of the collimator, and in the process c) at least one common dose image of the measurement element and of the radiation field is recorded by the EPID, d) on the basis of the recorded dose image, a respective dose profile is created for each direction within an EPID coordinate system, and e) a respective inflection point is determined over the course of the dose profile at the two expected element boundaries of the measurement element in the X-direction of the EPID coordinate system and at the two expected element boundaries of the measurement element in the Y-direction of the EPID coordinate system between a local dose minimum and a local dose maximum and between a local dose maximum and a local dose minimum, and f) the determined positions of the inflection points are assigned to the element boundaries of the measurement element in the X-direction and in the Y-direction, g) on the basis of the assigned element boundaries of the measurement element in the dose image, a position of the centre point of the measurement element relative to the EPID centre is determined, wherein steps d) to g) are carried out similarly for the field boundaries and the field centre point of the radiation field, and h) a differential vector is ascertained from both a positional deviation of the centre point of the measurement element from the EPID centre and a positional deviation of the field centre point of the radiation field from the EPID centre, and i) the vector components of the differential vector are used to correct the current radiological isocenter, wherein a global spatial isocenter of the radiotherapy machine is ascertained from the individual central-beam deviations in the three directions in space X, Y and Z of the three spatial isocenters of the supporting arm, of the collimator and of the patient support table, wherein all the machine-specific parameters of the radiotherapy machine that influence the spatial isocenters are optimised by minimising a predetermined target function. a) by means of the projection device, a measurement element is positioned at the projection position in the current radiological isocenter of the radiotherapy machine, b) a radiation field defined by the collimator is applied for at least one predetermined angular position of the supporting arm, of the patient support table and of the collimator, and in the process c) at least one common dose image of the measurement element and of the radiation field is recorded by the EPID, d) on the basis of the recorded dose image, a respective dose profile is created for each direction within an EPID coordinate system, and e) a respective inflection point is determined over the course of the dose profile at the two expected element boundaries of the measurement element in the X-direction of the EPID coordinate system and at the two expected element boundaries of the measurement element in the Y-direction of the EPID coordinate system between a local dose minimum and a local dose maximum and between a local dose maximum and a local dose minimum, and f) the determined positions of the inflection points are assigned to the element boundaries of the measurement element in the X-direction and in the Y-direction, g) on the basis of the assigned element boundaries of the measurement element in the dose image, a position of the centre point of the measurement element relative to the EPID centre is determined, wherein steps d) to g) are carried out similarly for the field boundaries and the field centre point of the radiation field, and h) a differential vector is ascertained from both a positional deviation of the centre point of the measurement element from the EPID centre and a positional deviation of the field centre point of the radiation field from the EPID centre, and i) the vector components of the differential vector are used to correct the current radiological isocenter, wherein a global spatial isocenter of the radiotherapy machine is ascertained from the individual central-beam deviations in the three directions in space X, Y and Z of the three spatial isocenters of the supporting arm, of the collimator and of the patient support table, wherein all the machine-specific parameters of the radiotherapy machine that influence the spatial isocenters are optimised by minimising a predetermined target function.