A data processing method for determining the consistency of registration of the position of a treatment body part to be treated by radiotherapy with a treatment beam arrangement of at least one position of a treatment beam issued by a treatment device, the treatment body part being a soft tissue part of an anatomical structure of a patient's body and the data processing method comprising the following steps which are constituted to be executed by a computer: a) acquiring CT data comprising predetermined computed tomography information about a position of the treatment body part relative to a bony structure of the patient's body and about a first position of the bony structure relative to the treatment beam arrangement; b) acquiring x-ray data comprising x-ray information about a second position of the bony structure relative to the treatment beam arrangement; c) determining, based on the x-ray data and the CT data by applying a fusion algorithm to the CT data and the x-ray data, bony structure position first transformation data comprising bony structure position first transformation information about a first transformation between the first position and the second position of the bony structure; d) acquiring CBCT data comprising cone-beam computed tomography information about a third position of the bony structure relative to the treatment beam arrangement and about the position of the treatment body part relative to the bony structure; e) determining (S 13), based on the CBCT data and the CT data by applying a fusion algorithm to the CBCT data and the CT data, bony structure position second transformation data comprising bony structure position second transformation information about a second transformation between the first position and a third position of the bony structure relative to the treatment beam arrangement; f) determining, based on the bony structure position first transformation data and the bony structure position second transformation data, transformation difference data comprising transformation difference information about a difference between the first and second transformations; and g) determining (S 13; S 14; S24), based on the transformation difference data and the CBCT data and the CT data, whether there is an offset between the position of the treatment body part relative to the bony structure as it is described by the CBCT data and the position of the treatment body part relative to the bony structure as it is described by the CT data. a) acquiring CT data comprising predetermined computed tomography information about a position of the treatment body part relative to a bony structure of the patient's body and about a first position of the bony structure relative to the treatment beam arrangement; b) acquiring x-ray data comprising x-ray information about a second position of the bony structure relative to the treatment beam arrangement; c) determining, based on the x-ray data and the CT data by applying a fusion algorithm to the CT data and the x-ray data, bony structure position first transformation data comprising bony structure position first transformation information about a first transformation between the first position and the second position of the bony structure; d) acquiring CBCT data comprising cone-beam computed tomography information about a third position of the bony structure relative to the treatment beam arrangement and about the position of the treatment body part relative to the bony structure; e) determining (S 13), based on the CBCT data and the CT data by applying a fusion algorithm to the CBCT data and the CT data, bony structure position second transformation data comprising bony structure position second transformation information about a second transformation between the first position and a third position of the bony structure relative to the treatment beam arrangement; f) determining, based on the bony structure position first transformation data and the bony structure position second transformation data, transformation difference data comprising transformation difference information about a difference between the first and second transformations; and g) determining (S 13; S 14; S24), based on the transformation difference data and the CBCT data and the CT data, whether there is an offset between the position of the treatment body part relative to the bony structure as it is described by the CBCT data and the position of the treatment body part relative to the bony structure as it is described by the CT data.