The invention is directed to 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 being constituted to be executed by a computer and comprising the following steps: g) acquiring CT data comprising predetermined CT 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; h) acquiring x-ray data comprising x-ray information about a second position of the bony structure relative to the treatment beam arrangement; i) determining, based on the x-ray data and the CT 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; j) acquiring CBCT data comprising CBCT information about the position of the treatment body part relative to the treatment beam arrangement or relative to the bony structure; k) determining, based on 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; 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.