The disclosure is directed to a radiotherapy system having a treatment device for treating a treatment body part of a patient with a treatment beam arrangement. The treatment device is included with a couch for placing the patient and includes a medical imaging devices for outputting three-dimensional cone-beam computed tomography images to a computer, and a medical imaging x-ray device for generating at least one x-ray image, if the patient is placed on the couch for treatment, and for outputting at least one x-ray image to the computer. The system will output movement control data to control the relative position of the treatment body part relative to the treatment beam if it is determined there is the offset between the position of the treatment body part relative to the bony structure as described by the image data and the position of the treatment body part relative to a bony structure.