The present invention refers to a data processing method for use in the field of radiation therapy and for determining a relative position between the position of the bony structure and a reference position at a monitoring time, the relative position being referred to as monitoring bone position, wherein an anatomical structure of a patient includes the bony structure and a treatment body part to be treated by at least one treatment beam of a treatment device, the reference position having a defined relative position with respect to an actual arrangement of at least one position of the at least one treatment beam the data processing method being constituted to be performed by a computer and comprising the following steps: • providing CBCT image data describing a three-dimensional CBCT image of the bony structure, the CBCT image representing the bony structure at a pre-alignment time • providing x-ray image data describing at least one two-dimensional x-ray image of the anatomical structure, the at least one two-dimensional x-ray image representing the bony structure (110) and the x-ray image data being generated at the monitoring time •providing imaging position data comprising at least one of CBCT position data and x-ray geometry data, CBCT position data describing the relative position between the CBCT image and the actual arrangement and the x-ray geometry data describing a positional relationship between the actual arrangement and at least one imaging geometry, referred to as x-ray imaging geometry, given for generating the at least one x-ray image •determining the relative position between the bony structure and the reference position at the monitoring time on the basis of the CBCT image data, the x-ray image data and the imaging position data.La présente invention porte sur un procédé de traitement de données destiné à être utilisé dans le domaine de la radiothérapie et pour déterminer une position relative entre la position de la structure osseuse et une posi
