The present invention relates to a data processing method for use in the field of radiation therapy and for determining a position of a treatment body part relative to an actual arrangement of at least one position of a treatment beam issued by a treatment device, the position being called monitoring tissue position and the treatment body part being a soft tissue part of an anatomical structure of a patient the data processing method being constituted to be performed by a computer and comprising the following steps: • providing CBCT data describing a three-dimensional CBCT image of the anatomical structure, the three dimensional CBCT image representing the treatment body part and a bony structure in a relative position to each other, called tissue-bone pre-alignment position, at a point in time, called pre-alignment time • providing x-ray data describing information on a position of the bony structure, called monitoring bone position, relative to the actual arrangement at a point in,time during treatment, called monitoring time, the monitoring time being after the pre-alignment time • determining the monitoring tissue position of the treatment body part relative to the actual arrangement º based on the tissue-bone pre-alignment position represented by the CBCT image data, º based on the information on the monitoring bone position described by the x- ray data, and º based on a predefined relationship between a first relative position between the treatment body part and the bony structure at the pre-alignment time and a second relative position between the treatment body part and the bony structure at the monitoring time.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 dune partie corporelle de traitement par rapport à un agencement réel dau moins une position dun faisceau de traitement émis par un dispositif de traitement, la position étant appelée position d
