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 ∘ based on the tissue-bone pre-alignment position represented by the CBCT image data, ∘ based on the information on the monitoring bone position described by the x-ray data, and ∘ 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.
