1. 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 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.