A computer program which, when executed by a computer of a radiotherapy system comprising a cone-beam computed tomography CBCT-device, causes the computer to perform a data processing method, wherein the data processing method is for use in the field of radiation therapy and is for determining a position (d 4at) 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 (d 4at) 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 (S6, S7) 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 (d 2tb) to each other, called tissue-bone pre-alignment position, the CBCT image being generated at a point in time, called pre-alignment time, wherein the pre-alignment time is a time at which the patient is placed for alignment with respect to the treatment device and wherein the pre-alignment time is after a planning time at which a pre-image as a basis for planning a relative position between the treatment body part and a planned arrangement of the at least one position of the treatment beam is generated; • providing x-ray data describing information on a position (d 1ba) of the bony structure, called monitoring bone position (d 1ba), 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 (d 4at) of the treatment body part relative to the actual arrangement ∘ based on the tissue-bone pre-alignment position (d 2tb) represented by the CBCT data, ∘ based on the information on the monitoring bone position (d 1ba) described by the x-ray data, and ∘ based on a position (d 3tb) called tissue-bone monitoring position which describes a relative position between the treatment body part and the bony structure at the monitoring time, the tissue-bone monitoring position being set based on a predefined function of identity between the tissue-bone pre-alignment position (d 2tb) and the tissue-bone monitoring position (d 3tb) as well as based on the tissue-bone pre-alignment position (d 2tb), wherein according to the function of identity the treatment body part performs a positional change which is identical with the positional change of the bony structure. • providing (S6, S7) 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 (d 2tb) to each other, called tissue-bone pre-alignment position, the CBCT image being generated at a point in time, called pre-alignment time, wherein the pre-alignment time is a time at which the patient is placed for alignment with respect to the treatment device and wherein the pre-alignment time is after a planning time at which a pre-image as a basis for planning a relative position between the treatment body part and a planned arrangement of the at least one position of the treatment beam is generated; • providing x-ray data describing information on a position (d 1ba) of the bony structure, called monitoring bone position (d 1ba), 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 (d 4at) of the treatment body part relative to the actual arrangement ∘ based on the tissue-bone pre-alignment position (d 2tb) represented by the CBCT data, ∘ based on the information on the monitoring bone position (d 1ba) described by the x-ray data, and ∘ based on a position (d 3tb) called tissue-bone monitoring position which describes a relative position between the treatment body part and the bony structure at the monitoring time, the tissue-bone monitoring position being set based on a predefined function of identity between the tissue-bone pre-alignment position (d 2tb) and the tissue-bone monitoring position (d 3tb) as well as based on the tissue-bone pre-alignment position (d 2tb), wherein according to the function of identity the treatment body part performs a positional change which is identical with the positional change of the bony structure. ∘ based on the tissue-bone pre-alignment position (d 2tb) represented by the CBCT data, ∘ based on the information on the monitoring bone position (d 1ba) described by the x-ray data, and ∘ based on a position (d 3tb) called tissue-bone monitoring position which describes a relative position between the treatment body part and the bony structure at the monitoring time, the tissue-bone monitoring position being set based on a predefined function of identity between the tissue-bone pre-alignment position (d 2tb) and the tissue-bone monitoring position (d 3tb) as well as based on the tissue-bone pre-alignment position (d 2tb), wherein according to the function of identity the treatment body part performs a positional change which is identical with the positional change of the bony structure.