The present invention relates to a data processing method performed by a computer (7) for determining the position of a soft tissue body part within a patient's body, which is to be treated by radiotherapy with a treatment beam arrangement of at least one position of a treatment beam issued by a treatment device (1), the data processing method comprising the following steps:a. acquiring CT-image data comprising information about the position of the body part within a coordinate system assigned to a transportable CT-device (3), wherein the patient's body is positioned relative to the treatment device (1), and wherein the CT-device (3) is configured to be positioned relative to the patient's body and/or relative to the treatment device (1)；b. acquiring first transformation data comprising information about a first transformation between the coordinate system assigned to the CT-device (3) and a coordinate system assigned to the treatment device (1):c. determining, based on the CT-image data and the first transformation data, position data comprising information about the position of the body part within the coordinate system assigned to the treatment device (1).The present invention further relates to a corresponding computer (7) and system for determining the position of a soft tissue body part within a patient's body.
