The present application relates to a data processing system and method for determining the position of a soft tissue body part within a patient's body. The data processing method includes acquiring CT-image data including information about the position of the body part within a coordinate system assigned to a transportable CT-device, wherein the patient's body is positioned relative to the treatment device, and wherein the CT-device is configured to be positioned relative to the patient's body and/or relative to the treatment device, acquiring first transformation data including information about a first transformation between the coordinate system assigned to the CT-device and a coordinate system assigned to the treatment device, and determining, based on the CT-image data and the first transformation data, position data including information about the position of the body part within the coordinate system assigned to the treatment device.