A medical data processing method for determining a change in the position of a soft tissue body part of a patient's body, the data processing method being constituted to be executed by a computer and comprising the following steps: a) acquiring (S10, S20) bony body part planned position data comprising bony body part planned position information describing a planned position of a bony body part of the patient's body; b) acquiring (S10, S20) soft tissue body part planned position data comprising soft tissue body part planned position information describing a planned position of the soft tissue body part; c) acquiring (S11, S21) bony body part actual position data comprising bony body part actual position information describing an actual position of the bony body part; d) acquiring (S11, S21) soft tissue body part actual position data comprising soft tissue body part actual position information describing an actual position of the soft tissue body part; e) determining (S12, S22), based on the bony body part planned position data and the bony body part actual position data, bony body part position transformation data comprising bony body part position transformation information describing a bony body part position transformation between the planned position and the actual position of the bony body part; f) determining (S13, S23), based on the bony body part position transformation data and the soft tissue body part actual position data, soft tissue transformed actual position data comprising soft tissue transformed actual position information describing a transformed actual position of the soft tissue body part; g) determining (S14, S24), based on the soft tissue transformed actual position data and the soft tissue body part planned position data, soft tissue body part position transformation data comprising soft tissue body part position transformation information describing a soft tissue body part position transformation between the planned position of the of the soft tissue body part and the transformed actual position of the soft tissue body part.