A computer-implemented medical data processing method for determining a difference in position of an imaged anatomical body part of a patient, the method comprising executing, on at least one processor of at least one computer, steps of: a) acquiring (S1.1, S.1.2), at the at least one processor, three-dimensional first patient image data of a patient describing a digital image of a first anatomical body part moving during inspiration and a second anatomical body part not moving during inspiration during a first phase of inspiration and the position of the first anatomical body part, the first phase of inspiration being a phase of deep inspiration breath hold at a first point in time, and the position of the second anatomical body part during the first phase of inspiration in a first reference system associated with the first image data; a1) acquiring, at the at least one processor, third patient image data of the patient describing a digital image of the second anatomical body part during a third phase of inspiration, for example an arbitrary phase of inspiration of the patient, and the position of the second anatomical body part during the third phase of inspiration in a second reference system associated with two-dimensional second image data of the patient, wherein the third patient image data is different from the first patient image data and the second patient image data; a2) acquiring (S1.3), by the at least one processor and based on the first patient image data and the third patient image data position transformation data describing a transformation between the first reference system and the second reference system; b) acquiring, at the at least one processor, the second patient image data different from the first patient image data describing a digital image of the first anatomical body part during a second phase of inspiration, the second phase of inspiration being a phase of deep inspiration breath hold at a second point in time which is later than the first point in time, and the position of the first anatomical body part during the second phase of inspiration in the second reference system; c) determining (S1.4), by the at least one processor and based on the first patient image data and the second patient image data and the position transformation data, position difference data describing a relative position between the position of the first anatomical body part during the first phase of inspiration and the position of the first anatomical body part during the second phase of inspiration. a) acquiring (S1.1, S.1.2), at the at least one processor, three-dimensional first patient image data of a patient describing a digital image of a first anatomical body part moving during inspiration and a second anatomical body part not moving during inspiration during a first phase of inspiration and the position of the first anatomical body part, the first phase of inspiration being a phase of deep inspiration breath hold at a first point in time, and the position of the second anatomical body part during the first phase of inspiration in a first reference system associated with the first image data; a1) acquiring, at the at least one processor, third patient image data of the patient describing a digital image of the second anatomical body part during a third phase of inspiration, for example an arbitrary phase of inspiration of the patient, and the position of the second anatomical body part during the third phase of inspiration in a second reference system associated with two-dimensional second image data of the patient, wherein the third patient image data is different from the first patient image data and the second patient image data; a2) acquiring (S1.3), by the at least one processor and based on the first patient image data and the third patient image data position transformation data describing a transformation between the first reference system and the second reference system; b) acquiring, at the at least one processor, the second patient image data different from the first patient image data describing a digital image of the first anatomical body part during a second phase of inspiration, the second phase of inspiration being a phase of deep inspiration breath hold at a second point in time which is later than the first point in time, and the position of the first anatomical body part during the second phase of inspiration in the second reference system; c) determining (S1.4), by the at least one processor and based on the first patient image data and the second patient image data and the position transformation data, position difference data describing a relative position between the position of the first anatomical body part during the first phase of inspiration and the position of the first anatomical body part during the second phase of inspiration.