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: acquiring, at the at least one processor, first patient image data describing a digital image of a first anatomical body part during a first phase of inspiration and the position of the first anatomical body part during the first phase of inspiration in a first reference system associated with the first image data； acquiring, at the at least one processor, second patient image data different from the first patient image data and describing a digital image of the first anatomical body part during a second phase of inspiration and the position of the first anatomical body part during the second phase of inspiration in a second reference system associated with the second image data； acquiring, at the at least one processor, position transformation data describing a transformation between the first reference system and the second reference system； and determining, 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.
