Disclosed is a medical data processing method of determining a transformation for determining a breathing state-dependent geometry of an anatomical body part of a patient's body, the method comprising executing, on at least one processor of at least one computer, steps of: a) acquiring, at a processor, planning image data describing a set of tomographic medical planning images describing each a different part of the anatomical body part in the same respiratory state called reference planning respiratory state (y), wherein the anatomical body part is subject to respiratory movement and wherein the planning images comprise a planning image called reference planning image describing a part of the anatomical body part which is called reference planning body part; b) acquiring, at a processor, breathing image data describing a set (1) of tomographic medical breathing images of the anatomical body part, wherein the breathing images comprise a reference breathing image (A) describing the reference planning body part in a respiratory state called reference breathing respiratory state (a), which is different from the reference planning respiratory state (y), and a target breathing image (C) describing at least another part of the anatomical body part, wherein the other part of the anatomical body part is called target body part, in a respiratory state called target respiratory state (c) which is different from the reference planning respiratory state; c) determining, by a processor and based on the planning image data and the breathing image data, reference transformation data describing a transformation, called reference transformation (R), between the geometry (Factor A) of the reference planning body part in the reference planning respiratory state (y) and the geometry of the reference planning body part in the reference breathing respiratory state (a); d) acquiring, at a processor, scaling factor data describing a scaling factor (sf) which describes a relationship between the geometry (Factor A) of the reference planning body part in the reference breathing respiratory state (a) and the geometry (Factor C) of the target body part in the target respiratory state (c); e) determining, by a processor and based on the reference transformation and the scaling factor data, derived transformation data describing a transformation called derived transformation (T) between the geometry of the target body part in the reference planning respiratory state (y), and the geometry (Factor C′) of the target body part in the reference breathing respiratory state (a).