1. A radiotherapy system or radiosurgery system, comprising: a treatment beam source; and at least one computer having at least one processor that executes instructions stored on a computer readable storage medium to determine a transformation for determining a breathing state-dependent geometry of an anatomical body part of a patient's body, the instructions causing the at least one processor to: acquire, by the at least one 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 a reference planning respiratory state, wherein the anatomical body part is subject to respiratory movement and wherein the planning image data comprises a planning image called a reference planning image describing a part of the anatomical body part which is called a reference planning body part; acquire, by the at least one processor, breathing image data describing a set of tomographic medical breathing images of the anatomical body part, wherein the breathing image data comprises a reference breathing image describing the reference planning body part in a respiratory state called a reference breathing respiratory state, which is different from the reference planning respiratory state, and a target breathing image describing at least another part of the anatomical body part, wherein the other part of the anatomical body part is called a target body part, in a respiratory state called a target respiratory state which is different from the reference planning respiratory state; determine, by the at least one processor and based on the planning image data and the breathing image data, a reference transformation data called a reference transformation, between the geometry of the reference planning body part in the reference planning respiratory state and the geometry of the reference planning body part in the reference breathing respiratory state; acquire, by the at least one processor, scaling factor data describing a scaling factor which describes a relationship between the geometry of the reference planning body part in the reference breathing respiratory state and the geometry of the target body part in the target respiratory state; determine, by the at least one processor and based on the reference transformation and the scaling factor data, derived transformation data called a derived transformation between the geometry of the target body part in the reference planning respiratory state, and the geometry of the target body part in the reference breathing respiratory state wherein the at least one computer is operatively connected to the treatment beam source and controlling the treatment beam source to emit a treatment beam based on the derived transformation data.