1. A radiotherapy system, comprising: a computer, for processing static medical image data and patient model data and for determining data; an irradiation unit for emitting treatment radiation; the computer having a digital processor and a stored computer program which, when running on the computer or when loaded onto the computer, causes the computer to perform steps for determining a change of an image of an anatomical body part of a patient's body, the steps being executed by the computer and the steps comprising: a) acquiring static medical image data comprising static medical image information describing the anatomical body part in a first anatomical spatial state and a third anatomical spatial state of an anatomical vital spatial change of the anatomical body part; b) acquiring patient model data comprising patient model information describing a model body part corresponding to the anatomical body part, wherein the patient model information describes the model body part in a plurality of model spatial states of a model vital spatial change corresponding to the anatomical vital spatial change; c) determining spatial state mapping data comprising spatial state mapping information describing at least one of a first mapping from the model body part in a first model spatial state of the plurality of model spatial states to the model body part in a second, different model spatial state of the plurality of model spatial states, the first model spatial state corresponding to the first anatomical spatial state, and a second mapping from the model body part in the first model spatial state to the anatomical body part in the first anatomical spatial state and wherein the spatial state mapping information further describes at least one of a third mapping from the model body part in at least one of the first model spatial state and the second model spatial state to the model body part in a third model spatial state which corresponds to the third anatomical spatial state and a fourth mapping from the model body part in the third model spatial state to the anatomical body part in the third anatomical spatial state; d) determining, based on the static medical image data and the spatial state mapping data, transformed medical image data comprising transformed medical image information describing the anatomical body part in a second anatomical spatial state and the third anatomical spatial state of the anatomical vital spatial change, the second anatomical spatial state corresponding to the second model spatial state, wherein the transformed medical image information is determined based on at least one of applying the third mapping to the anatomical body part in the first anatomical spatial state and the fourth mapping to the model body part in the third model spatial state, and determining, based on comparing the third anatomical spatial state described by the static medical image information to the third anatomical spatial state described by the transformed medical image information, correction mapping data comprising correction mapping information describing a correction mapping from the third anatomical spatial state described by the static medical image information to the third anatomical spatial state described by the transformed medical image information, and when the correction mapping information indicates that the correction mapping is not equal to identity, the transformed medical image data is determined further based on the correction mapping data by applying the correction mapping to the anatomical body part in the second anatomical spatial state described by the transformed medical image information.