A computer-implemented medical method of determining an assignment between a region of interest in a medical image of an anatomical body part to a virtual object defined in an image-based model, the method comprising the following steps: a) patient image data is acquired which describes a medical patient image of the anatomical body part including information about the position of an image representation in the medical patient image of a region of interest; b) atlas data is acquired which describes an image-based model of the anatomical body part which comprises virtual objects, each virtual object describing a subset of the anatomical body part; c) atlas region registration data is determined based on the patient image data and the atlas data, wherein the atlas region registration data describes a mapping between the virtual objects defined in the image-based model and image constituents of the medical patient image, wherein the image constituents include the region of interest; d) region ratio data is determined based on the atlas registration data and the patient image data, wherein the region ratio data describes a ratio between the volume of the region of interest and the volume of an intersection of the region of interest with each one of the virtual objects onto which an image constituent including the region of interest is mapped by the mapping between the virtual objects defined in the image-based model and constituents of the medical patient image; e) envelope data is determined based on the patient image data, wherein the envelope data describes the position of an envelope of the region of interest determined by deducting the region of interest from a dilation of the region of interest; f) atlas envelope registration data is determined based on the envelope data and the atlas data, wherein the atlas envelope registration data describes a mapping between the virtual objects defined in the image-based model and the envelope; g) envelope ratio data is determined based on the envelope data and the atlas registration data, wherein the envelope ratio data describes a ratio between the volume of the envelope and the volume of an intersection of the envelope with each one of the virtual objects onto the surface of which the outer surface of the envelope is mapped by the mapping between the virtual objects defined in the image-based model and the envelope; h) score data is determined based on the region ratio data and the envelope ratio data, wherein the score data describes a score value being the greater of the ratio described by the region ratio data and the ratio described by the envelope ratio data; i) object assignment data is determined based on the score data, wherein the object assignment data describes at least one of the virtual objects onto which the region of interest or the envelope is mapped and for which the score value exceeds a predetermined threshold value. a) patient image data is acquired which describes a medical patient image of the anatomical body part including information about the position of an image representation in the medical patient image of a region of interest; b) atlas data is acquired which describes an image-based model of the anatomical body part which comprises virtual objects, each virtual object describing a subset of the anatomical body part; c) atlas region registration data is determined based on the patient image data and the atlas data, wherein the atlas region registration data describes a mapping between the virtual objects defined in the image-based model and image constituents of the medical patient image, wherein the image constituents include the region of interest; d) region ratio data is determined based on the atlas registration data and the patient image data, wherein the region ratio data describes a ratio between the volume of the region of interest and the volume of an intersection of the region of interest with each one of the virtual objects onto which an image constituent including the region of interest is mapped by the mapping between the virtual objects defined in the image-based model and constituents of the medical patient image; e) envelope data is determined based on the patient image data, wherein the envelope data describes the position of an envelope of the region of interest determined by deducting the region of interest from a dilation of the region of interest; f) atlas envelope registration data is determined based on the envelope data and the atlas data, wherein the atlas envelope registration data describes a mapping between the virtual objects defined in the image-based model and the envelope; g) envelope ratio data is determined based on the envelope data and the atlas registration data, wherein the envelope ratio data describes a ratio between the volume of the envelope and the volume of an intersection of the envelope with each one of the virtual objects onto the surface of which the outer surface of the envelope is mapped by the mapping between the virtual objects defined in the image-based model and the envelope; h) score data is determined based on the region ratio data and the envelope ratio data, wherein the score data describes a score value being the greater of the ratio described by the region ratio data and the ratio described by the envelope ratio data; i) object assignment data is determined based on the score data, wherein the object assignment data describes at least one of the virtual objects onto which the region of interest or the envelope is mapped and for which the score value exceeds a predetermined threshold value.