A method for generating synthetic electron density information for use in radiation treatment planning, said synthetic electron density information being generated based on an acquired magnetic resonance, MR, image stack, said method comprising: analyzing the acquired MR image stack in order to automatically segment the acquired MR image stack such that structure information defining locations and/or geometry of structures in the acquired MR image stack is determined; generating warp information for a template dataset, said template dataset comprising at least one template electron density information map and at least one template structure information, said generating comprising utilizing said structure information and said template structure information representing locations and/or geometry of template structures in order to determine transformations bringing the template structures in spatial correspondence with the structures in the acquired MR image stack, wherein said generated warp information describes at least one transformation bringing the at least one template electron density information map in spatial correspondence with the acquired MR image stack, wherein the generated warp information comprises deformation fields utilized in the transformations bringing the at least one template electron density information map in spatial correspondence with the acquired MR image stack; and forming synthetic electron density information corresponding to the acquired MR image stack based on at least part of the at least one template electron density information map, wherein the at least part of the at least one template electron density information map is warped using the generated warp information; wherein a set of template electron density maps are used and wherein said generated warp information describes transformations bringing each template electron density information map in the set of template electron density information maps in spatial correspondence with the acquired MR image stack, and wherein said forming comprises fusioning parts of the template electron density information maps, wherein the parts of the template electron density information maps are warped using the warp information, to form synthetic electron density information corresponding to the acquired MR image stack, wherein only the parts that are to be fused are warped. analyzing the acquired MR image stack in order to automatically segment the acquired MR image stack such that structure information defining locations and/or geometry of structures in the acquired MR image stack is determined; generating warp information for a template dataset, said template dataset comprising at least one template electron density information map and at least one template structure information, said generating comprising utilizing said structure information and said template structure information representing locations and/or geometry of template structures in order to determine transformations bringing the template structures in spatial correspondence with the structures in the acquired MR image stack, wherein said generated warp information describes at least one transformation bringing the at least one template electron density information map in spatial correspondence with the acquired MR image stack, wherein the generated warp information comprises deformation fields utilized in the transformations bringing the at least one template electron density information map in spatial correspondence with the acquired MR image stack; and forming synthetic electron density information corresponding to the acquired MR image stack based on at least part of the at least one template electron density information map, wherein the at least part of the at least one template electron density information map is warped using the generated warp information; wherein a set of template electron density maps are used and wherein said generated warp information describes transformations bringing each template electron density information map in the set of template electron density information maps in spatial correspondence with the acquired MR image stack, and wherein said forming comprises fusioning parts of the template electron density information maps, wherein the parts of the template electron density information maps are warped using the warp information, to form synthetic electron density information corresponding to the acquired MR image stack, wherein only the parts that are to be fused are warped.