A device for expanding a particle energy distribution of a therapeutic particle beam installation and method of producing the same is provided. The device for expanding a particle energy distribution of a therapeutic particle beam installation comprises an expansion element with a structured surface. The expansion element comprises a flexible material. The method for producing an expansion element of a device for expanding a particle beam distribution of a therapeutic particle installation includes forming a negative mold that is a rigid material, filling the negative mold with liquid or jelly material; processing the material into a solid state; and removing the solid state material from the negative mold.