Abstract: Composition in the form of a shaped device, for use as a fiducial marker in tissues in the animal body, in radiotherapy and/or radiosurgery, characterized in that it comprises: —a core consisting of a colloidal dispersion of metal nanoparticles and/or oxides or metal salts having X-ray-contrast properties, where said nanoparticles are stabilized with surfactants, polymers or capping agents in a liquid vehicle, and —a polymeric casing that encapsulates the core, said device having a minimum size of not less than 500 microns and a maximum size not greater than 3000 microns.