The disclosure relates to a radiolabelled material comprising a polymer preferably a polystyrene sulfonate cationic exchange resin, a radioactive metal isotope, and an immobilizing agent selected from tannins, tannic acid and theaflavin-3-gallate, wherein the immobilizing agent is capable of immobilizing the radioactive isotope on or in the polymer, and wherein the immobilizing agent is a macromolecule comprising electron donating groups. The disclosure also relates to a process for making a radiolabelled material, to use of a radiolabelled material for the preparation of medicaments for treating cancer and/or for radiation imaging, and to use of a radiolabelled material in the treatment of cancer. There is further described use of an immobilizing agent to immobilize a radioactive isotope on or in a polymer.
