The invention relates to a radiolabelled material comprising a polymer, a radioactive isotope, and an immobilizing agent, 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 a polycation with multiple pendant metal-chelating side-chains. The invention 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.