a ferric hydroxide colloid having an alpha - emitting radionuclide essentially on the outer surfaces and a method of forming same . the method includes oxidizing a ferrous hydroxide to ferric hydroxide in the presence of a preselected radionuclide to form a colloid having the radionuclide on the outer surface thereof , and thereafter washing the colloid , and suspending the washed colloid in a suitable solution . the labelled colloid is useful in cancer therapy and for the treatment of inflamed joints .