Radiolytic breakdown of sodium iodide (131I) in sodium iodide (131I) capsules.

Sodium iodide (131I) capsules are widely used for both diagnosis and therapy. The radiochemical purity of both diagnostic and therapeutic capsules in use in Australia was studied by gel chromatography, high voltage electrophoresis and paper chromatography. It was found that in two of the three bands of therapeutic capsules examined, significant quantities of a labelled high molecular weight component was produced with the result that these capsules failed to meet British Pharmacopoeia, BP, and United States Pharmacopoeia, USP, requirements for radiochemical purity well before their quoted expiry time. The nature of the impurity has not been identified but it is thought to be either iodinated gelatin or an iodinated capsule component.