Actinium-225 is a key alpha-emitting isotope used in targeted alpha therapy, a cancer treatment modality that delivers highly localized radiation to tumor cells. Its short range in tissue minimizes collateral damage, making it attractive for oncology applications. Global supply is extremely limited because Ac-225 is produced in only a handful of nuclear reactors and accelerators worldwide, creating significant supply-chain concentration risk for pharmaceutical developers.

Heading 284442 enumerates specific isotopes by name, and only those explicitly listed—including californium-253—fall here. Other californium isotopes not enumerated in this heading would be classified under the residual radioactive elements heading (284443). Importers should verify the exact isotope identity on the commercial invoice and certificate of analysis to ensure correct classification, as misclassification between these headings can affect licensing documentation requirements.