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Terbium is a rare-earth metal remarkable in nuclear medicine for offering four medically useful radioisotopes of a single element. Terbium-161 is a synthetic radioisotope produced for treatment, with a half-life of about 6.9 days.
Terbium-161 emits beta particles much like lutetium-177, but with an important addition: it also releases large numbers of very low-energy conversion and Auger electrons. These deposit extra dose over extremely short distances, which may make it especially effective against small tumour deposits, micrometastases, and single cancer cells.
Terbium-161 is the most clinically advanced member of the terbium quartet and is used in radioligand therapy in much the same way as lutetium-177. Active trials include PSMA ligands for prostate cancer and somatostatin analogues for neuroendocrine tumours where the added short-range electron dose offers potential benefit. Its terbium imaging siblings allow fully element-matched theranostics.
Terbium-161 is produced by neutron activation of an enriched gadolinium-160 target, which forms gadolinium-161 that decays to terbium-161, followed by radiochemical separation. Its supply therefore depends on neutron-irradiation capacity and on scarce enriched gadolinium-160.
StandardX is developing accelerator-driven production to strengthen supply and reduce this dependence on legacy neutron production.