Terbium-161

Terbium-161 decays by beta-minus emission with a half-life of 6.95 days (very close to lutetium-177's 6.65 days) to stable dysprosium-161, and shares much of 177Lu's chemistry (both are trivalent lanthanides suited to DOTA-type chelators), so it can be substituted directly into existing DOTATATE, DOTATOC and PSMA-617-type ligands.

What distinguishes it is a large yield of low-energy conversion and Auger electrons -- roughly 2.3 electrons above 3 keV per decay, with ranges from nanometres to a few tens of nanometres -- alongside its 154 keV mean-energy beta particles, which give it a mean tissue range similar to 177Lu (about 0.7 mm) but potentially more damage to single cells and micrometastases through the additional short-range electrons. It also emits imageable gamma/X-rays around 48.9 keV (17%) and 74.6 keV (10%), suitable for SPECT, including simultaneous dual-isotope imaging alongside 177Lu.

Production is analogous to no-carrier-added 177Lu: thermal-neutron irradiation of enriched gadolinium-160 targets produces short-lived 161Gd (half-life 3.66 minutes), which beta-decays to 161Tb; the Tb is then chemically separated from the bulk Gd. It therefore depends on the same high-flux research reactors used for 177Lu (a European PRISMAP network of ILL, SCK CEN and NCBJ reactors, plus PSI's SINQ spallation source, and a University of Utah TRIGA/University of Missouri collaboration in the US), and remains available only in gigabecquerel, research-scale monthly batches. Terbium-161-labelled PSMA and somatostatin-receptor agents are in early clinical investigation and are not approved by the FDA or EMA.