Terbium-161

Tb-161 undergoes beta-minus decay with a half-life of 6.953(2) days, transforming into stable Dysprosium-161 (Dy-161). Its half-life is slightly—less than 5%—longer than that of Lutetium-177 (Lu-177). As a beta-emitter is only deposits its energy within a short range, decreasing collateral damaging effects to normal tissues.

In addition to beta particles, Tb-161 emits a significant number of low-energy conversion electrons and Auger electrons, resulting in the release of approximately 2.27 electrons per decay with energies exceeding 3 keV. Furthermore, Tb-161 emits both gamma and X-rays, most notably with a 43.1(5)% emission around 48 keV (±10%) and a 10.3(2)% emission at 74.6 keV. These photon energies make Tb-161 well-suited for SPECT imaging.

Terbium-161 could be a  substitute of Lu-177 PSMA for radionuclide therapy of metastasized castration-resistant prostate cancer (mCRPC). The properties of Tb-161 are similar to those of Lu-177. It is believed, however, that the co-emission of conversion and Auger electrons make terbium-161 superior. These short-ranged electrons may effectively eliminate microscopic metastases that are not even visible on a PET image, but responsible for relapse and metastatic spread.

Similar to other recently introduced therapeutic radionuclides, such as Actinium-225 (Ac-225) and Lead-212 (Pb-212), Tb-161 and Lu-177 undergo decay to stable isotopes (Dysprosium-161 and Hafnium-177, respectively), thereby avoiding  the risk associated with particle-emitting daughter products.

NRG|PALLAS produces Tb-161 in our High Flux Reactor (HFR) through neutron irradiation  of enriched Gadolinium-160 (Gd-160). Upon capturing a neutron, Gd-160 is converted into Gd-161, an unstable isotope that subsequently undergoes  beta decay. This process results in the transformation of Tb-161, accompanied by the  emission of beta particles and gamma radiation.

The key reaction can be represented as follows:

Gd-160 (n,γ) Gd-161 -> Tb-161 + β- + γ

The produced Tb-161 should be separated from the Gd160, purified and processed to meet the required quality and safety standards before it can be used in medical settings.                                                             

Tb-161, one of four medically relevant terbium isotopes, is considered a natural successor to Lu-177 due to their closely related chemical and nuclear characteristics. Compared to Lu-177, Tb-161 offers enhanced therapeutic potential by delivering a higher localized radiation dose, particularly within micrometer-scale distances—more than doubling the dose per decay in some cases. This advantage has been demonstrated in various preclinical studies using different radiopharmaceuticals. In addition, Tb-161 emits photons with distinct energies from Lu-177, allowing for dual-radionuclide SPECT imaging. Clinical imaging protocols have already been proposed, and first-in-human use of Tb-161-DOTATOC—including SPECT/CT imaging up to 113 hours post-injection—has been reported. Beyond therapy and diagnostics, Tb-161 can also serve as a source for Mößbauer spectroscopy, enabling precise characterization of terbium’s chemical state in solid or frozen biological samples.