Chemical properties

Tb-155 is a radiolanthanide, most often in trivalent state and chemically very similar to lutetium. The ionic radius of Tb³⁺ is 92 pm, i.e., just 7% larger than Lu³⁺, which explains that macrocyclic chelators suitable for Lu-177 can be directly employed for Tb-155 too, in particular DOTA. Also radiolabelling of heat-sensitive molecules with Tb radionuclides has been reported REF.

Nuclear properties

Tb-155 decays by electron-capture with a half-life of 5.235(4) days to stable Gd-155. Its half-life of several days is close to the physical half-life of the therapeutic Tb-161 radiolanthanide and of Lu-177, therefore allowing for extended imaging studies.

Tb-155 emits low energy conversion and Auger electrons with an average of 2.0 electrons (with energies above 4 keV) per decay.

Tb-155 emits γ and X rays, notably 107(1) % around 45(+/-11) keV, and 32.0(18) % at 86.6 keV and 25.1% at 105.3 keV REF. These make Tb-155 suitable for SPECT-imaging.

The mean electron energy emitted per decay is 38 keV, the mean photon energy per decay is 175 keV REF.

Production

Tb-155 is produced by proton irradiation of Gd targets at ARRONAX (Nantes, France) followed by offline mass separation at CERN-MEDICIS (Geneva, Switzerland/France). The mass-separated ions are implanted into a solid matrix, e.g. an Al, Zn or NaCl layer on a noble metal backing.

Tb-155 is also produced by proton irradiation of enriched Gd-155 targets at Hevesy Laboratory at DTU. The Tb/Gd separation is performed at Institute for Energy Technology (IFE).

Distribution

Activity will be shipped either from IFE (Kjeller, Norway) as a TbCl₃ solution, ready for labelling of, e.g., DOTA-based compounds, or from MEDICIS (CERN, Switzerland/France) as mass-separated ¹⁵⁵Tb. The latter will be provided either as a TbCl₃ or Tb(NO₃)₃ solution, or in solid form, implanted into a NaCl, Al, or Zn layer on a metallic foil.

Examples of use

• Tb-155 is part of the terbium radionuclide quadruplet [10.2967/jnumed.112.107540], and can offer SPECT imaging capacity over extended periods, notably for peptides and antibodies REF.