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, but also others. Also radiolabelling of heat-sensitive molecules with Tb radionuclides has been reported .

Nuclear properties

Tb-155 decays by electron-capture with a half-life of 5.32(6) 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 gamma- 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 . 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.

Production

Tb-155 is produced by high energy proton induced spallation of tantalum targets at the production facility MEDICIS at CERN. An off-line mass separation is performed at MEDICIS at CERN. The mass-separated ions are implanted into a solid matrix, e.g. an Al, Zn or NaCl layer on a noble metal backing. The implanted layer is then dissolved and used for an additional radiochemical separation to obtain optimum radionuclidic purity by removing the pseudo-isobar Ce-139 that is mass-separated as oxide ions on mass 155.

Distribution

Radiochemical separations are performed at the PRISMAP production facilities PSI SINQ and Injector 2 (PSI, Villigen, Switzerland), BR2 (SCK CEN, Mol, Belgium) or MARIA (NCBJ, Otwock-Swierk, Poland). Activity will be shipped from one of these locations to the users in form of TbCl₃ solution, ready for labelling of e.g. DOTA-compounds.

Radiochemical separations is performed directly at MEDICIS at CERN, Switzerland. Activity will be shipped from there in form of TbCl₃ solution, ready for labelling of e.g. DOTA-compounds.

Examples of use

• Tb-155 is part of the terbium radionuclide quadruplet , and can offer SPECT imaging capacity over extended periods, notably for peptides and antibodies .