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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-155 is a synthetic radioisotope produced for SPECT imaging, with a half-life of about 5.3 days.
Terbium-155 decays by electron capture, emitting gamma photons suited to SPECT imaging, with early studies showing excellent image quality even at low doses. Its multi-day half-life allows imaging over an extended window.
As one member of the terbium quadruplet, it shares the identical chemistry of its therapeutic siblings, so a targeting molecule can be imaged with terbium-155 and then treated with a therapeutic terbium isotope using the same chemistry and biodistribution.
Terbium-155 is produced mainly at large research facilities by high-energy proton accelerators and mass separation. Cyclotron-based production is also possible, though is current-limited. Supply is the key constraint on wider use.
StandardX is developing accelerator-based production to make terbium-155 available at clinical scale.