Tb-161 · Terbium-161
Beta emitter with abundant conversion and Auger electrons. Half-life 6.953 days.
Tb-161 behaves much like Lu-177: a similar half-life, the same chelating chemistry, and beta emission. It also emits many low-energy conversion and Auger electrons, which deposit their energy over very short distances, and its photons are lower in energy. Day-to-day handling looks familiar; measurement settings and surveys need attention.
What changes in practice
- Handled much like Lu-177, with the same chemistry and a similar half-life.
- Photons are low in energy (about 48 and 75 keV), so meters with a poor low-energy response under-read. Check the energy response.
- The dose calibrator needs a setting for Tb-161, and low-energy photons make the reading sensitive to the container. Use a factor for the actual vial and syringe.
- Produced from enriched Gd-160 without carrier, so there is no long-lived metastable impurity like the Lu-177m found in some Lu-177.
Decay chain
Tb-161 · 6.953 d
Beta minus to stable Dy-161
Main emissions
| Type | From | Energy | Per Tb-161 decay |
|---|---|---|---|
| Electrons | Tb-161 | 197 keV mean electron energy per decay (beta, conversion and Auger) | every decay |
| Electrons | Tb-161 | conversion and Auger electrons above 3 keV | about 2.27 per decay |
| Gamma | Tb-161 | about 48 keV (several lines) | 43.1% |
| Gamma | Tb-161 | 74.6 keV | 10.3% |
Detection and measurement
- Contamination: a GM pancake detects the betas well.
- Low-energy photon probes (thin NaI) also work; check what the meter reads at 48 and 75 keV before relying on it.
- Dose calibrator: establish the Tb-161 setting with the supplier’s reference and your own containers.
- Imaging: SPECT on the 48 and 75 keV photons is possible, with energy windows set for them.
Shielding
- Low-energy photons are stopped well by thin lead; standard syringe and vial shields designed for Lu-177 are expected to be adequate, but confirm with a survey.
- Use plastic (acrylic) close to beta sources, with lead outside it if needed.
Contamination control
- As for Lu-177: gloves, absorbent liners, and hand and clothing surveys before leaving the area.
- Conversion and Auger electrons matter for dose to cells, not for external shielding. Skin contamination should still be removed promptly.
Waste and decay-in-storage
- Ten half-lives is about 70 days, within the NRC 120-day limit for decay-in-storage (10 CFR 35.92). Agreement States may differ.
- Check the supplier certificate for radionuclidic impurities before planning release.
Licensing
- The license must authorize Tb-161 in the form used. A license amendment and Authorized User review may be needed before the first patient.
Compared with Lu-177
- Half-life 6.95 d against 6.65 d: scheduling, release planning and decay-in-storage look much the same.
- Same chelators and targeting molecules, so radiopharmacy steps are similar.
- More conversion and Auger electrons per decay; the radiobiology differs, not the shielding.
- Lower-energy photons: a different dose calibrator setting, and meters that under-read at low energies.
Sources
Decay data checked against these sources in October 2026. Practice notes are general guidance for planning; your license, your procedures and your Radiation Safety Officer decide how Tb-161 is handled at your facility.
Preparing to use Tb-161?
License amendment, shielding evaluation, procedures and staff training, with a medical physicist.