What is Terbium-161 and what is it used for?
Terbium-161 (Tb-161) is a next-generation medical isotope used in targeted radioligand therapy (TRT) for treating cancers such as prostate cancer (via PSMA ligands), neuroendocrine tumors (via DOTATATE), and emerging targets like FAPI in other solid tumors. It emits beta particles similar to Lutetium-177 (Lu-177), but also releases Auger and conversion electrons—tiny, high-energy emissions that can destroy microscopic residual cancer cells responsible for relapse.
Is Terbium-161 a substitute for Lu-177 or a bridge to Alpha therapy?
Tb-161 shares many properties with Lu-177, including similar half-life (6.95 days for Tb-161 vs. 6.6 days for Lu-177) and compatibility with existing ligands. However, Tb-161 offers a ~40% higher radiation dose delivery per unit without increasing toxicity, thanks to its additional Auger and conversion electron emissions. Clinically, this positions it as a bridge between Lu-177 and more aggressive Alpha therapies like Actinium-225. Whether it replaces Lu-177 is currently under clinical investigation.
How is Tb-161 produced and where is it sourced from?
Tb-161 is produced by neutron irradiation of enriched Gadolinium-160 (¹⁶⁰Gd) in a nuclear reactor. The leading global supplier is TerThera based in the Netherlands. At our centre in Fortis Memorial Research Institute (FMRI), Gurugram, we receive Terbium shipments twice a month, ensuring regular availability for patient therapy planning.
What cancers is Tb-161 therapy suitable for?
Currently, Tb-161 is used to treat:
- Prostate cancer (via PSMA ligands)
- Neuroendocrine tumors (via DOTATATE)
- Other solid tumors where FAPI or other ligands are applicable
It may also be beneficial in patients who did not adequately respond to Lu-177, although more data is awaited.
How is Tb-161 therapy administered?
Tb-161 therapy is very similar in protocol to Lu-177 therapy:
- Patients undergo a PSMA or DOTATOC scan for eligibility.
- It is administered as a short intravenous infusion via a cannula.
- Supportive care includes hydration and sometimes amino acids to protect the kidneys.
- * Post-therapy, patients are scanned multiple times (same day, next day, and after 3–4 days) for image-based dosimetry.
This helps assess dose delivery to tumor, kidneys, and bone marrow.
Patients are typically admitted for daycare or 1-night admission.
How many cycles are given and at what intervals?
Patients usually undergo 4 to 6 cycles of Tb-161 therapy. These are spaced at 8–12 week intervals, depending on dosimetry results, patient response, and tolerance.
What are the benefits of Tb-161 over Lu-177?
- 40% more radiation per unit molecule to tumor cells
- Lower systemic toxicity, as less isotope is needed for the same effect
- Superior ability to eliminate residual tumor cells, reducing recurrence risk
- Compatible with existing ligands (PSMA, DOTATATE, FAPI)
Are there ongoing trials with Tb-161?
Yes. One of the most significant trials is the VIOLET study, a Phase I/II clinical trial led by Prof. Michael Hofman at the Peter MacCallum Cancer Centre in Melbourne, Australia. This study is evaluating the safety, dosimetry, and efficacy of [¹⁶¹Tb]Tb-PSMA-I&T in patients with metastatic castration-resistant prostate cancer (mCRPC).
The trial completed recruitment for its initial dose escalation cohort and showed no dose-limiting toxicities across six cycles up to 7.4 GBq. A new cohort using a higher dose of 9.5 GBq has been initiated to further explore safety and efficacy.
You can read more about the study and early results here:
https://jnm.snmjournals.org/content/65/8/1231
What are the common side effects of Tb-161 therapy?
Side effects are generally mild to moderate and similar to Lu-177. They may include:
- Dry mouth (xerostomia)
- Fatigue
- Nausea or vomiting
- Appetite loss
- Low blood counts (anemia, thrombocytopenia)
- Kidney strain (hence amino acid support)
Because of its enhanced tumor specificity, Tb-161 is expected to have lower toxicity overall.
What is the response rate of Tb-161 PSMA therapy?
Initial studies show encouraging tumor control and symptom relief, with theoretical advantages in eliminating micrometastases. While long-term clinical response data is still emerging, preclinical models suggest lower recurrence compared to Lu-177.
Is Tb-161 therapy safe for patients who didn’t respond to Lu-177?
While formal comparative studies are limited, the theoretical model supports its use in such patients. Some patients who show progression post Lu-177 may benefit from Tb-161 due to its enhanced emission spectrum. However, more clinical validation is required before it becomes a standard.
What is the decay scheme of Tb-161?
Tb-161 decays via beta-minus (β-) emission to stable Dysprosium-161 (¹⁶¹Dy). This decay is accompanied by Auger electrons and conversion electrons, which enhance therapeutic effectiveness in small tumor clusters.
Is Tb-161 approved and in clinical use?
Tb-161 is currently used under clinical protocols and in select research centres. At our Nuclear Medicine Centre at FMRI, we offer Tb-161 therapy to eligible patients through structured and safe treatment protocols.
Who can I consult for Tb-161 therapy?
You can consult Dr. Ishita B. Sen, Senior Director – Nuclear Oncology & Molecular Imaging at Fortis Memorial Research Institute (FMRI), Gurgaon. She and her team are among the first in India to offer Tb-161 therapy with customized dosimetry and precise patient selection.
If you are a patient or caregiver looking for Terbium-161 therapy or want to know whether this option suits your case, please feel free to reach out to our expert team at info@nuclearmedicinetherapy.in