Last reviewed by Dr. Dharmender Malik on 8 May 2026 · this article reflects the published evidence and current clinical practice at FMRI Gurugram.
Introduction
Patients arrive in the consultation having read about Terbium-161 and ask the question reasonably: should I wait for it instead of starting Lu-177 PSMA today? Sometimes the question comes from genuine medical-literacy curiosity. Sometimes it comes from a clinic that has overstated what the evidence shows. This article is the answer we give in the consultation room, written down — careful, evidence-anchored, and honest about what is settled and what is still being studied.
The short summary, before the longer one: Tb-161 is a real and interesting radioisotope with theoretical advantages over Lu-177 at the cellular scale, supported by promising preclinical data and limited early clinical data. Whether those theoretical advantages translate into clinical benefit, and in which patients, is the question the ongoing trials are designed to answer. Today, in 2026, Tb-161 is investigational. Lu-177 is the established standard. The clinical decision for an individual patient should be made on the basis of evidence available now, not on the basis of what may be available later.
What Tb-161 is
Terbium-161 is a radioactive isotope of terbium, a lanthanide element that sits next to lutetium on the periodic table. The chemical similarity to lutetium means that Tb-161 can be incorporated into the same chelator structures (DOTATATE, PSMA-617, FAPI variants) that have been developed for Lu-177. From a pharmacology standpoint, Tb-161 PSMA-617 and Lu-177 PSMA-617 behave almost identically in the body — same biodistribution, same tumour uptake, same clearance pathways. The difference is in what each isotope emits as it decays.
The Auger physics
Lu-177 emits beta particles with a maximum energy of around 0.5 MeV and a tissue range of about 2.5 mm. The decay scheme is dominated by the beta emission. This is what gives Lu-177 its useful clinical profile — beta range matches small-to-medium tumour deposits, and the crossfire effect treats neighbouring cells.
Tb-161 emits beta particles of similar energy and range. But Tb-161 also emits a substantial component of Auger and conversion electrons — low-energy electrons with very short range, often less than the diameter of a single cell. Quantitatively, around half of the dose deposited by Tb-161 comes from the Auger and conversion-electron component, with the other half from beta. The dose density at the cellular scale is therefore higher with Tb-161 than with Lu-177.
"At the cellular scale, Tb-161 deposits more dose than Lu-177. At the multi-cellular scale, the two are similar. The therapeutic implication depends entirely on what scale of disease you are trying to treat."
The implication: for very small tumour deposits — micrometastases of a few cells, or single circulating tumour cells — the Auger component of Tb-161 may deliver more lethal dose per disintegration than Lu-177's beta alone. For large tumours, where beta-particle crossfire is what dominates the dose anyway, the two isotopes should perform similarly.
What the preclinical evidence shows
Müller and colleagues published a series of preclinical comparisons of Tb-161 against Lu-177 in matched chelator complexes.[1] The headline observations:
- In cell-culture models of small clusters of tumour cells, Tb-161 showed greater cytotoxicity per administered activity than Lu-177.
- In small-animal models with micrometastatic disease patterns, Tb-161 produced greater tumour control than Lu-177.
- In larger established tumour models, Tb-161 and Lu-177 performed similarly — supporting the prediction that the Auger advantage is scale-dependent.
Borgna, Champion, and others have published supporting work in similar models. The preclinical evidence base is now substantial enough to motivate clinical trials.
Clinical evidence so far
Clinical experience with Tb-161 in radioligand therapy is currently limited to small case series, dosimetry studies, and ongoing clinical trials. As of mid-2026:
- Pilot studies of Tb-161 PSMA-617 in advanced prostate cancer have demonstrated feasibility and safety, with biodistribution and dosimetry comparable to Lu-177 PSMA-617.
- Preliminary efficacy signals exist but the patient numbers are too small for confident comparison with Lu-177.
- No randomised head-to-head trial data exists.
- Tb-161 is not FDA-approved; it is available in trials and at a small number of specialised centres internationally.
The VIOLET trial
VIOLET (NCT05521412) is the most prominent ongoing clinical trial of Tb-161 PSMA-617 in metastatic castration-resistant prostate cancer. The trial is evaluating safety, dosimetry, and efficacy endpoints in a defined patient population. Recruitment began in 2022. As of mid-2026, mature outcome data has not yet been published.
VIOLET is the trial that will move Tb-161 from "interesting preclinical agent" to "established clinical option" or, if the data does not support the hypothesis, to "useful but not transformative". Either result is informative. In either case, the responsible clinical posture for today is to acknowledge VIOLET is in flight and not to base treatment decisions on the assumption of its outcome.
An honest Tb-161 vs Lu-177 comparison
Where Tb-161 might genuinely matter:
- In patients with predominantly micrometastatic disease (low volume, small lesions on imaging) where the Auger component might deliver superior dose at the cellular scale.
- In adjuvant or maintenance settings, where small residual disease is the target.
- In cells that have reduced radiosensitivity to beta — though this is currently theoretical.
Where Lu-177 remains the standard:
- In patients with established radiographic disease — where beta-particle range matches the lesion scale and crossfire dominates the dose.
- Wherever clinical trial evidence is required to support a treatment decision, given that Lu-177 has the VISION and NETTER-1 trial backing and Tb-161 does not yet.
- For routine, non-trial clinical use today.
The honest summary: Tb-161 is a promising next-generation isotope being properly studied. It is not currently a substitute for Lu-177 outside of clinical trials. When the trial data matures, this conversation will be revisited. Patients waiting for definitive Tb-161 evidence while their disease progresses are not making a wise trade-off; the right move today, for an appropriate candidate, is Lu-177 with the option to consider Tb-161 if and when the evidence supports it.
Considering radioligand therapy and unsure which isotope?
Send your imaging and clinical summary. We respond with a written, evidence-referenced opinion on Lu-177, Ac-225, and (where appropriate) Tb-161 trial options.
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For patients & referring clinicians
Frequently asked questions
Q01
What is terbium-161?
Tb-161 is a lanthanide radioisotope being investigated for radioligand therapy. It is the periodic-table neighbour of Lu-177. Tb-161 emits beta particles plus a substantial Auger and conversion electron component, with potential advantages at the cellular scale.
Q02
How is Tb-161 different from Lu-177?
Both emit beta particles of similar energy and range. Tb-161 also emits Auger and conversion electrons with very short range that may deliver more dose at the cellular scale. The clinical implication is scale-dependent — likely advantageous for very small lesions, similar for larger disease.
Q03
Is Tb-161 better than Lu-177?
In preclinical models, Tb-161 has shown superiority over Lu-177 in some experimental settings. Clinical superiority is not yet established. Direct head-to-head clinical trials are ongoing. As of 2026, claiming Tb-161 outperforms Lu-177 in clinical practice goes beyond what the evidence supports.
Q04
Is Tb-161 FDA-approved?
No. Tb-161 is not FDA-approved for any indication. It is available in clinical trials and at a limited number of specialised centres internationally.
Q05
What is the VIOLET trial?
VIOLET (NCT05521412) is an ongoing clinical trial evaluating Tb-161 PSMA-617 in metastatic castration-resistant prostate cancer. Recruitment began in 2022. Mature outcome data has not yet been published as of mid-2026.
Q06
Who is a candidate for Tb-161 therapy?
Currently, trial participants and patients at specialised centres under defined protocols. Selection mirrors Lu-177 radioligand therapy criteria. The Helsinki Declaration framework applies, with written informed consent reflecting the early-evidence nature of the therapy.
Citations & references
Müller C, Reber J, Haller S, et al. Direct in vitro and in vivo comparison of 161Tb and 177Lu using a tumour-targeting folate conjugate. Eur J Nucl Med Mol Imaging. 2019;46(6):1382-1389.
Champion C, Quinto MA, Morgat C, et al. Comparison between three promising β-emitting radionuclides, 67Cu, 47Sc and 161Tb, with emphasis on doses delivered to minimal residual disease. Theranostics. 2016;6(10):1611-1618.
Borgna F, Haller S, Rodríguez JMM, et al. Combination of terbium-161 with somatostatin receptor antagonists — a potential paradigm shift for the treatment of neuroendocrine neoplasms. Eur J Nucl Med Mol Imaging. 2022;49(4):1113-1126.
VIOLET Clinical Trial Registration. NCT05521412. ClinicalTrials.gov.
Bernhardt P, Forssell-Aronsson E, Jacobsson L, Skarnemark G. Low-energy electron emitters for targeted radiotherapy of small tumours. Acta Oncol. 2001;40(5):602-608.
About the Author
Dr. Ishita B. Sen
MBBS · MD (Nuclear Medicine) · DNB · Post-doctoral Fellowship, Memorial Sloan Kettering Cancer Center, New York
Director and Chief of Nuclear Medicine at Fortis Memorial Research Institute. Co-founder of Theranostic Physicians Private Limited (TPPL). Active interest in next-generation radioligand therapy isotopes including Tb-161 in the appropriate clinical-trial and Helsinki-Declaration framework.