Summary (10 sec read)
Terbium-161 (Tb-161) is a promising breakthrough in prostate cancer treatment, offering superior precision and effectiveness compared to Lutetium-177 (Lu-177). This targeted radionuclide therapy utilizes Auger electrons to deliver high-energy radiation directly to cancer cells while minimizing damage to healthy tissues. Dr. Ishita B. Sen, a leading nuclear oncology expert at Fortis Memorial Research Institute, is pioneering this advanced treatment for metastatic castration-resistant prostate can
By Dr. Ishita B. Sen, Senior Director, Nuclear Oncology and Molecular Imaging at Fortis Memorial Research Institute.
As a specialist in nuclear oncology with over 20 years of experience, I am excited to introduce Terbium-161 (Tb-161) as a groundbreaking advancement in PSMA-targeted radionuclide therapy for metastatic castration-resistant prostate cancer (mCRPC). While Lutetium-177 (Lu-177) has been the gold standard, offering significant survival benefits, my clinical experience suggests that Tb-161 presents a superior alternative, particularly in addressing micrometastatic disease. Unlike other centers in India and globally still relying on Lu-177, I am eager to offer Tb-161 to my patients, emphasizing its improved impact on treatment outcomes.
Radiophysical and Radiobiological Properties of Tb-161
Tb-161 decays via electron capture, emitting Auger electrons (mean energy: 5–25 keV; path length: 2–500 nm) and conversion electrons (mean energy: 50–60 keV; path length: ~1 mm). This dual emission profile allows for a combination of ultra-short-range (Auger) and short-range (conversion electron) radiation. Auger electrons, in particular, deposit energy over nanometers to micrometers, requiring close proximity to DNA for optimal effect. This necessitates PSMA ligand internalization into tumor cells—a process inherent to PSMA biology—ensuring precise DNA damage within the nucleus.
In contrast, Lu-177 emits medium-energy beta particles (mean path length: 0.2–2 mm), which are effective for larger tumors but less efficient in sterilizing single cells or micrometastases due to their longer range and lower linear energy transfer (LET). Computational simulations highlight that low-energy electrons from Tb-161 increase energy deposition within target cells by 40–60% compared to Lu-177, enhancing cytotoxicity in submillimeter lesions.
Preclinical Evidence Supporting Tb-161
Preclinical studies underscore Tb-161’s superiority in heterogeneous tumor models. In vitro, Tb-161-PSMA ligands demonstrated 2–3 times greater DNA double-strand breaks per decay than Lu-177, attributable to Auger electrons’ high LET. Animal models of disseminated PSMA-positive tumors revealed improved tumor control with Tb-161, particularly in lesions <1 mm, where Lu-177’s beta emissions often spare peripheral tumor cells due to the "crossfire" effect.
Notably, simulations of Tb-161’s dose distribution show that Auger electrons contribute >50% of the total energy in cellular/subcellular targets, maximizing lethality while sparing adjacent healthy tissue. This is critical for reducing nephrotoxicity and bone marrow suppression, common limitations of Lu-177.
Clinical Implications and Comparison with Lu-177
Lu-177 remains effective for bulky tumors, but its utility in oligometastatic or micrometastatic settings is constrained. Tb-161’s Auger electrons bridge this gap, offering a "targeted microtherapy" approach. Furthermore, Tb-161’s gamma emissions (25–50 keV) permit post-therapy SPECT imaging, similar to Lu-177’s 113 keV gamma photons, enabling dosimetry and treatment monitoring.
Early-phase clinical trials are evaluating Tb-161-PSMA in mCRPC. Preliminary data suggest comparable pharmacokinetics to Lu-177, with enhanced PSA responses in patients with low-volume disease. Dosimetric analyses indicate higher tumor-to-kidney and tumor-to-salivary gland ratios for Tb-161, correlating with its favorable toxicity profile.
Future Prospects of Tb-161 in Nuclear Medicine
With the introduction of Tb-161 at Fortis Memorial Research Institute, I anticipate significant improvements in prostate cancer treatment, particularly for patients with micrometastases or oligometastatic disease. The ability of Tb-161 to deliver high-energy Auger electrons directly to the DNA of tumor cells translates to superior therapeutic outcomes with minimal collateral damage. While Lu-177 remains effective for larger tumors, I strongly believe that Tb-161 offers a more precise and impactful alternative, and I look forward to further research and clinical applications solidifying its role in nuclear medicine.
About Dr. Ishita B. Sen
Dr. Ishita B. Sen is the Director and Head of the Department of Nuclear Medicine & Molecular Imaging at Fortis Memorial Research Institute (FMRI), Gurgaon. An alumna of Lady Hardinge Medical College & Associated Hospitals, she has over 20 years of experience in nuclear medicine. Dr. Sen has received specialized training in the medical management of thyroid cancer at Memorial Sloan Kettering Cancer Centre in New York, as well as in myocardial SPECT imaging in Beijing, China. Her areas of expertise include nuclear oncology, radionuclide therapy, PET-guided interventional procedures, and infection imaging. She has authored numerous reviews and clinical communications in imaging and oncology and has been an invited speaker at national and international conferences. Dr. Sen currently serves as the President of the Association of Nuclear Medicine Physicians of India and is a member of the International Task Force to prepare Standard Operating Procedures on the Use of Clinical Practice, International Energy Agency, Vienna.
References
- J Nucl Med. 2021;62(10):1391. URL: https://jnm.snmjournals.org/content/62/10/1391.
- PMC7237560. URL: https://pmc.ncbi.nlm.nih.gov/articles/PMC7237560/.
- ClinicalTrials.gov (NCT05521412). URL: https://clinicaltrials.gov/ct2/show/NCT05521412.
- Additional preprints and radiobiology textbooks on Auger electron emitters.