First-in-Human Application of Terbium-161: A Feasibility Study Using 161Tb-DOTATOC.

161Tb has similar decay properties as 177Lu but, additionally, emits a substantial number of conversion and Auger electrons. The aim of this study was to apply 161Tb in a clinical setting and to investigate the feasibility to visualize the physiological and tumor biodistribution of 161Tb-DOTATOC. \nMethods:161Tb was shipped from Paul Scherrer Institute, Switzerland, to Zentralklinik Bad Berka, Germany, where it was used for the radiolabeling of DOTATOC. In two separate studies, 596 MBq and 1300 MBq 161Tb-DOTATOC were administered to a 35-year-old male patient with metastatic, well differentiated, non-functional malignant paraganglioma and a 70-year-old male patient with a metastatic, functional neuroendocrine neoplasm of the pancreatic tail, respectively. Whole-body planar γ-scintigraphies were acquired over a period of several days for dosimetry calculations. SPECT/CT images were reconstructed, using a recently-established protocol and visually analyzed. Patients were checked for adverse events after application of 161Tb-DOTATOC. \nResults: The radiolabeling of DOTATOC with 161Tb was readily achieved with high radiochemical purity suitable for patient application. Planar images and dosimetry provided the expected time-dependent biodistribution of 161Tb-DOTATOC in liver, kidneys, spleen and urinary bladder. SPECT/CT images were of high quality and visualized even small metastases in the liver and bones. Application of 161Tb-DOTATOC was well tolerated and no related adverse events were reported. \nConclusion: This study demonstrated the feasibility to image even small metastases after injection of relatively low activities of 161Tb-DOTATOC using γ-scintigraphy and SPECT. Based on this essential first step to translate 161Tb to clinics, further efforts will be directed towards the application of 161Tb for therapeutic purposes.