First-in-Humans Application of ¹⁶¹Tb: A Feasibility Study Using ¹⁶¹Tb-DOTATOC

¹⁶¹Tb has decay properties similar to those of 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 of visualizing the physiologic and tumor biodistributions of ¹⁶¹Tb-DOTATOC. <b>Methods:</b>¹⁶¹Tb was shipped from Paul Scherrer Institute, Villigen-PSI, Switzerland, to Zentralklinik Bad Berka, Bad Berka, Germany, where it was used for the radiolabeling of DOTATOC. In 2 separate studies, 596 and 1,300 MBq of ¹⁶¹Tb-DOTATOC were administered to a 35-y-old male patient with a metastatic, well-differentiated, nonfunctional malignant paraganglioma and a 70-y-old male patient with a metastatic, functional neuroendocrine neoplasm of the pancreatic tail, respectively. Whole-body planar γ-scintigraphy images 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 observed for adverse events after the application of ¹⁶¹Tb-DOTATOC. <b>Results:</b> The radiolabeling of DOTATOC with ¹⁶¹Tb was readily achieved with a high radiochemical purity suitable for patient application. Planar images and dosimetry provided the expected time-dependent biodistribution of ¹⁶¹Tb-DOTATOC in the liver, kidneys, spleen, and urinary bladder. SPECT/CT images were of high quality and visualized even small metastases in bones and liver. The application of ¹⁶¹Tb-DOTATOC was well tolerated, and no related adverse events were reported. <b>Conclusion:</b> This study demonstrated the feasibility of imaging even small metastases after the injection of relatively low activities of ¹⁶¹Tb-DOTATOC using γ-scintigraphy and SPECT/CT. On the basis of this essential first step in translating ¹⁶¹Tb to clinics, further efforts will be directed toward the application of ¹⁶¹Tb for therapeutic purposes.