Schematic representation of the fate of antibodies (internalized or non-internalized) labeled with residualizing or non-residualizing radionuclides. a. a non-internalized antibody labeled with a non-residualizing radionuclide. Antibodies bind to their antigens in a dynamic association–dissociation equilibrium fashion. b. an internalized antibody labeled with a nonresidualizing radionuclide. After internalization and degradation of the antibody conjugates and receptors, non-residualizing radionuclides (such as 124I, 76Br) are released from the cell and enter the extracellular space. c. an internalized antibody linked with a residualizing radionuclide. After internalization and degradation of the antibody conjugates and receptors, residualizing radionuclides (such as 64Cu, 86Y, 89Zr) can be trapped in the lysosome. Even if released from lysosome, it can transchelate to intracellular proteins and remain in the cell.