ERBB2 and breast cancer: We conclude that [111In]In-DOTA-trastuzumab F(ab')2 and [225Ac]Ac-DOTA-trastuzumab F(ab')2 exhibited superior properties as a theranostic pair for SPECT/CTimaging and α-particle RIT of HER2-positive 164/8-1B/H2N.luc+ human BC tumors in NRG mice compared to [111In]In-DOTA-trastuzumabIgG and [225Ac]Ac-DOTA-trastuzumab IgG or [111In]In-DOTA-trastuzumab Fab and [225Ac]Ac-DOTA-trastuzumabFab.