Biodistribution and SPECT/CT imaging studies were performed in NRG mice with s.c. 164/8-1B/H2N.luc+ HER2-positive BC xenografts co-injected with 7–8 MBq of [111In]In-DOTA-trastuzumab and 4 kBq of [225Ac]Ac-DOTA-trastuzumab (total mass = 40 μg) since it was not feasible to measure these very small amounts of 225Ac by using the γ-emissions of the 213Bi daughter product. The gene discussed is ERBB2; the disease is breast cancer.