Since the SF of SK-BR-3 cells exposed to [225Ac]Ac-DOTA-trastuzumab was decreased > 100-fold compared to treatment in vitro with [177Lu]Lu-DOTA-trastuzumab reported by Rasaneh et al. (Rasaneh et al. 2010) (ie. SF < 0.001 vs. 0.10, respectively) and this decreased SF was achieved at a 123-fold lower SA of [225Ac]Ac-DOTA-trastuzumab than [177Lu]Lu-DOTA-trastuzumab (ie. 0.754 kBq/μg vs. 0.0925 MBq/μg, respectively), these results indicate that the α-particles emitted by 225Ac were far more effective than the β-particles emitted by 177Lu for killing HER2-positive BC cells. Here, ERBB2 is linked to breast cancer.