Mortimer JE, et al. - Experts sought to characterize the relationship between tumor uptake of ⁶⁴Cu-DOTA-trastuzumab as measured by PET/CT and standard, immunohistochemistry (IHC)-based, histopathologic classification of human epidermal growth factor receptor 2 (HER2) status in women with metastatic breast cancer (MBC). Uptake of ⁶⁴Cu-DOTA-trastuzumab in MBC was strongly correlated with patient human epidermal growth factor receptor 2 (HER2) status by 1 d after injection. It appeared to be indicative of binding to HER2. The role of ⁶⁴Cu-DOTA-trastuzumab PET/CT in optimizing treatments that included trastuzumab was revealed through the variability within and among HER2+ patients, as well as the overlap between the HER2+ and HER2- groups.