ERBB2 and breast cancer: Although initial evaluations of the heterogeneity of HER2 expression in breast cancer used [89Zr]Zr-trastuzumab PET alone (9–11), by combining it with additional imaging, such as [18F]FDG PET/CT, the ZEPHIR trial demonstrated improved positive predictive values with trastuzumab emtansine compared with either modality alone (13).