Background
breast cancers, aggressive disease, breast cancer
SECONDARY, METASTATIC BREAST CANCER, BREAST CANCER
Human epidermal growth factor receptor 2 (HER2)-positive breast cancer is an aggressive disease subtype, which accounts for 15–20% of breast cancers [Definition of primary trastuzumab resistance is derived from and serves for clinical trials, while its value of guiding clinical practice is not clarified. The cutoff to differentiate primary resistance from secondary resistance is not completely consistent in different clinical trials [Primary resistance mostly stands for intrinsic resistance and HER2 independency, while secondary resistance reflects enquired loss of sensitivity or presence of dominant resistant subclones [Pyrotinib is an irreversible pan-HER TKI that targets EGFR, HER2, and HER4. The phase 3 PHOEBE study confirmed the superiority of pyrotinib plus capecitabine over lapatinib plus capecitabine in patients with trastuzumab-taxane-treated, HER2-positive metastatic breast cancer [