Treatment
All eligible patients were treated with pyrotinib (240–400 mg orally once daily), trastuzumab (6 mg per kilogram of body weight intravenously per 21 days, with an initial loading dose of 8 mg per kilogram) and single chemotherapeutic agent (nab-paclitaxel, 260 mg per square meter of body-surface area intravenously on day 1 of each 21-day cycle; or capecitabine, 1000 mg per square meter of body-surface area orally twice daily on days 1 to 14 of each 21-day cycle; or gemcitabine, 1000 mg per square meter of body-surface area intravenously on days 1 and 8 of each 21-day cycle; or vinorelbine, 60 mg per square meter of body-surface area orally on days 1 and 8 of each 21-day cycle and dose elevation to 80 mg per square meter of body-surface area was allowed if well tolerated).