Antibodies, Monoclonal, Humanized/therapeutic use; Antineoplastic Agents/therapeutic use; Breast Neoplasms/drug therapy; Maytansine/analogs & derivatives; Receptor, ErbB-2/antagonists & inhibitors; Ado-Trastuzumab Emtansine; Antibodies, Monoclonal, Humanized/administration & dosage; Antibodies, Monoclonal, Humanized/adverse effects; Antibodies, Monoclonal, Humanized/economics; Antineoplastic Agents/administration & dosage; Antineoplastic Agents/adverse effects; Antineoplastic Agents/economics; Breast Neoplasms/enzymology; Breast Neoplasms/mortality; Breast Neoplasms/secondary; Disease Progression; Disease-Free Survival; Drug Administration Schedule; Drug Interactions; Female; Humans; Infusions, Intravenous; Maytansine/administration & dosage; Maytansine/adverse effects; Maytansine/economics; Maytansine/therapeutic use; Receptor, ErbB-2/metabolism; Time Factors; Trastuzumab; Treatment Outcome