A Study of Trastuzumab Emtansine Versus Trastuzumab as Adjuvant Therapy in Patients With HER2-Positive Breast Cancer Who Have Residual Tumor in the Breast or Axillary Lymph Nodes Following Preoperative Therapy (KATHERINE)

Inclusion Criteria: * Adult patient, \>/= 18 years of age * HER2-positive breast cancer * Histologically confirmed invasive breast carcinoma * Clinical stage T1-4/N0-3/M0 at presentation (patients with T1a/bN0 tumors will not be eligible) * Completion of preoperative systemic chemotherapy and HER2-directed treatment consisting of at least 6 cycles of chemotherapy with a total duration of at least 16 weeks, including at least 9 weeks of trastuzumab and at least 9 weeks of taxane-based therapy * Adequate excision: surgical removal of all clinically evident disease in the breast and lymph nodes as specified in protocol * Pathological evidence of residual invasive carcinoma in the breast or axillary lymph nodes following completion of preoperative therapy * An interval of no more than 12 weeks between the date of surgery and the date of randomization * Known hormone-receptor status * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 * Adequate hematologic, renal and liver function * Screening Left ventricular ejection fraction (LVEF) \>/= 50% on echocardiogram (ECHO) or multiple-gated acquisition (MUGA) after receiving neoadjuvant chemotherapy and no decrease in LVEF by more than 15% absolute points from the pre-chemotherapy LVEF. Or, if pre-chemotherapy LVEF was not assessed, the screening LVEF must be \>/= 55% after completion of neoadjuvant chemotherapy. * For women who are not postmenopausal or surgically sterile: agreement to remain abstinent or use single…