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 or combined contraceptive methods that result in a failure
   rate of < 1% per year during the treatment period and for at least 7 months after the
   last dose of study drug

   - Documentation of hepatitis B virus and hepatitis C virus serology is required

Exclusion Criteria:

   - Stage IV (metastatic) breast cancer

   - History of any prior (ipsi- or contralateral breast cancer except lobular carcinoma in
   situ

   - Evidence of clinically evident gross residual or recurrent disease following
   preoperative therapy and surgery

   - Progressive disease during preoperative systemic therapy

   - Treatment with any anti-cancer investigational drug within 28 days prior to commencing
   study treatment

   - History of other malignancy within the last 5 years except for appropriately treated
   carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer,
   or other non-breast malignancies with a similar outcome to those mentioned above

   - Patients for whom radiotherapy would be recommended for breast cancer treatment but
   for whom it is contraindicated because of medical reasons

   - Current NCI CTCAE (Version 4.0) Grade >/= 2 peripheral neuropathy

   - History of exposure to the following cumulative doses of anthracyclines: Doxorubicin >
   240 mg/m2; Epirubicin or Liposomal Doxorubicin-Hydrochloride (Myocet®) > 480 mg/m2;
   For other anthracyclines, exposure equivalent to doxorubicin > 240 mg/m2

   - Cardiopulmonary dysfunction as defined by protocol

   - Prior treatment with trastuzumab emtansine

   - Current severe, uncontrolled systemic disease

   - Pregnant or lactating women

   - Any known active liver disease, e.g. due to HBV, HCV, autoimmune hepatic disorders, or
   sclerosing cholangitis

   - Concurrent serious uncontrolled infections requiring treatment or known infection with
   HIV

   - History of intolerance, including Grade 3 to 4 infusion reaction or hypersensitivity
   to trastuzumab or murine proteins or any components of the product