Key Inclusion Criteria:

          -  Adults ≥18 years old (local regulatory requirements will apply if the legal age of
             consent for study participation is >18 years old)

          -  Pathologically documented HER2-positive breast cancer (BC):

               -  HER2-positive expression defined as an immunohistochemistry (IHC) score of 3+
                  and/or positive by in situ hybridization (ISH) confirmed prior to study
                  randomization

          -  Histologically confirmed invasive breast carcinoma

          -  Clinical stage at disease presentation: T1-4, N0-3, M0; patients presenting with T1N0
             tumors are not eligible

          -  Pathologic evidence of residual invasive carcinoma in the breast and/or axillary lymph
             nodes following completion of neoadjuvant therapy meeting one of the following
             high-risk criteria:

               -  Inoperable breast cancer at presentation (prior to neoadjuvant therapy), defined
                  as clinical stages T4, N0-3, M0 or T1-3, N2-3, M0

               -  Operable at presentation, defined as clinical stages T1-3,N0-1,M0, with axillary
                  node positive disease (ypN1-3) following neoadjuvant therapy

          -  Completion of neoadjuvant systemic chemotherapy, including taxane and HER2-directed
             treatment prior to surgery

               -  Systemic therapy must consist of at least 6 cycles of chemotherapy with a total
                  duration of at least 16 weeks, including at least 9 weeks of trastuzumab (±
                  pertuzumab) and at least 9 weeks of taxane based chemotherapy. Patients may have
                  received an anthracycline as part of neoadjuvant therapy in addition to taxane
                  chemotherapy.

          -  Adequate excision as confirmed per medical records: surgical removal of all clinically
             evident disease in the breast and lymph nodes.

          -  An interval of no more than 12 weeks between the date of last surgery and the date of
             randomization.

          -  Known hormone receptor (HR) status, per local laboratory assessment, as defined by
             ASCO-CAP guidelines (≥1%): HR positive status defined by either positive estrogen
             receptor (ER) and/or positive progesterone receptor (PR) status. HR-negative status
             defined by both known negative ER and known negative PR.

          -  Left ventricular ejection fraction (LVEF) ≥50% within 28 days prior to randomization.

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 at Screening.

          -  Has adequate organ function within 14 days before randomization.

        Key Exclusion Criteria:

          -  Stage IV (metastatic) BC

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

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

          -  Prior treatment with T-DXd, T-DM1 or other anti-HER2 antibody-drug conjugate (ADC)

          -  History of exposure to the following cumulative doses of anthracyclines:

               -  Doxorubicin > 240 mg/m^2

               -  Epirubicin or Liposomal Doxorubicin-Hydrochloride > 480 mg/m^2

               -  For other anthracyclines, exposure equivalent to doxorubicin > 240 mg/m^2

          -  History of other malignancy within the last 5 years except for appropriately treated
             CIS of the cervix, nonmelanoma skin carcinoma, Stage I melanoma skin carcinoma, Stage
             I uterine cancer, or other appropriately treated non-breast malignancies

          -  History of (noninfectious) interstitial lung disease (ILD)/pneumonitis that required
             steroids and/or has ILD/pneumonitis noted on computed tomography (CT) scan of the
             chest at Screening (asymptomatic interstitial changes confined to recent radiation
             therapy fields are not excluded)

          -  Known pulmonary compromise resulting from intercurrent pulmonary illnesses including,
             but not limited to, any underlying pulmonary disorder (eg, pulmonary emboli within
             three months prior to randomization, severe asthma, severe chronic obstructive
             pulmonary disease [COPD], restrictive lung disease).

          -  Any autoimmune, connective tissue or inflammatory disorders with pulmonary involvement
             (eg, Rheumatoid arthritis, Sjogren's, sarcoidosis, etc.), or prior lobectomy or
             pneumonectomy

          -  Medical history of myocardial infarction (MI) within 6 months before randomization,
             symptomatic congestive heart failure (CHF) (New York Heart Association Class II to
             IV), troponin levels consistent with MI as defined according to the manufacturer 28
             days prior to randomization