ATEMPT 2.0: Adjuvant T-DM1 vs TH

Inclusion Criteria: * Patients must have HER2-positive Stage I histologically confirmed invasive carcinoma of the breast. Patients must have node-negative (N0) or micrometastases (N1mic) breast cancer according to the AJCC 8th edition anatomic staging table. * If the patient has had a negative sentinel node biopsy, then no further axillary dissection is required, and the patient is determined to be node-negative. If an axillary dissection without sentinel lymph node biopsy is performed to determine nodal status, at least six axillary lymph nodes must be removed and analyzed, and determined to be negative, for the patient to be considered node-negative. Axillary nodes with single cells or tumor clusters ≤ 0.2 mm by either H\&E or immunohistochemistry (IHC) will be considered node-negative. * Any axillary lymph node with tumor clusters between 0.02 and 0.2 cm is considered a micrometastasis. Patients with a micrometastasis are eligible. An axillary dissection is not required to be performed in patients with a micrometastasis found by sentinel node evaluation. In cases where the specific pathologic size of lymph node involvement is subject to interpretation, the principal investigator will make the final determination as to eligibility. The investigator must document approval in the patient medical record. * Patients who have an area of a T1aN0, ER+ (defined as \>10%), HER2-negative cancer in addition to their primary HER2-positive tumor are eligible. * HER2-positive by …