ShortStop-HER2: 12 Months vs. 6 Months of HER2-targeted Medications for People With HER2+ Breast Cancer Who Had a Pathologic Complete Response After Chemotherapy Plus Trastuzumab

Inclusion Criteria: * Patients (females and males) with clinical stage T1c-T3 (or Tx) and nodal stage N0-N1 (except T3N1 tumors, which are not eligible) * Patients must have no residual invasive disease in the breast or lymph nodes after the completion of neoadjuvant therapy. Residual ductal carcinoma in situ (DCIS) is allowed. Patients with residual isolated tumor cells at surgery are considered node-positive and are not eligible * HER2+ by American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines. Central pathology review is not required. In cases where there were multiple tumor sites in breast/nodes that had HER2 testing at diagnosis, at least one site must have been HER2+ AND the treating investigator must feel it is in the patient's best interest to be treated as having HER2+ breast cancer * Known hormone receptor status as defined by ASCO/CAP guidelines. Estrogen receptor (ER) and progesterone receptor (PR) of any values are allowed. Hormone receptor positive status can be determined by either known positive ER or known positive PR status; hormone receptor negative status must be determined by both known negative ER and known negative PR * If invasive disease was present in both breasts, participation in the study is permitted as long as the eligibility criteria are met for both tumors/breasts (including the requirement that at least one biopsied site on each side must have been HER2+) * Age ≥ 18 years * Eastern Cooperative Oncology…