Patient description: The patient is a 42 year-old postmenopausal woman who had a screening sonogram which revealed an abnormality in the right breast. She had no palpable masses on breast exam. Core biopsy was done and revealed a 1.8 cm infiltrating ductal breast carcinoma in the left upper outer quadrant. Lumpectomy was done and the surgical margins were clear. The tumor was HER2-positive and ER/PR negative. Axillary sampling revealed 1 positive lymph node out of 12 sampled. CXR was unremarkable. She is using “well women” multivitamins daily and no other medication. She smokes frequently and consumes alcohol occasionally. She is in a relation with only one partner and has a history of 3 pregnancies and live births. She breastfed all three children.

Clinical trial eligibility criteria: eligible ages (years): 60.0-999.0,  Breast Cancer Participants must have histologically or cytologically confirmed HER2-positive disease by local pathology, defined as immunohistochemistry (IHC) 3+ or amplification by FISH (HER2/CEP17 ratio ≥2 or an average of ≥6 HER2 gene copies per nucleus) AND confirmed by Central Pathology Review (Dr. Deborah Dillon at Brigham and Women's Hospital, Boston, MA) prior to patient being registered to begin protocol therapy. See section 3.4. http://ascopubs.org/doi/full/10.1200/jco.2013.50.9984 NOTE: DCIS components should not be counted in the determination of HER2 status Age ≥60 years at the time of study registration (men and women eligible) Participants must have histologically or cytologically confirmed Stage I-III breast cancer with the following met If node-negative or if node status unknown (because it was not assessed), tumor must be >5 mm of any hormone receptor subtype (document ER/PR status: if some ER/PR staining is present, ER and PR negative are defined as being positive in <10% cells [per local pathology read]) If node-positive (N1-N3), T1mi, T1a, T1b, T1c, T2, or T3 tumors are eligible (see below for further details on defining node-negative disease) Definition of node-negative disease (when node status known): If the patient has had a negative sentinel node biopsy and/or a negative axillary dissection, then the patient is determined to be node-negative. Axillary nodes with single cells or tumor clusters ≤ 0.2 mm by either H&E or IHC will be considered node-negative. Any axillary lymph node with tumor clusters between 0.02 and 0.2cm is considered a micrometastasis. Patients with a micrometastasis are eligible even if their tumor is </= 5mm. An axillary dissection is not required to be performed in patients with a positive sentinel node and management of the axilla will be left up to the treating provider. 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. In these special situations, the investigator must document this approval in the patient medical record ER/PR determination assays performed by IHC methods according to the local institution standard protocol Standard chemotherapy/trastuzumab declined by patient OR patient is deemed by physician for any reason to not be a candidate for standard therapy (i.e. patient and/or provider choose not to pursue standard trastuzumab-based chemotherapy regimen because of concerns related to toxicity or provider/patient preference) For patients with bilateral or multifocal/multicentric breast cancers, one of the following must be met to enroll: (1) each cancer individually meets for enrollment (only ONE tumor has to undergo central confirmation for HER2), (2) at least one tumor meets (per tumor size/nodes/subtype outlined above) and the other foci in the ipsilateral or contralateral breast are also HER2-positive but are too small for enrollment (e.g., a patient is eligible if a cancer is T2N0 and HER2-positive in one breast, but the contralateral breast has a T1a HER2+ cancer that isn't eligible on its own, (3) there is at least one qualifying tumor of >5mm but there are other small foci of disease that are too small to test for ER/PR/HER2 and are felt to be a part of the same tumor or similar tumor, OR (4) at least one tumor meets and the other foci in the ipsilateral or contralateral breast are HER2-negative and do not meet for adjuvant chemotherapy per provider discretion (e.g. if a patient has a HER2-positive tumor meeting but also has a second, HER2-negative, small, node-negative, ER+, low grade cancer present, she is still eligible for enrollment). However, in the specific case that a second breast cancer is stage III and HER2-negative, that patient is excluded (because the second cancer is high-risk and likely will require non-HER2-directed therapy) All tumor removed by either a modified radical mastectomy or a segmental mastectomy (lumpectomy)  Evidence of metastatic disease Patients will not require baseline staging PET or CT chest, abdomen, pelvis or bone scan to rule out metastatic disease prior to enrollment. Any staging scans will be ordered at the treating provider's discretion. If metastatic disease is found on any staging studies done, patients will not be eligible for enrollment Locally advanced tumors at diagnosis (T4), including tumors fixed to the chest wall, peau d'orange, skin ulcerations/nodules, or clinical inflammatory changes (diffuse brawny cutaneous induration with an erysipeloid) Patients with stage III, HER2-negative cancer in the contralateral breast (see 3.1.6 above) Positive Hepatitis B (Hepatitis B surface antigen and antibody) and/or Hepatitis C (Hepatitis C antibody test) as indicated by serologies conducted ≤3 months prior to registration if liver function tests are outside of the normal institutional range. NOTE: A hepatitis panel is required of all participants as part of screening. Patients with positive Hepatitis B or C serologies indicating active infection without known active disease must meet the requirements for ALT, AST, total bilirubin, INR, PTT, and alkaline phosphatase on at least two consecutive occasions, separated by at least 1 week. Patients with laboratory evidence of vaccination to Hepatitis B (e.g., positive antibodies) are eligible Active liver disease, for example, due to autoimmune hepatic disorder, or sclerosing cholangitis Significant, active cardiopulmonary dysfunction (i.e. uncontrolled heart issues)as indicated by MUGA or echocardiogram performed ≤60 days prior to registration and/or by presence of any of the following History of NCI CTCAE (Version 4.0) Grade ≥3 symptomatic congestive heart failure (CHF) or NYHA Class ≥ II Angina pectoris requiring anti-anginal medication, serious cardiac arrhythmia not controlled by adequate medication, severe conduction abnormality, or clinically significant valvular disease High-risk uncontrolled arrhythmias (i.e., atrial tachycardia with a heart rate > 100/min at rest, significant ventricular arrhythmia [ventricular tachycardia], or higher-grade atrioventricular [AV]-block [second degree AV-block Type 2 [Mobitz 2] or third degree AV-block]); if adequately and safely treated, patient may be eligible