HER2-positive Breast Cancer With Brain Metastasis (GCC 1345)

Inclusion Criteria: * Histologically proven diagnosis HER2-positive breast cancer. Her2-positive is defined as follows: \* Validated IHC assay score of 3+ (defined as uniform, intense staining of \>30% of invasive tumor cells) * OR- Average HER2 gene copy number of \>6 * OR- Gene amplified (HER2:D17Z1 ratio \>2.20). * Patients with 1-10 newly diagnosed brain metastases * The contrast-enhancing intraparenchymal brain tumor must be well circumscribed and must have a maximum diameter of ≤ 4.0 cm in any direction on the enhanced scan. If multiple lesions are present and one lesion is at the maximum diameter, the other(s) must not exceed 3.0 cm in maximum diameter. * History and physical with neurological examination, steroid documentation, height, and weight within 14 days of registration. * A diagnostic contrast-enhanced MRI of the brain must be performed within 28 days prior to registration. * Eligibility for treatment with SRS confirmed by a radiation oncologist. * Performance Status 0-2 * Age ≥ 18. * CBC with differential obtained within 14 days prior to registration, with adequate bone marrow function defined as follows: * Absolute neutrophil count (ANC) ≥ 1,100 cells/mm3. * Platelets ≥ 75,000 cells/mm3. * Hemoglobin ≥ 9.0 g/dl (Note: The use of transfusion or other intervention to achieve Hgb ≥9.0 g/dl is acceptable). * Adequate renal function within 14 days prior to registration, as defined below: * BUN ≤ 30 mg/dl. * Creatinine ≤ 1.5 x ULN * Creatinine clearance ≥30 mL/min. Adequate hepatic function within 14 days prior to registration, as defined below: * Total Bilirubin ≤1.5 x ULN * ALT/AST ≤ 2.5 x upper limit of normal (ULN). * Systolic blood pressure ≤ 160 mg Hg or diastolic pressure ≤ 90 mg Hg within 14 days prior to registration. * Prothrombin time/international normalized ratio (PT INR) \< 1.4 for patients not on warfarin confirmed by testing within 14 days prior to registration. Patients on full-dose anticoagulants (e.g., warfarin or LMW heparin) must meet both of the following criteria: * No active bleeding or pathological condition that carries a high risk of bleeding (e.g., tumor involving major vessels or known varices). * In-range INR (between 2.5 and 3.5) on a stable dose of warfarin-based oral anticoagulant; or on a stable dose of low molecular weight heparin; or INR between 1.5 and 2 if a Greenfield filter is in place. * Patient must provide study specific informed consent prior to study entry. * For women of child-bearing potential, negative serum pregnancy test within 14 days prior to registration. * Women of childbearing potential and male participants must practice adequate contraception. * Echocardiogram or MUGA scan with ejection fraction within normal institution limits within 28 days of registration Exclusion Criteria: * Prior invasive malignancy (except for non-melanomatous skin cancer) unless disease free for ≥ 3 years. For example, carcinoma in situ of the oral cavity and cervix are all permissible. * Leptomeningeal metastases * Previous treatment with all of the following: lapatinib, trastuzumab, pertuzumab, and trastuzumab emtansine. (Patients are eligible if treated with 3 or less of these agents.) * Prior cranial radiotherapy. * Prior resection of cerebral metastases * Allergy to gadolinium Severe, active co-morbidity, defined as follows: * Unstable angina and/or congestive heart failure within the last 6 months. * Transmural myocardial infarction within the last 6 months. * New York Heart Association grade II or greater congestive heart failure requiring hospitalization within 12 months prior to registration. * History of stroke, cerebral vascular accident (CVA) or transient ischemic attack within 6 months. * Serious and inadequately controlled cardiac arrhythmia. * Significant vascular disease (e.g., aortic aneurysm, history of aortic dissection) or clinically significant peripheral vascular disease. * Evidence of bleeding diathesis or coagulopathy. * Serious or non-healing wound, ulcer, or bone fracture or history of abdominal fistula, gastrointestinal perforation or intra-abdominal abscess, major surgical procedure or significant traumatic injury within 28 days prior to registration, with the exception of the craniotomy for tumor resection or follow-on craniotomies to manage complications of brain tumor management such as hemorrhage or infection. * Bacterial or fungal infection requiring intravenous antibiotics at the time of registration. * Chronic obstructive pulmonary disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of registration. * Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects; note, however, that laboratory tests for coagulation parameters are not required for entry into this protocol. * Active connective tissue disorders, such as lupus or scleroderma, that in the opinion of the treating physician may put the patient at high risk for radiation toxicity. * Any other major medical illnesses or psychiatric impairments that in the investigator's opinion will prevent administration or completion of protocol therapy. * Cognitive impairment that precludes a patient from acting as his or her own agent to provide informed consent. * Women of childbearing potential who are sexually active and not willing/able to use medically acceptable forms of contraception; this exclusion is necessary because the chemotherapeutic treatment involved in this study is potentially teratogenic. * Pregnant or lactating women, due to possible adverse effects on the developing fetus or infant due to study treatment. * Patients treated on any other therapeutic clinical protocols within 30 days prior to study entry or during participation in the study. * Inability to undergo MRI (e.g., due to safety reasons, such as presence of a pacemaker). * Inability to undergo SRS due to claustrophobia