Radiation Therapy Followed by Intrathecal Trastuzumab/Pertuzumab in HER2+ Breast Leptomeningeal Disease

Inclusion Criteria: * Confirmation of HER2 positivity. All patients with HER2+ cancers will be allowed to enroll if they have leptomeningeal disease (LMD). Patients may be IHC 3+ and/or FISH-positive. IHC 2+ HER2 patients are eligible with reflex FISH-positive testing with the ratio ≥2.0. And/or patients with HER2 positive cells in the cerebral spinal fluid. * Participants may have concomitant brain metastases * Cerebrospinal fluid (CSF) sampling is required to document LMD if not documented by MRI. Participants are still eligible CSF is negative but LMD disease is documented on MRI * Life expectancy greater than 8 weeks * Consent to pretreatment tumor biopsy or retrieval of archival tissue * Normal renal (creatinine \<1.5 × upper limit of normal \[ULN\]), liver (bilirubin \< 1.5 × ULN, transaminases \<3.0 × ULN, except in known hepatic disease, wherein may be \<5 × ULN) and blood counts (white blood cells ≥2.5, neutrophils ≥1000, platelets ≥75,000, hemoglobin ≥8) * LVEF \>50% * KPS \>/= 60 * Patients with surgery within 14 days should have recovered from all effects of the surgery and be cleared by their surgeon * There is no limit on prior systemic or IT therapies * Must be willing to have an Ommaya reservoir placed and a candidate for an Ommaya reservoir placement * Women of childbearing potential and sexually active males must commit to the use of effective contraception while on study. Contraception methods should start a minimum of 14 days before the first administra…