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

Participants will receive radiation therapy (RT), either whole brain radiation therapy or focal brain/spine RT. The goal of RT is to palliate symptoms and improve the flow of Intrathecal (IT) therapy through the cerebrospinal fluid (CSF). As such, various RT schedules and targets are permitted. It is expected that the majority of patients will receive WBRT with 30 Gy in 10 fractions. However, shorter fractions of 20 Gy in 5 fractions of WBRT are permitted. In those patients who have more localized leptomeningeal disease in the spine, focal RT in up to 10 fractions may be administered with the exact dose left up to the discretion of the treating radiation oncologist.

Participants will be treated at 1 of 4 dose levels of pertuzumab, beginning at 10 mg and increasing up to 80 mg or Maximum Tolerated Dose (MTD).

Participants will be treated at a fixed dose of 80 mg trastuzumab.