T-DM1 Alone Versus T-DM1 and Metronomic Temozolomide in Secondary Prevention of HER2-Positive Breast Cancer Brain Metastases Following Stereotactic Radiosurgery

Background: Sometimes breast cancer spreads (metastasizes) to the brain. Researchers want to study new treatments for brain metastases. The drug Temozolomide is approved to treat brain tumors. Researchers want to see if combining it with the drug trastuzumab emtansine (T-DMI) prevents the formation of new metastases in the brain. Objective: To study if Temozolomide with T-DM1 lowers the chance of having new metastases in the brain. Eligibility: Adults at least 18 years old with a human epidermal growth factor receptor 2 (HER2)-positive breast cancer that has spread to the brain and was recently treated with stereotactic radiation or surgery. Design: Participants will be screened with * Medical history * Physical exam * Heart tests * A scan (computed tomography (CT) that makes a picture of the body using a small amount of radiation * A scan (magnetic resonance imaging (MRI) that uses a magnetic field to make an image of the brain * Blood tests. * Pregnancy test. The study will be done in 3-week cycles. All participants will get T-DM1 on Day 1 of every cycle through a small plastic tube inserted in an arm vein. Some participants will also take Temozolomide capsules by mouth every day. Participants will keep a medication diary. During the study, participants will also: * Repeat most of the screening tests. * Answer questions about their general well-being and functioning. Participants will have lumbar puncture at least 2 times. A needle is inserted into the spinal canal low in the back and cerebrospinal fluid is collected. This will be done with local anesthesia and with the help of images. Participants will be asked to provide tumor samples when available. Participants will have a follow-up visit about 1 month after stopping the study drug. They will be contacted by telephone or email every 3 months after that.