Lapatinib in Combination With Radiation Therapy in Patients With Brain Metastases From HER2-Positive Breast Cancer

* Participant's will be given a study medication-dosing calendar for each treatment cycle. Each treatment cycle lasts four weeks. On the first day of the treatment cycle, participants will take 1 lapatinib orally twice per day, 12 hours apart. After the first day, lapatinib will be taken once a day in the morning. * Whole brain radiation treatments will begin approximately 1-8 days after the first dose of lapatinib. The radiation treatments will follow standard guidelines and will be supervised by a radiation oncologist. Radiation will be given in 15 treatments over a period of three weeks. * Some participants will also undergo stereotactic radiosurgery (SRS). SRS is a highly focused and intense form of radiation treatment generally done as an outpatient procedure in a single treatment. * After whole brain radiation treatments are completed, lapatinib will be continued at the same dose for one more week. After that, the lapatinib dose may change. In addition at the same time, Herceptin will be started. Participants will continue with both lapatinib and herceptin together as long as there is evidence that they are benefitting from it. * During all treatment cycles participants will have a physical exam and be asked general questions about their health. Photographs will be taken of the tumor, if visible, to assess the response of the tumor to the treatment. An assessment of the tumor by CT scan of the body, and MRI imaging of the brain will be performed every two months. An assessment of heart function by MUGA scan or echocardiogram will be performed every 8 weeks. The participant will also be asked to complete a brief questionnaire measuring quality of life and asking about symptoms related to the cancer at baseline, 6 months, and one year. Blood tests will be performed every 4 weeks.