Whole-Brain Radiation Therapy or Stereotactic Radiosurgery With or Without Lapatinib Ditosylate in Treating Patients With Brain Metastasis From HER2-Positive Breast Cancer

PRIMARY OBJECTIVES: I. To determine if there is a signal for an increase in complete response (CR) rate in the measurable brain metastases at 12 weeks post radiation therapy (RT) (whole brain or stereotactic radiosurgery \[SRS\]) as determined by magnetic-resonance imaging (MRI) scan of the brain, with the addition of lapatinib (lapatinib ditosylate) to whole-brain radiation therapy (WBRT)/SRS compared to WBRT/SRS alone. SECONDARY OBJECTIVES: I. To evaluate CR rate of the measurable brain metastases at 4 weeks post RT (WBRT/SRS) as determined by MRI scan of the brain, with the addition of lapatinib to WBRT/SRS compared to WBRT/SRS alone. II. To evaluate objective response rate of measurable brain metastases at 4 and 12 weeks post RT (WBRT/SRS) as determined by MRI scan of the brain, with the addition of lapatinib to WBRT/SRS compared to WBRT/SRS alone. III. To evaluate targeted lesion-specific objective response rate (CR + partial response \[PR\]) at 4 and 12 weeks post WBRT/SRS. IV. To evaluate central nervous system (CNS) progressive disease outside the targeted measurable disease with addition of lapatinib to WBRT/SRS compared to WBRT/SRS alone. V. To evaluate targeted lesion-specific progression at 4 and 12 weeks post WBRT/SRS. VI. To evaluate treatment related adverse events when adding lapatinib to WBRT/SRS compared to WBRT/SRS alone. VII. To evaluate overall CNS complete response: disappearance of all CNS target lesions sustained for at least 4 weeks; with no new lesions, no use of corticosteroids, and patient is stable or improved clinically, when adding lapatinib to WBRT/SRS compared to WBRT/SRS alone. VIII. To evaluate overall CNS progressive disease (within or outside targeted measurable disease) with addition of lapatinib to WBRT/SRS compared to WBRT/SRS alone. IX. To evaluate overall survival when adding lapatinib to WBRT/SRS compared to WBRT/SRS alone. OUTLINE: Patients are randomized to 1 of 2 treatment arms. ARM A: Patients undergo WBRT 5 days a week for 3 weeks for a total of 15 treatments or SRS for 1 treatment. ARM B: Patients undergo WBRT or SRS as in Arm A. Patients also receive lapatinib ditosylate orally (PO) once daily (QD) for 6 weeks. After completion of study treatment, patients are followed up at 4 and 12 weeks and then every 12 weeks thereafter.