18F-FLT-PET Imaging of the Brain in Patients With Metastatic Breast Cancer to the Brain Treated With Whole Brain Radiation Therapy With or Without Sorafenib: Comparison With MR Imaging of the Brain

Inclusion Criteria: * Histologically-confirmed (confirmation done at MSKCC) metastatic adenocarcinoma of the breast * Radiologic evidence of new and/or progressive brain metastases ((≥10 mm in longest dimension) by MRI imaging of the Brain * Planned WBRT based on number (≥ 3 lesions) and/or size (≥ 1 cm) of brain metastases. * Age ≥18 years; males and females * Patients who require additional clinically indicated stereotactic radiosurgery (SRS) in addition to WBRT will also be eligible. * Life expectancy of \>12 weeks. * Karnofsky Performance Status (KPS) ≥ 70%. * Creatinine ≤2.0 times the upper limit of normal. * Women of childbearing potential must have a negative serum pregnancy test performed within 7 days prior to enrollment, must be non-lactating and must agree to use adequate contraception prior to enrollment and for the duration of study participation. * No limit to prior therapies with last anti-cancer treatment ≥2 weeks from initiation of WBRT. Please note: there is no washout period required for trastuzumab, pertuzumab, for patients who have developed new parenchymal brain metastases while on these agents. Exclusion Criteria: * Leptomeningeal metastases Please note: leptomeningeal metastases may be allowed if it is limited to cranial metastasis (MRI spine should be completed, within 4 weeks of enrollment, to show that no other leptomeningeal metastases is present) and is not the only metastasis present in the brain. * Concurrent administration of lapatinib or other tyrosine kinase inhibitors other than sorafenib * Craniotomy or any other major surgery, open biopsy, or significant traumatic injury within 4 weeks of randomization. * Concurrent anti-cancer therapy (chemotherapy, hormonal therapy, radiation therapy, surgery, immunotherapy, biologic therapy, or tumor embolization) other than sorafenib, and protocol-specified whole-brain radiotherapy. * Use of any investigational drug within 30 days or 5 half-lives, whichever is longer, preceding enrollment. * Inability to comply with protocol and /or not willing or not available for follow-up assessments. * Any condition which in the investigator's opinion makes the patient unsuitable for the study participation. * Patient is incontinent of urine or stool (which would make them unable to tolerate lying still for 60 minutes). * Claustrophobia * Known allergic reaction to Gd-DTPA * Renal insufficiency with recent (\<3 month old) creatinine \>2.0