PRIMARY OBJECTIVES: I. Study fludeoxyglucose [18F] 2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil-clevudine (FMAU) (fluorine F 18-clevudine ) in 10 patients with known breast cancer to obtain data on safety, circulating metabolite, tumor imaging feasibility, and radiation dosimetry. II. Assess the potential utility of FMAU-PET in the imaging evaluation of chemotherapeutic response in 15 patients with breast cancer. OUTLINE: Patients receive fluorine F18-clevudine intravenously (IV) over 1 minute and then undergo PET/CT scan at baseline. Patients with human epidermal growth factor receptor 2 positive (HER2+) breast cancer also undergo fluorine F 18-clevudine PET/CT scan 2-3 weeks after the first course of treatment and after completion of treatment. After completion of study treatment, patients are followed up at 24 hours.