These are tumors which expresses estrogen receptors. Estrogen receptors are found within breast cancer cells and are an important marker of which medicines may best treat a breast cancer. This study is being performed to imaging the amount of estrogen receptors expressed in the tumor. A research agent named 16α-18F-fluoroestradiol (FES) finds estrogen receptors and binds to them. Then tumors with estrogen receptors can be imaged by a PET/CT scanner, a machine that can take pictures of where the FES is and how much FES is present. The results of this study may help researchers know whether FES can be used to monitor changes in estrogen receptors in tumors during treatment.