A radiopharmaceutical consisting of an estradiol analogue radiolabeled with the positron-emitting isotope fluorine F 18. F-18 16 alpha-fluoroestradiol is actively taken up in tumor cells expressing the estrogen receptor (ER), allowing visualization of ER-positive tumor cells with positron emiision tomography (PET). Uptake of this agent depends upon the ER status of target tissues. or A radiolabeled substance being studied as an imaging agent in breast cancer. F-18 16 alpha-fluoroestradiol binds to estrogen receptors and gives off radiation that can be detected by a PET scan. The PET scan forms an image that shows where cancer cells with estrogen receptors can be found in the body. It is a type of radioimaging agent. Also called F-18 FES.