A drug that is used to plan cancer treatment by measuring oxygen levels in tumor cells. OR A fluorinated derivative of the 2-nitroimidazole etanidazole. EF5 is effective in accessing oxygen levels in tumor tissue through its adduct formation to intracellular macromolecules in the absence of oxygen. Reduction of this agent is carried out by a diverse group of enzymes in the cytoplasm, microsomes and mitochondria. Tissue hypoxia detection via EF5 has been reported in several cancers, including squamous cell carcinoma of the cervix and the head and neck, and in sarcoma.