a radioactive substance being studied as an imaging agent in head and neck cancers and other types of cancer. it binds to large molecules in tumor cells that have a low level of oxygen. radiation given off by 18f-miso is detected by a pet scan. the amount of 18f-miso in the tumor may help decide the best treatment and help predict whether the cancer will come back after treatment. 18f-miso is a type of radioimaging agent. also called 18f-fluoromisonidazole and fmiso.