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The branch of medicine that uses radioisotopes for diagnosis and treatment is called nuclear medicine. A radioactive tracer is an isotope that emits radiation that can be detected to track the movement of the isotope through a system such as a patient's body. Radioactive tracers are generally used in nuclear medicine for diagnostic purposes. For example, fluorine-18 (${}^{18}\rm {F}$) is often used to evaluate the progression of cancers. Cancer cells take in amino acids at increased rates. An amino acid can be synthesized that contains the ${}^{18}\rm {F}$ atom, and this labeled ${}^{18}\rm {F}$ amino acid is absorbed by cells the same way normal amino acids are. Cells higher in ${}^{18}\rm {F}$ concentration indicate sites with cancerous cells.
External radiation therapy is radiation therapy that administers high-energy radiation, such as gamma rays from a cobalt-60 (${}^{60}{\rm{Co}}$) source, directed at the targeted area. Internal radiation therapy is radiation therapy that administers radiation such as gamma rays or beta particles to the target area of the body through ingestion or implantation at the target site. For example, iodine-131 (${}^{131}\rm{I}$) is frequently injected to treat thyroid cancer. Radiation therapy is distinct from chemotherapy, which is the use of cytotoxic drugs to shrink or eliminate tumors. Radiation therapy and chemotherapy are often combined in the treatment of cancer.
Applications for radioisotopes exist outside of medicine, as well. A radiolabel is an isotope that can be added to a molecule to aid in detection of the molecule. Radiolabels can be used in pathology, biotechnology, and biological research. Radioisotopes are used in gamma radiography, the process of testing the integrity of pipelines and other infrastructure by placing a high-energy radiation source on one side of a structure and a sensor on the other side to examine the structure for defects. Damaged areas in the structure have less matter and a lower absorption rate of gamma rays or X-rays. The differences in the amount of radiation that can penetrate the material allows for imaging of the structure. Radioisotopes are also used in sensors and gauges, such as smoke detectors, which contain americium-241 (${}^{241}{\rm{Am}}$). The power of the uranium decay series is harnessed in nuclear reactors to generate electricity. Many other industrial and research applications for isotopes exist, with new applications being discovered all the time.