Radiation can be put to good use killing cancer cells. This area of medicine is called radiation therapy. Gamma rays are the most common form of radiation, often from the isotope ${}^{60}\text{Co}$. As FIGURE 42.25 shows, the gamma rays are directed along many different lines, all of which intersect the tumor. The goal is to provide a lethal dose to the cancer cells without overexposing nearby tissue. The patient and the radiation source are rotated around each other under careful computer control to deliver the proper dose.

FIGURE 42.25 Radiation therapy is designed to deliver a lethal dose to the tumor without damaging nearby tissue.

Other tumors are treated by surgically implanting radioactive “seeds” within or next to the tumor. Alpha particles, which are very damaging locally but don’t penetrate far, can be used in this fashion.

Radioactive isotopes are also used as tracers in diagnostic procedures. This technique is based on the fact that all isotopes of an element have identical chemical behavior. As an example, a radioactive isotope of iodine is used in the diagnosis of certain thyroid conditions. Iodine is an essential element in the body, and it concentrates in the thyroid gland. A doctor who suspects a malfunctioning thyroid gland gives the patient a small dose of sodium iodide in which some of the normal ${}^{127}\text{I}$ atoms have been replaced with ${}^{131}\text{I}$. (Sodium iodide, which is harmless, dissolves in water and can simply be drunk.) The ${}^{131}\text{I}$ isotope, with a half-life of eight days, undergoes beta decay and subsequently emits a gamma-ray photon that can be detected.

The radioactive iodine concentrates inside the thyroid gland within a few hours. The doctor then monitors the gamma-ray photon emissions over the next few days to see how the iodine is being processed within the thyroid and how quickly it is eliminated from the body.

Other important radioactive tracers include the chromium isotope ${}^{51}\text{Cr}$, which is taken up by red blood cells and can be used to monitor blood flow, and the xenon isotope ${}^{133}\text{Xe}$, which is inhaled to reveal lung functioning. Radioactive tracers are non-invasive, meaning that the doctor can monitor the inside of the body without surgery.