Vicki L. arrived early at the nuclear medicine department of the health center. As she sat in an isolated cubicle, a doctor in full sterile dress approached with a small metal canister marked with numerous warnings. The doctor carefully unscrewed the top, inserted a straw, and watched as the young woman sipped the fluid within. It tasted like stale water, but was actually a solution containing a radioactive isotope, iodine-131.
Vicki's thyroid gland has been removed three months earlier, and this test was to determine whether any active thyroid tissue remained. the thyroid is the only part of the body to metabolize iodine, so if Vicki's body retained any of the radioactive drink, it would mean that some of her cancerous thyroid gland remained. By using a radioactive isotope, her physicians could detect iodine uptake using a scanning device called a scintillation counter.
The next day, Vicki returned for the scan, which showed that a small amount of thyroid tissue was indeed left and was functioning. This meant another treatment would be necessary. Vicki would drink more of the radioactive iodine, enough to destroy the remaining tissue. This time Vicki had her stale drink in an isolation room, which was lined with paper to keep her from contaminating the floor, walls, and furniture. The same physician administered the radioactive iodine. Vicki's physician had this job because his own thyroid had been removed many years earlier, and therefore the radiation couldn't harm him.
After two days in isolation, Vicki was sent home with a list of rather odd directions. She was to stay away from her children and pets, wash her clothing separately, use disposable utensils and plates, and flush the toilet three times each time she used it. These precautions would minimize her contaminating her family--mom was radioactive!
Iodine-131 is a medically useful radioactive isotope because it has a short half-life, a measurement of the time it takes for half of an amount of an isotope to decay to a nonradioactive form. The half-life of iodine-131 is 8.1 days. With the amount of radiation in Vicki's body dissipating by half every 8.1 days, after three months there would be hardly any left. Doctors hoped that along with the radioactive iodine would go the remaining unhealthy thyroid cells.
Isotopes of other elements have different half-lives. The half-life of iron-59 is 45.1 days; that of phosphorus-32 is 14.3 days; that of cobalt-60 is 5.26 years; and that of radium-226 is 1,620 years.
A form of thallium-201 with a half-life of 73.5 hours is commonly used to detect disorders in the blood vessels supplying the heart muscle or to locate regions of damaged heart tissue after a heart attack. Gallium-67, with a half-life of 78 hours, is used to detect and monitor the progress of certain cancers and inflammatory illnesses. These medical procedures involve injecting the isotope into the blood and following its path using detectors that record images on paper or film.
Radioactive isotopes are also used to assess kidney function, estimate the concentrations of hormones in body fluids, measure blood volume and red blood cell mass, and study changes in bone density. Cobalt-60 is a radioactive isotope used to treat some cancers. The cobalt emits radiation that damages cancer cells more readily than it does healthy cells.