Chap 20 - Applications of Radioactive Isotopes

Medical Therapy and Diagnosis Using Radioactive Isotopes

The use of radioactive isotopes has had a profound effect on the practice of medicine. Radioisotopes were first used in medicine in the treatment of cancer. This treatment is based on the fact that rapidly dividing cells, such as those in cancer, are more adversely affected by radiation from radioactive substances than are cells that divide more slowly. Radium-226 and its decay product radon-222 were used for cancer therapy a few years after the discovery of radioactivity. Today gamma radiation from cobalt-60 is more commonly used.

Cancer therapy is only one of the ways in which radioactive isotopes are used in medicine. The greatest advances in the use of radioactive isotopes have been in the diagnosis of disease. Radioactive isotopes are used for diagnosis in two ways. They are used to develop images of internal body organs so that their functioning can be examined. And they are used as tracers in the analysis of minute amounts of substances, such as a growth hormone in blood, to deduce possible disease conditions.


Figure 20.12 A technetium-99m generator. Left: A conceptual view of the generator. Molybdenum-99, in the form of MoO₄^2- ion adsorbed on alumina, decays to technetium-99m. The technetium is leached from the generator with a salt solution (saline charge) as TcO₄^-. Right: A technetium-99m generator, with lead case and vials of saline charge.

Technetium-99m is the radioactive isotope used most often to develop images (pictures) of internal body organs. It has a half-life of 6.02 h, decaying by gamma emission to technetium-99 in its nuclear ground state. The image is prepared by scanning part of the body for gamma rays with a scintillation detector. (Figure 20.1 shows the image of a person’s skeleton obtained with technetium-99m.)

The technetium isotope is produced in a special container, or generator, shown in Figure 20.12. The generator contains radioactive molybdate ion, MoO422, adsorbed on alumina granules. Radioactive molybdenum-99 is produced from uranium-235 nuclear fission products (see Section 20.7). This radioactive molybdenum, adsorbed on alumina, is placed in the generator and sent to the hospital. Pertechnetate ion is obtained when the molybdenum-99 nucleus in MoO422 decays. The nuclear equation is

⁹₄⁹₂Mo ^____ ⁹₄⁹₃^mTc + _-⁰₁e
Each day pertechnetate ion, TcO₄^-, is leached from the generator with a salt solution whose osmotic pressure is the same as that of blood. One use of this pertechnetate ion is in assessing the performance of a patient’s heart. The physician injects tin(II) ion into a vein and, a few minutes later, administers a similar injection of the pertechnetate ion. In the presence of tin(II) ion, the pertechnetate ion binds to the red blood cells. The heart then becomes visible in gamma-ray imaging equipment, because of the quantity of blood in the heart. Technetium-99m pyrophosphate (Tc₂P₂O₇) is another technetium species useful for gamma-ray imaging. This compound binds especially strongly to recently damaged heart muscle, so these gamma-ray images can be used to assess the extent of damage from a heart attack.

Thallium-201 is a radioisotope used to determine whether a person has heart disease (caused by narrowing of the arteries to the heart). This isotope decays by electron capture and emits x rays and gamma rays, which can be used to obtain images similar to those obtained from technetium-99m (Figure 20.13). Thallium-201 injected into the blood binds particularly strongly to heart muscle. Diagnosis of heart disease depends on the fact that only tissue that receives sufficient blood flow binds thallium-201. When someone exercises strenuously, some part of the person’s heart tissue may not receive sufficient blood because of narrowed arteries. These areas do not bind thallium-201 and show up on an image as dark spots.

Figure 20.13
Using thallium-201 to diagnose heart disease. Left: A patient is undergoing a heart scan using a portable thallium-201 scintillation counter (on the pivoted arm). Gamma-ray scintillations are counted and collected by a computer (in the foreground) and then presented on the screen as an image. Right: A series of cross-sectional images of a patient’s heart after exercise (labeled “stress”) and then some time afterward (labeled “rest”). By comparing the stress and rest images, a physician can see if there is impaired blood flow to an area of the heart (the area is dark in the stress image but light in the rest image) or if the heart muscle has been damaged through a heart attack (the area is dark in both stress and rest images).

More than a hundred different radioactive isotopes have been used in medicine. Besides technetium-99m and thallium-201, other examples include iodine-131, used to measure thyroid activity; phosphorus-32, used to locate tumors; and iron-59, used to measure the rate of formation of red blood cells.

Radioimmunoassay is a technique for analyzing blood and other body fluids for very small quantities of biologically active substances. The technique depends on the reversible binding of the substance to an antibody. Antibodies are produced in animals as protection against foreign substances. They protect by binding to the substance and countering its biological activity. Consider, for example, the analysis for insulin in a sample of blood from a patient. Before the analysis, a solution of insulin-binding antibodies has been prepared from laboratory animals. This solution is combined with insulin containing a radioactive isotope, in which the antibodies bind with radioactive insulin. Now the blood sample containing an unknown amount of insulin is added to the antibody–radioactive insulin mixture. The nonradioactive insulin replaces some of the radioactive insulin bound to the antibody. As a result, the antibody loses some of its radioactivity. The loss in radioactivity can be related to the amount of insulin in the blood sample.