How Are Radioisotopes Used?
A radioisotope may be used either as a source of radiation energy (energy is always released during decay), or as a tracer: an identifying and readily detectable marker material. The location of this material during a given treatment can be determined with a suitable instrument even though an unweighably small amount of it is present in a mixture with other materials. On the following pages we will discuss medical uses of individual radioisotopes—first those used as tracers and then those used for their energy. In general, tracers are used for analysis and diagnosis, and radiant-energy emitters are used for treatment (therapy).
Radioisotopes offer two advantages. First, they can be used in extremely small amounts. As little as one-billionth of a gram can be measured with suitable apparatus. Secondly, they can be directed to various definitely known parts of the body. For example, radioactive sodium iodide behaves in the body just the same as normal sodium iodide found in the iodized salt used in many homes. The iodine concentrates in the thyroid gland where it is converted to the hormone thyroxin. Other radioactive, or “tagged”, atoms can be routed to bone marrow, red blood cells, the liver, the kidneys, or made to remain in the blood stream, where they are measured using suitable instruments.[6]
Of the three types of radiation, alpha particles (helium nuclei) are of such low penetrating power that they cannot be used for measurement from outside the body. Beta particles (electrons) have a moderate penetrating power, therefore they produce useful therapeutic results in the vicinity of their release, and they can be detected by sensitive counting devices. Gamma rays are highly energetic, and they can be readily detected by counters—radiation measurement devices—used outside the body.
Relative penetration of alpha, beta, and gamma radiation.
For comparison, a sheet of paper stops alpha particles, a block of wood stops beta particles, and a thick concrete wall stops gamma rays.
In one way or another, the key to the usefulness of radioisotopes lies in the energy of the radiation. When radiation is used for treatment, the energy absorbed by the body is used either to destroy tissue, particularly cancer, or to suppress some function of the body. Properly calculated and applied doses of radiation can be used to produce the desired effect with minimum side reactions. Expressed in terms of the usual work or heat units, ergs or calories, the amount of energy associated with a radiation dose is small. The significance lies in the fact that this energy is released in such a way as to produce important changes in the molecular composition of individual cells within the body.