Radioisotopes in Medicine is an educational booklet published in 1966 as part of the Understanding the Atom series by the United States Atomic Energy Commission. Written in clear language for the general public, the booklet covers the diagnostic and therapeutic uses of radioactive isotopes like technetium 99m and iodine 131.
By : Earl W. Phelan (1900 - ) and United States Atomic Energy Commission
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Introduction
History
The history of the use of radioisotopes for medical purposes is filled with names of Nobel Prize winners. It is inspiring to read how great minds attacked puzzling phenomena, worked out the theoretical and practical implications of what they observed, and were rewarded by the highest honor in science.
For example, in 1895 a German physicist, Wilhelm Konrad Roentgen, noticed that certain crystals became luminescent when they were in the vicinity of a highly evacuated electric-discharge tube. Objects placed between the tube and the crystals screened out some of the invisible radiation that caused this effect, and he observed that the greater the density of the object so placed, the greater the screening effect. He called this new radiation X rays, because x was the standard algebraic symbol for an unknown quantity. His discovery won him the first Nobel Prize in physics in 1901.
A French physicist, Antoine Henri Becquerel, newly appointed to the chair of physics at the Ecole Polytechnique in Paris, saw that this discovery opened up a new field for research and set to work on some of its ramifications. One of the evident features of the production of X rays was the fact that while they were being created, the glass of the vacuum tube gave off a greenish phosphorescent glow. This suggested to several physicists that substances which become phosphorescent upon exposure to visible light might give off X rays along with the phosphorescence.
Becquerel experimented with this by exposing various crystals to sunlight and then placing each of them on a black paper envelope enclosing an unexposed photographic plate. If any X rays were thus produced, he reasoned, they would penetrate the wrapping and create a developable spot of exposure on the plate. To his delight, he indeed observed just this effect when he used a phosphorescent material, uranium potassium sulfate. Then he made a confusing discovery. For several days there was no sunshine, so he could not expose the phosphorescent material. For no particular reason (other than that there was nothing else to do) Becquerel developed a plate that had been in contact with uranium material in a dark drawer, even though there had been no phosphorescence. The telltale black spot marking the position of the mineral nevertheless appeared on the developed plate! His conclusion was that uranium in its normal state gave off X rays or something similar.
At this point, Pierre Curie, a friend of Becquerel and also a professor of physics in Paris, suggested to one of his graduate students, his young bride, Marie, that she study this new phenomenon. She found that both uranium and thorium possessed this property of radioactivity, but also, surprisingly, that some uranium minerals were more radioactive than uranium itself. Through a tedious series of chemical separations, she obtained from pitchblende (a uranium ore) small amounts of two new elements, polonium 3and radium, and showed that they possessed far greater radioactivity than uranium itself. For this work Becquerel and the two Curies were jointly awarded the Nobel Prize in physics in 1903.
At the outset, Roentgen had noticed that although X rays passed through human tissue without causing any immediate sensation, they definitely affected the skin and underlying cells. Soon after exposure, it was evident that X rays could cause redness of the skin, blistering, and even ulceration, either in single doses or in repeated smaller doses. In spite of the hazards involved, early experimenters determined that X rays could destroy cancer tissues more rapidly than they affected healthy organs, so a basis was established quite soon for one of Medicine’s few methods of curing or at least restraining cancer.
The work of the Curies in turn stimulated many studies of the effect of radioactivity. It was not long before experimenters learned that naturally radioactive elements—like radium—were also useful in cancer therapy. These elements emitted gamma rays, which are like X rays but usually are even more penetrating, and their application often could be controlled better than X rays. Slowly, over the years, reliable methods were developed for treatment with these radioactive sources, and instruments were designed for measuring the quantity of radiation received by the patient.
The next momentous advance was made by Frederic Joliot, a French chemist who married Irene Curie, daughter of Pierre and Marie Curie. He discovered in 1934 that when aluminum was bombarded with alpha particles from a radioactive source, emission of positrons (positive electrons) was induced. Moreover, the emission continued long after the alpha source was removed. This was the first example of artificially induced radioactivity, and it stimulated a new flood of discoveries. Frederic and Irene Joliot-Curie won the Nobel Prize in chemistry in 1935 for this work.
Others who followed this discovery with the development of additional ways to create artificial radioactivity were two Americans, H. Richard Crane and C. C. Lauritsen, the British scientists, John Cockcroft and E. T. S. Walton, and an American, Robert J. Van de Graaff. Ernest O. Lawrence, an American physicist, invented the cyclotron (or “atom smasher”), a powerful source of high-energy particles that induced radioactivity in whatever target materials they impinged upon. Enrico Fermi, an Italian physicist, seized upon the idea of using the newly discovered neutron (an electrically neutral particle) and showed that bombardment with neutrons also could induce radioactivity in a target substance. Cockcroft and Walton, Lawrence, and Fermi all won Nobel Prizes for their work.
Patient application of these new sources of bombarding particles resulted in the creation of small quantities of hundreds of radioactive isotopic species, each with distinctive characteristics. In turn, as we shall see, many ways to use radioisotopes have been developed in medical therapy, diagnosis, and research. By now, more than 3000 hospitals hold licenses from the Atomic Energy Commission to use radioisotopes. In addition, many thousands of doctors, dentists, and hospitals have X-ray machines that they use for some of the same broad purposes. One of the results of all this is that every month new uses of radioisotopes are developed.
More persons are trained every year in methods of radioisotope use and more manufacturers are producing and packaging radioactive materials. This booklet tells some of the successes achieved with these materials for medical purposes.
What Is Radiation?
Radiation is the propagation of radiant energy in the form of waves or particles. It includes electromagnetic radiation ranging from radio waves, infrared heat waves, visible light, ultraviolet light, and X rays to gamma rays. It may also include beams of particles of which electrons, positrons, neutrons, protons, deuterons, and alpha particles are the best known.
What Is Radioactivity?
It took several years following the basic discovery by Becquerel, and the work of many investigators, to systematize the information about this phenomenon. Radioactivity is defined as the property, possessed by some materials, of spontaneously emitting alpha or beta particles or gamma rays as the unstable (or radioactive) nuclei of their atoms disintegrate.
What Are Radioisotopes?
In the 19th Century an Englishman, John Dalton, put forth his atomic theory, which stated that all atoms of the same element were exactly alike. This remained unchallenged for 100 years, until experiments by the British chemist, Frederick Soddy, proved conclusively that the element neon consisted of two different kinds of atoms. All were alike in chemical behavior but some had an atomic weight (their mass relative to other atoms) of 20 and some a weight of 22. He coined the word isotope to describe one of two or more atoms having the same atomic number but different atomic weights.
Radioisotopes are isotopes that are unstable, or radioactive, and give off radiation spontaneously. Many radioisotopes are produced by bombarding suitable targets with neutrons now readily available inside atomic reactors. Some of them, however, are more satisfactorily created by the action of protons, deuterons, or other subatomic particles that have been given high velocities in a cyclotron or similar accelerator.
Radioactivity is a process that is practically uninfluenced by any of the factors, such as temperature and pressure, that are used to control the rate of chemical reactions. The rate of radioactive decay appears to be affected only by the structure of the unstable (decaying) nucleus. Each radioisotope has its own half-life, which is the time it takes for one half the number of atoms present to decay. These half-lives vary from fractions of a second to millions of years, depending only upon the atom. We shall see that the half-life is one factor considered in choosing a particular isotope for certain uses.
Most artificially made radioisotopes have relatively short half-lives. This makes them useful in two ways. First, it means that very little material is needed to obtain a significant number of disintegrations. It should be evident that, with any given number of radioactive atoms, the number of disintegrations per second will be inversely proportional to the half-life. Second, by the time 10 half-lives have elapsed, the number of disintegrations per second will have dwindled to ¹/₁₀₂₄ the original number, and the amount of radioactive material is so small it is usually no longer significant. (Note the decrease in the figure above.)
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.
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.
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.
What Do We Mean by Tracer Atoms?
When a radioisotope is used as a tracer, the energy of the radiation triggers the counting device, and the exact amount of energy from each disintegrating atom is measured. This differentiates the substance being traced from other materials naturally present.
With one conspicuous exception, it is impossible for a chemist to distinguish any one atom of an element from another. Once ordinary salt gets into the blood stream, for example, it normally has no characteristic by which anyone can decide what its source was, or which sodium atoms were added to the blood and which were already present. The exception to this is the case in which some of the atoms are “tagged” by being made radioactive. Then the radioactive atoms are readily identified and their quantity can be measured with a counting device.
A radioactive tracer, it is apparent, corresponds in chemical nature and behavior to the thing it traces. It is a true part of it, and the body treats the tagged and untagged material in the same way. A molecule of hemoglobin carrying a radioactive iron atom is still hemoglobin, and the body processes affect it just as they do an untagged hemoglobin molecule. The difference is that a scientist can use counting devices to follow the tracer molecules wherever they go.
It should be evident that tracers used in diagnosis—to identify disease or improper body function—are present in such small quantities that they are relatively harmless. Their effects are analogous to those from the radiation that every one of us continually receives from natural sources within and without the body. Therapeutic doses—those given for medical treatment—by contrast, are given to patients with a disease that is in need of control, that is, the physician desires to destroy selectively cells or tissues that are abnormal. In these cases, therefore, the skill and experience of the attending physician must be applied to limit the effects to the desired benefits, without damage to healthy organs.
This booklet is devoted to these two functions of radioisotopes, diagnosis and therapy; the field of medical research using radioactive tools is so large that it requires separate coverage.
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