 Section 1 of Radioisotopes in Medicine. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Read Ballarian Walden. Radioisotopes in Medicine by Earl W. Feelen. The Understanding the Atoms series. Nuclear energy is playing a vital role in the life of every man, woman, and child in the United States today. In the years ahead, it will affect, increasingly, all the peoples of the Earth. It is essential that all Americans gain an understanding of this vital force. If they are to discharge thoughtfully, their responsibilities as citizens. And if they are to realize fully the myriad benefits that nuclear energy offers them. The United States Atomic Energy Commission provides this booklet to help you achieve such understanding. Edward J. Bruningkant, Director, Division of Technical Information. The cover. This multi-detector positron scanner is used to locate tumors. A radioisotope-labeled substance is injected into the body and subsequently concentrates in the tumor tissue. The radioisotope emits positrons that immediately decay and produce two gamma rays that travel in opposite directions. These rays are detected simultaneously on a pair of opposing detection crystals. And a line is established, along which the tumor is located. This method is one of many ways doctors use radioisotopes to combat disease. In this, as in many other procedures described in this booklet, the patient remains comfortable at all times. The author. Earl W. Feelen is Professor of Chemistry at Tusculum College, Greenville, Tennessee. From 1952 to 1965, he served as staff assistant in the laboratory director's office at Argonne National Laboratory, where his duties included editing the Argonne reviews and supplying information to students. For 22 years prior to moving to Argonne, he served as head of the chemistry department of the Valdosta State College in Georgia. He received his B.S. and Ph.D. degrees from Cornell University. End of section one. Section two of Radioisotopes in Medicine by Earl W. Feelen. The Slipperovac recording is in the public domain. Read by Laurie Ann Walden. 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 Conrad Röntgen, 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 École 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 the uranium material and 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 pitch blend a uranium ore small amounts of two new elements, polonium and 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, Rinkin 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. Footnote. The early dangers from use of X-rays due to incomplete understanding and inadequate shielding have now been eliminated. End footnote. 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. Footnote. Gamma rays are high-energy electromagnetic radiation. End footnote. 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 Frédéric Joliot, a French chemist who married Irène Curie, daughter of Pierre and Marie Curie. He discovered in 1934 that when aluminum was bombarded with alpha particles from a radioactive source, a mission of positrons, positive electrons, was induced. Footnote. Alpha particles are large, positively charged particles identical to helium nuclei. For definitions of unfamiliar words, see nuclear terms of brief glossary, a companion booklet in this series. End footnote. 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. Frédéric and Irène 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. Vandegraaff. 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 could also 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 3,000 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. Footnote. For detailed descriptions of these waves and particles, see Our Atomic World, a companion booklet in this series. In footnote. 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. Footnote. An equivalent statement is that nuclei of isotopes have the same number of protons but different numbers of neutrons. End footnote. 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 1,024, the original number. And the amount of radioactive material is so small, it is usually no longer significant. 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 unwaverly 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 1 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 bloodstream where they are measured using suitable instruments. Footnote. See appendix for a description of types of radiation detection instruments. In footnote. 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 radio isotopes 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 bloodstream, 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. Footnote. See Radioisotopes and Life Processes, another booklet in this series, for a discussion of one area of biomedical research. End footnote. End of section two. Section three of Radioisotopes in Medicine by Earl W. Philan. This librivox recording is in the public domain. Read by Laurie Ann Walden. Diagnosis, pinpointing disease. Mr. Peters, 35-year-old father of four and a resident of Chicago's northwest side, went to a Chicago hospital one winter day after persistent headaches had made his life miserable. Routine examinations showed nothing amiss, and his doctor ordered a brain scan in the hospital's Department of Nuclear Medicine. 30 minutes before scan time, Mr. Peters was given, by intravenous injection, a minute amount of radioactive technetium. This radiochemical had been structured so that if there were a tumor in his cranium, the radioisotopes would be attracted to it. Then he was positioned so an instrument called a scanner could pass close to his head. As the motor-driven scanner passed back and forth, it picked up the gamma rays being emitted by the radioactive technetium, much as a Geiger counter detects other radiation. These rays were recorded as black blocks on sensitized film inside the scanner. The result was a piece of exposed film that, when developed, bore an architectural likeness or image of Mr. Peters' cranium. Mr. Peters, who admitted to no pain or other adverse reaction from the scanning, was photographed by the scanner from the front and both sides. The procedure took less than an hour. The developed film showed that the technetium had concentrated in one spot, indicating definitely that a tumor was present. Comparison of front and side views made it possible to pinpoint the location exactly. Surgery followed to remove the tumor. Today, thanks to sound and early diagnosis, Mr. Peters is well and back on the job. His case is an example of how radioisotopes are used in hospitals and medical centers for diagnosis. In one representative hospital, 17 different kinds of radioisotope measurements are available to aid physicians in making their diagnoses. All the methods use tracer quantities of materials. Other hospitals may use only a few of them. Some may use even more. In any case, they are merely tools to augment the doctor's skill. Examples of measurements that can be made include blood volume, blood circulation rate, red blood cell turnover, glandular activity, location of cancerous tissue, and rates of formation of bone tissue or blood cells. Of the more than 100 different radioisotopes that have been used by doctors during the past 30 years, five have received by far the greatest attention. These are iodine-131, phosphorus-32, gold-198, chromium-51, and iron-59. Some others have important uses too, but have been less wildly employed than these five. The use of individual radioisotopes in making important diagnostic tests makes a fascinating story. Typical instances will be described in the following pages. Arsenic-74. Brain tumors tend to concentrate certain ions, charged atoms, or molecules. When these ions are gamma-ray emitters, it is possible to take advantage of the penetrating power of their gamma rays to locate the tumor with a scanning device located outside the skull. Arsenic-74 and Copper-64 are isotopes emitting positrons, which have one peculiar property. Footnote. Opositron is an anti-electron. It has the mass of an electron, but a positive charge. End footnote. Immediately after opositron is emitted from a nucleus, it decays, producing two gamma rays that travel in exactly opposite directions. The scanning device has two detectors called scintillation counters, one mounted on each side of the patient's head. The electrical circuitry in the scanner is such that only those gamma rays are counted that impinge simultaneously on both counters. This procedure eliminates most of the noise or scattered and background radiation. Chromium-51. Because chromium in the molecule sodium chromate attaches itself to red blood cells, it is useful in several kinds of tests. The procedures are slightly complicated, but yield useful information. In one, a sample of the patient's blood is withdrawn, stabilized with heparin to prevent clotting, and incubated with a tracer of radioactive sodium chromate. Excess chromate that is not taken up by the cells is reduced and washed away. Then the radioactivity of the cell is measured just before injection into the patient. After a suitable time to permit thorough mixing of the added material throughout the bloodstream, a new blood sample is taken and its radioactivity is measured. The total volume of red blood cells can then be calculated by dividing the total radioactivity of the injective sample by the activity per milliliter of the second sample. In certain types of anemia, the patient's red blood cells die before completing the usual red cell lifetime of about 120 days. To diagnose this, red cells are tagged with chromium-51 in the manner just described. Then some of them are injected back into the patient and an identical sample is injected into a compatible normal individual. If the tracer shows that the cell survival time is too short in both recipients to the same degree, the conclusion is that the red cells themselves must be abnormal. On the other hand, if the cell survival time is normal in the normal individual and too short in the patient, the diagnosis is that the patient's blood contains some substance that destroys the red cells. When chromium trichloride, CRCL3, is used as the tagging agent, the chromium is bound almost exclusively to plasma proteins rather than the red cells. Chromium-51 may thus be used for estimating the volume of plasma circulating in the heart and blood vessels. The same type of computation is carried on for red cells after correction for a small amount of chromium taken up by the red blood cells. This procedure is easy to carry out because the radioactive chromium chloride is injected directly into a vein. An ingenious automatic device has been devised for computing a patient's total blood volume using the chromium-51 measurement of the red blood cell volume as its basis. This determination of total blood volume is, of course, necessary in deciding whether blood or plasma transfusions are needed in cases involving bleeding, burns, or surgical shock. This chromium-51 procedure was used during the Korean War to determine how much blood had been lost by wounded patients and helped to save many, many lives. For several years, iodine-131 has been used as a tracer in determining cardiac output, which is the rate of blood flow from the heart. It has appeared recently that red blood cells tagged with chromium-51 are more satisfactory for this measurement than iodine-labeled albumin in the blood serum. It is obvious that the blood flow rate is an extremely important physiological quantity and a doctor must know it to treat either heart ailments or circulatory disturbances. In contrast to the iodine-131 procedure, which requires that an artery be punctured and blood samples be removed regularly for measurement, chromium labeling merely requires that a radiation counter be mounted on the outside of the chest over the aorta, main artery leaving the heart. A sample of labeled red blood cells is introduced into a vein and the recording device counts the radioactivity appearing in the aorta as a function of time. Eventually, of course, the counting rate, the number of radioactive disintegrations per second levels off when the indicator sample has become mixed uniformly in the bloodstream. From the shape of the curve on which the data are recorded during the measurements taken before that time, the operator calculates the heart output per second. Obstetricians caring for expectant mothers use red cells tagged with chromium-51 to find the exact location of the placenta. For example, in the condition known as placenta previa, the placenta, the organ within the uterus by which nourishment is transferred from the mother's blood to that of the unborn child, may be placed in such a position that fatal bleeding can occur. A radiation counting instrument placed over the lower abdomen gives information about the exact location of the placenta. If an abnormal situation exists, the attending physician is then alert and ready to cope with it. The advantages of chromium over iodine-131, which has also been used, are that smaller doses are required and that there is no transfer of radioactivity to the fetal circulation. Still another common measurement using chromium-51 labeled red blood cells is the determination of the amount and location of bleeding from the gastrointestinal tract, the stomach and bowels. The amount is found by simple measurement of chromium in the blood that appears in the stools. To find the location is slightly more complicated. The intestinal contents are sampled at different levels through an inserted tube and the radiation of the samples determined separately. Finally, gastrointestinal loss of protein can be measured with the aid of chromium-51 labeled blood serum. The serum is treated with chromium trichloride and then injected into a vein. In several very serious ailments, there is serious loss of blood protein through the intestines. In these conditions, the chromium-51 level in the intestinal excretions is high and this alerts the doctor to apply remedial measures. Cobalt-60. Vitamin B12 is a cobalt compound. Normally, the few milligrams of B12 in the body are stored in the liver and released to the bloodstream as needed. In pernicious anemia, a potentially fatal but curable disease, the B12 content of the blood falls from the usual level of 300 to 900 micro micrograms per milliliter to zero to 100 micro micrograms per milliliter. The administration of massive doses of B12 is the only known remedy for this condition. If the B12 is labeled with radioactive cobalt, its passage into the bloodstream may be observed by several different methods. The simplest is to give the B12 by mouth and after about eight hours study the level of cobalt radioactivity in the blood. Cobalt-60 has been used for several years but recently cobalt-58 has been found more satisfactory. It has a half-life of 72 days while cobalt-60 has a 5.3 year half-life. This reduces greatly the amount of radiation to the patient's liver by the retained radioactivity. Iodine-131. Like chromium-51, iodine is a versatile tracer element. It is used to determine blood volume, cardiac output, plasma volume, liver activity, fat metabolism, thyroid cancer metastases, brain tumors, and the size, shape, and activity of the thyroid gland. Because of its unique connection with the thyroid gland, iodine-131 is most valuable in measurements connected with that organ. Thyroxin, an iodine compound, is manufactured in the thyroid gland and transferred by the bloodstream to the body tissues. The thyroxin helps to govern the oxygen consumption of the body and therefore helps control its metabolism. Proper production of thyroxin is essential to the proper utilization of nutrients. Lowered metabolism means increased body weight. Lowered thyroid activity may mean expansion of the gland causing one form of goiter. Iodine-131 behaves in the body just as the natural non-radioactive isotope, Iodine-127 does. But the radioactivity permits observation from outside the body with some form of radiation counter. Iodine can exist in the body in many different chemical compounds. And the counter can tell where it is, but not in what form. Hence, chemical manipulation is necessary in applying this technique to different diagnostic procedures. The thyroid gland, which is located at the base of the neck, is very efficient in trapping inorganic iodide from the bloodstream, concentrating and storing the iodine-containing material and gradually releasing it to the bloodstream in the form of protein-bound iodine. One of the common diagnostic procedures for determining thyroid function, therefore, is to measure the percentage of an administered dose of Iodine-131 that is taken up by the gland. Usually, the patient is given a very small dose of radioactive sodium iodide solution to drink. And two hours later, the amount of iodine in the gland is determined by measuring the radiation coming from the neck area. In hyperthyroidism or high thyroid gland activity, the gland removes iodide ions from the bloodstream more rapidly than normal. This simple procedure has been used widely. One difficulty in using it is that its success is dependent upon the time interval between injection and measurement. An overactive gland both concentrates iodine rapidly and also discharges it back to the bloodstream as protein-bound iodine more rapidly than normal. Modifications of the tests have been made to compare the amount of Iodine-131 that was administered with the amount circulating in the blood as protein-bound iodine. The system acquires chemical separation of the two forms of iodine from a sample of blood removed from a vein, followed by separate counting. This computation of the conversion ratio of radioactive plasma protein-bound iodine to plasma total iodine-131 gives results that are less subject to misinterpretation. To determine local activity in small portions of the thyroid, an automatic scanner is used. A collimator shields the detector, a Geiger-Mueller tube, or scintillating crystal so that only those impulses originating within a very small area are accepted by the instrument. Footnote. A collimator is a focusing device consisting of a series of slits between blocks of shielding material. Consult the appendix for descriptions of other instruments mentioned here. End footnote. The detector is then moved back and forth slowly over the entire area and the radiation is automatically recorded at definite intervals, creating a map of the active area. In cases where lops or nodules have been discovered in the thyroid, the map is quite helpful in distinguishing between cancerous and benign nodules. The former are almost always less radioactive than surrounding tissues. Fragments of cancerous thyroid tissue may migrate to other parts of the body and grow there. These new cancers are known as metastatic cancers and are a signal of an advanced state of disease. In such a situation, even complete surgical removal of the original cancer may not save the patient. If these metastases are capable of concentrating iodine, less than 10% of them are, they can be located by scanning the whole body in the manner that was just described. When a thyroid cancer is discovered, therefore, a doctor may look for metastases before deciding to operate. Human blood serum albumin, labeled with iodine 131, is used for measurement of the volume of circulating plasma. The procedure is quite similar to that used with radioactive chromium. Iodinated human serum albumin, labeled with iodine 131, is injected into a vein. Then, after allowing time for complete mixing of the sample with the blood, a second sample is counted using a scintillation counter. For many years, a dye known as rose bingle has been used in testing liver function. About 10 years ago, this procedure was improved by labeling the dye with iodine 131. When this dye is injected into a vein, it goes to the liver, which removes it from the bloodstream and transfers it to the intestines to be excreted. The rate of disappearance of the dye from the bloodstream is therefore a measure of liver activity. Immediately after administration of the radioactive dye, counts are recorded, preferably continuously from several sites with shielded, collimated detectors. One counter is placed over the side of the head, or the thigh, to record the clearance of the dye from the bloodstream. A second is placed over the liver, and a third over the abdomen to record the passage of the dye into the small intestine. Human serum albumin, labeled with iodine 131, is sometimes used for location of brain tumors. It appears that tumors alter a normal barrier between the brain and blood in such a manner that the labeled albumin can penetrate tumorous tissues, although it would be excluded from healthy brain tissue. The brain behaves almost uniquely among body tissues in that a blood-brain barrier exists so that substances injected into the bloodstream will not pass into brain cells, although they will pass readily into muscular tissue. This blood-brain barrier does not exist in brain tumors. A systematic scanning of the skull then permits location of these cancerous hotspots. Iron 59. Iron is a necessary constituent of red blood cells, so its radioactive form, iron 59, has been used frequently in measurement of the rate of formation of red cells, the lifetime of red cells, and red cell volumes. The labeling is more difficult than labeling with chromium for the same purposes, so this procedure no longer has the importance it wants add. On the other hand, direct measurement of absorption of iron by the digestive tract can be accomplished only by using iron 59. In aclorehydria, the gastric juice in the stomach is deficient in hydrochloric acid, and this condition has been shown to lower the iron absorption. A normal diet contains much more iron than the body needs, but in special cases, sometimes called tired blood in advertising for medicines, iron compounds are prescribed for the patient. If iron 59 is included, its appearance in the bloodstream can be monitored and the effectiveness of the medication noted. Phosphorus 32. The phosphate ion is a normal constituent of the blood. In many kinds of tumors, phosphates seem to be present in the cancerous tissue and a concentration several times that of the surrounding healthy tissue. This offers a way of using phosphorus 32 to distinguish between cancer cells and their neighbors. Due to the fact that phosphorus 32 gives off beta rays but no gammas, the counter must be placed very close to the suspected tissue since beta particles have very little penetrating power. This fact limits the use of the test to skin cancers or to cancers exposed by surgery. Some kinds of brain tumors, for instance, are difficult to distinguish visually from the healthy brain tissue. In such cases, the patient may be given phosphorus 32 labeled phosphate intravenously some hours before surgery. A tiny beta-sensitive probe counter can then be moved about within the operative site to indicate to the surgeon the limits of the cancerous area. Sodium 24. Normal blood is about 1% sodium chloride or ordinary salt. This fact makes possible the use of sodium 24 in some measurements of the blood and other fluids. The figure illustrates this technique. A sample of sodium 24 chloride solution is injected into a vein in an arm or leg. The time the radioisotope arrives at another part of the body is detected with a shielded radiation counter. The elapsed time is a good indication of the presence or absence of constrictions or obstructions in the circulatory system. The passage of blood through the heart may also be measured with the aid of sodium 24. Since this isotope emits gamma rays, measurement is done using counters on the outside of the body, placed at appropriate locations above the different sections of the heart. Technetium 99M. Because of its short half-life of six hours, Technetium 99M is coming into use for diagnosis using scanning devices, particularly for brain tumors. Footnote. The superscript M after this isotope indicates an excited state of the atom. End footnote. It lasts such a short time it obviously cannot be kept in stock, so it is prepared by the baited decay of molybdenum 99. Footnote. As radioactive nuclei disintegrate, they change to other radioactive forms, their daughter products. Every radioisotope is thus part of a chain or a series of steps that ends with a stable form. Technetium 99M is a daughter product of molybdenum 99. It decays by a process known as isomeric transition to a state of lower energy and longer half-life. End footnote. A stock of molybdenum is kept in a shielded container in which it undergoes radioactive decay yielding technetium. Every morning, as the technetium is needed, it is extracted from its parent by a brine solution. This general procedure of extracting a short-lived isotope from its parent is also used in other cases. We shall see later that radon gas is obtained by an analogous method from its parent, radium. Thulium 170 and gamma radiography. For years it has been recognized that there would be many uses for a truly portable device for taking x-ray pictures, one that could be carried by the doctor to the bedside or to the scene of an accident. Conventional x-ray equipment has been in use by doctors for many years and highly efficient apparatus has become indispensable, especially in treating bone conditions. There is, however, a need for a means of examining patients who cannot be moved to a hospital x-ray room and are located where electric current sources are not available. A few years ago, a unit was devised that weighed only a few pounds and could take x-ray pictures, actually gamma radiographs, using the gamma rays from the radioisotope Thulium 170. The Thulium source is kept inside a lead shield, but a photographic shutter release cable can be pressed to move it momentarily over an open port in the shielding. The picture is taken with an exposure of a few seconds. A somewhat similar device uses Strontium-90 as the source of beta radiation that in turn stimulates the emission of gamma rays from a target within the instrument. Still more recently, Iodine 125 has been used very successfully in a portable device as a low-energy gamma source for radiography. The gamma rays from this source are sufficiently penetrating for photographing the arms and legs, and the necessary shielding is easily supplied to protect the operator. By contrast with larger devices, the gamma ray source can be as small as one-tenth millimeter in diameter, virtually a point source. This makes possible maximum sharpness of image. The latest device, using up to one curie of Iodine 125, weighs two pounds, yet has adequate shielding for the operator. It is truly portable. Footnote, the curie is the basic unit of radiation intensity. One curie is approximately the amount of radioactivity in one gram of radium. In footnote. If this X-ray source is combined with a rapid-developing photographic film, a physician can be completely freed from dependence upon the hospital laboratory for emergency X-rays. A finished print can be ready for inspection in 10 seconds. The doctor thus can decide quickly whether it is safe to move an accident victim, for instance. In military operations, similarly, it becomes a simple matter to examine wounded soldiers in the field where conventional equipment is not available. Tritium. More than 30 years ago, when deuterium, heavy hydrogen, was first discovered, heavy water, D2O, was used for the determination of total body water. A small sample of heavy water was given, either intravenously or orally, and time was allowed for it to mix uniformly with all the water in the body, about four to six hours. A sample was then obtained of the mixed water and analyzed for its heavy water content. This procedure was useful, but it was hard to make an accurate analysis of low concentrations of heavy water. More recently, however, tritium, radioactive hydrogen, has been produced in abundance. Its oxide, tritiated water, is chemically almost the same as ordinary water, but physically it may be distinguished by the beta rays given off by the tritium. This very soft, low energy, beta ray, requires the use of special counting equipment, either a windowless, flow gas counter or a liquid scintillator, but with the proper techniques, accurate measurement is possible. The total body water can then be computed by the general isotope dilution formula used for measuring blood plasma volume. Activation analysis. Another booklet in this series, Neutron Activation Analysis, discusses a new process by which microscopic quantities of many different materials may be analyzed accurately. Neutron irradiation of these samples changes some of their atoms to radioactive isotopes. A multi-channel analyzer instrument gives a record of the concentration of any of about 50 of the known elements. One use of this technique involved the analysis of a hair from Napoleon's head. More than 100 years after his death, it was shown that the French emperor had been given arsenic in large quantities and that this possibly cost his death. The ways in which activation analysis can be applied to medical diagnosis are at present largely limited to toxicology, the study of poisons, but the future may bring new possibilities. Knowledge is still being sought, for example, about the physiological role played by minute quantities of some of the elements found in the body. The ability to determine accurately a few parts per million of trace elements in the various tissues and body fluids is expected to provide much useful information as to the functions of these materials. Summary. A large number of different radioisotopes have been used for measurement of disease conditions in the human body. They may measure liquid volumes, rates of flow or rates of transfer through organs or membranes. They may show the behavior of internal organs. They may differentiate between normal and malignant tissues. Hundreds of hospitals are now making thousands of these tests annually. This does not mean that all the diagnostic problems have been solved. Much of the work is on an experimental rather than a routine basis. Improvements and techniques are still being made. As quantities of radioisotopes available for these purposes grow and as the cost continues to drop, it is expected there will be still more applications. Finally, this does not mean we no longer need the doctor's diagnostic skill. All radioisotope procedures are merely tools to aid the skilled physician. As the practice of medicine has changed from an art to a science, radioisotopes have played a useful part. End of section three. Section four of radioisotopes in medicine by Earl W. Fielin. The Slippervocht recording is in the public domain. Read by Laurie Ann Walden. Therapy, a successful case. A doctor recently told this story about a cancer patient who was cured by irradiation with cobalt 60. A 75-year-old white male patient who had been hoarse for one month was treated unsuccessfully with the usual medications given for a bad cold. Finally, examination of his larynx revealed an ulcerated swelling on the right vocal cord. A biopsy, microscopic examination of a tissue sample, was made and it was found the swelling was a squamous cell cancer. Daily radiation treatment using a cobalt 60 device was started and continued for 31 days. This was in September, 1959. The cobalt 60 unit is one that can be operated by remote control. It positions radioactive cobalt over a collimator, which determines the size of the radiation beam reaching the patient. The machine may be made to rotate around the patient or can be used at any desired angle or position. When the treatment series was in progress, the patient's voice was temporarily made worse but it returned to normal within two months after the treatment ended. The radiation destroyed the cancerous growth and frequent examinations over six years since have failed to reveal any regrowth. The treatment spared the patient's vocal cords and his voice, airway and food passage were preserved. This dramatic tale with a happy ending is a good one with which to start a discussion of how doctors use radioisotopes for treatment of disease. General Principles. Radioisotopes have an important role in the treatment of disease, particularly cancer. It is still believed that cancer is not one but several diseases with possible multiple causes. Great progress is being made in development of chemicals for relief of cancer. Nevertheless, radiation and surgery are still the main methods for treating cancer and there are many conditions in which relief can be obtained through use of radiation. Moreover, the imaginative use of radioisotopes gives much greater flexibility in radiation therapy. This is expected to be true for some years to come even as progress continues. Radioisotopes serve as concentrated sources of radiation and frequently are localized within the diseased cells or organs. The dose can be computed to yield the maximum therapeutic effect without harming adjacent healthy tissues. Let us see some of the ways in which this is done. Iodine-131 and Iodine-132. Iodine, as was mentioned earlier, concentrates in the thyroid gland and is converted there to protein-bound iodine that is slowly released to the bloodstream. Iodine-131 and concentrations much higher than those used in diagnostic tests will irradiate thyroid cells, thereby damage them, and reduce the activity of an overactive thyroid, hyperthyroidism. The energy is released within the affected gland and much of it is absorbed there. Iodine-131 has a half-life of 8.1 days. In contrast, Iodine-132 has a half-life of only 2.33 hours. What this means is that the same weight of radioactive Iodine-132 will give a greater radiation dose than Iodine-131 would and lose its activity rapidly enough to present much less hazard by the time the Iodine is released to the bloodstream. Iodine-132 is therefore often preferred for treatment of this sort. Boron-10. Boron-10 has been used experimentally in the treatment of inoperable brain tumors. Glioblastoma multiforme, a particularly malignant form of cancer, is an invariably fatal disease in which the patient has a probable life expectancy of only one year. The tumor extends roots into normal tissues to such an extent that it is virtually impossible for the surgeon to remove all malignant tissue, even if he removes enough normal brain to affect the functioning of the patient seriously. With or without operation, the patient dies within months. This is therefore a case in which any improvement at all is significantly helpful. The blood-brain barrier that was mentioned earlier minimizes the passages of many materials into normal brain tissues. But when some organic or inorganic compounds, such as the boron compounds, are injected into the bloodstream, they will pass readily into brain tumors and not move into normal brain cells. Boron-10 absorbs slow-neutrons readily and becomes Boron-11, which disintegrates almost immediately into alpha particles and a lithium isotope. Alpha particles, remember, have very little penetrating power, so all the energy of the alpha radioactivity is expended within the individual tumor cells. This is an ideal situation, for it makes possible destruction of tumor cells with virtually no harm to normal cells, even when the two kinds are closely intermingled. Slow-neutrons pass through the human body with very little damage, so a fairly strong dose of them can be safely applied to the head. Many of them will be absorbed by the boron-10 and maximum destruction of the cancer will occur, along with minimum hazard to the patient. This treatment is accomplished by placing the head of the patient in a beam of slow-neutrons emerging from a nuclear reactor a few minutes after the boron-10 compound has been injected into a vein. Figure. Sequence of events in neutron capture therapy using boron-10. Neutron capture treatment of a brain tumor using the Brookhaven National Laboratory Research Reactor Center. One. A lead shutter shields the patient from reactor neutrons. Two. A compound containing the stable element boron is injected into the bloodstream. The tumor absorbs most of the boron. Three. After eight minutes when the tumor is saturated the shutter is removed and neutrons bombard the brain, splitting boron atoms so that fragments destroy tumor tissue. Four. Twenty minutes later the shutter is closed and the treatment ends. End Figure. The difficulty is that most boron compounds themselves are poisonous to human tissues and only small concentrations can be tolerated in the blood. Efforts have been made with some success to synthesize new boron compounds that have the greatest possible degree of selective absorption by the tumors. Both organic and inorganic compounds have been tried and the degree of selectivity has been shown to be much greater for some than for others. So far it is too early to say that any cures have been brought about but results have been very encouraging. The ideal drug, one which will make possible complete destruction of the cancer without harming the patient, is probably still to be devised. Phosphorus 32. Another disease which is peculiarly open to attack by radioisotopes is polycythemia vira. This is an insidious ailment of a chronic, slowly progressive nature characterized by an abnormal increase in the number of red blood cells an increase in total blood volume, enlargement of the spleen and a tendency for bleeding to occur. There is some indication that it may be related to leukemia. Until recent years, there was no very satisfactory treatment of this malady. The ancient practice of bleeding was as useful as anything, giving temporary relief but not striking at the underlying cause. There is still no true cure but the use of phosphorus 32 is very effective in causing disappearance of symptoms for periods from months to years, lengthening the patient's life considerably. The purpose of the phosphorus 32 treatment, using a sodium radiophosphate solution, is not to destroy the excess of red cells as had been tried with some drugs but rather to slow down their formation and thereby get at the basic cause. Phosphorus 32 emits pure beta rays having an average path in tissue only two millimeters long. Its half-life is 14.3 days. When it is given intravenously, it mixes rapidly with the circulating blood and slowly accumulates in tissues that utilize phosphates in their metabolism. This brings appreciable concentration in the blood-forming tissues about twice as much in blood cells as in general body cells. One other pertinent fact is rapidly dividing hematopoietic cells are extremely sensitive to radiation. Hematopoietic cells are those that are actively forming blood cells and are therefore those that should be attacked selectively. The dose required is, of course, many times that needed for diagnostic studies and careful observation of the results is necessary to determine that exactly the desired effect has been obtained. There exists some controversy over this course of treatment. No one denies that the lives of patients have been lengthened notably. Nevertheless, since the purpose of the procedure is to reduce red cell formation, there exists the hazard of too great a reduction and the possibility of causing leukemia, a disease of too few red cells. There may be a small increase in the number of cases of leukemia, among those treated with phosphorus 32, compared with the general population. The controversy arises over whether the phosphorus 32 treatment caused the leukemia or whether it merely prolonged the lives of the patients until leukemia appeared, as it would have in these persons even without treatment. This is probably quibbling and many doctors believe that the slight unproven risk is worth taking to produce the admitted lengthy freedom from symptoms. Gold 198. The last ailment we shall discuss in this section is the accumulation of large quantities of excess fluid in the chest and abdominal cavities from their linings as a consequence of the growth of certain types of malignant tumors. Frequent surgical drainage was at one time the only very useful treatment. And of course this was both uncomfortable and dangerous. The use of radioactive colloidal suspensions, primarily colloidal Gold 198, has been quite successful in palliative treatment. It does not cure, but it does give marked relief. Radioactive colloids, a colloid is a suspension of one very finely divided substance in some other medium, can be introduced into the abdominal cavity, where they may remain suspended or settle out upon aligning. In either case, since they are not dissolved, they do not pass through the membranes or cell walls, but remain within the cavity. With a starting effect on the cancer cells, the radiation inhibits the oozing of fluids. Gold 198 offers several advantages in such cases. It has a short half-life, 2.7 days. It is chemically inert, and therefore non-toxic. And it emits beta and gamma radiation that is almost entirely absorbed by the tissues in its immediate neighborhood. The results have been very encouraging. There is admittedly no evidence of any cures, or even lengthening of life. But there has been marked reduction of discomfort and control of the oozing and over two-thirds of the cases treated. Beads, needles, and applicators. Radium salts were the first materials to be used for radiation treatment of cancer. Being both very expensive and very long-lived, they could not be injected, but were used in temporary implants. Radium salts in the body, radium salts in powder form were packed into tiny hollow needles about 1 centimeter long, which were then sealed tightly to prevent the escape of radon gas. As radium decays, half-life 1,620 years, it becomes gaseous radon. The latter is also radioactive, so it must be prevented from escaping. These gold needles could be inserted into tumors and left there until the desired dosage had been administered. One difficulty in radium treatment was that the needles were so tiny that on numerous occasions they were lost, having been thrown out with the dressings. Then, both because of their value and their hazard, a frantic search ensued when this happened, not always ending successfully. The fact that radon, the daughter of radium, is constantly produced from its parent, helped to eliminate some of this difficulty. Radium could be kept in solution, decaying constantly to yield radon. The latter, with a half-life of 4 days, could be sealed into gold seeds 3 by 0.5 millimeters and left in the patient without much risk, even if he failed to return for its removal at exactly the appointed time. The cost was low even if the seeds were lost. During the last 20 years, other highly radioactive sources have been developed that have been used successfully. Cobalt-60 is one popular material. Cobalt-59 can be neutron irradiated in a reactor to yield Cobalt-60 with such a high specific activity that a small cylinder of it is more radioactive than the entire world's supply of radium. Cobalt-60 has been encapsulated in gold or silver needles, sometimes of special shapes for adaptation to specific tumors such as carcinoma of the cervix. Sometimes needles have been spaced at intervals on plastic ribbon that adapts itself readily to the shape of the organ treated. Gold 198 is also an interesting isotope. Since it is chemically inert in the body, it needs no protective coating and, as is the case with radon, its short half-life makes its use simpler and that the time of removal is not of critical importance. Ceramic beads made of yttrium nani oxide are a moderately new development. One very successful application of this material has been for the destruction of the pituitary gland. Cancer may be described as the runaway growth of cells. The secretions of the pituitary gland serve to stimulate cell reproduction. So it was reason that destruction of this gland might well slow down growth of the tumor elsewhere in the body. It was that the pituitary is small and located at the base of the brain. Surgical removal had brought dramatic relief, not cure to many patients, but the surgery itself was difficult and hazardous. Tiny yttrium nani oxide beads glass-like in nature can be implanted directly in the gland with much less difficulty and risk and do the work of destroying the gland with little damage to its surroundings. The key to the success of yttrium 90 is the fact that it is a beta emitter and beta rays have so little penetrating power that their effect is limited to the immediate area of the implant. Teletherapy Over 200 teletherapy units are now in use in the United States for treatment of patients by using very high intensity sources of cobalt 60, usually, or cesium 137. Units carrying sources with intensities of more than a thousand curies are common. Since a curie is the amount of radioactivity and a gram of radium that is in equilibrium with its decay products, a 1000 curie source is comparable to two pounds of pure radium. Neglecting for the moment the scarcity and enormous cost of that much radium millions of dollars, we have to consider that it would be large in volume and consequently difficult to apply. Radiation from such a quantity cannot be focused. Consequently, either much of it will fall upon healthy tissue surrounding the cancer, or much of it will be wasted if a narrow passage through the shield is aimed at the tumor. In contrast, a tiny cobalt source provides just as much radiation and more if it can be brought to bear upon the exact spot to be treated. Most interesting of all is the principle by which internal cancers can be treated with a minimum of damage to the skin. Deep X irradiation has always been the approved treatment for deep lying cancers. But until recently this required very cumbersome units. With the modern rotational device shown in the diagram, a very narrow beam is aimed at the patient while the source is mounted upon a carrier that revolves completely around him. The patient is positioned carefully so that the lesion to be treated is exactly at the center of the circular path of the carrier. The result is that the beam strikes its internal target during the entire circular orbit. But the same amount of radiation is spread out over a belt of skin and tissue all the way around the patient. The damage to any one skin cell is minimized. The advantage of this device over an earlier device in which the patient was revolved in a stationary beam is that the mechanical equipment is much simpler. End of section four. Section five of Radioisotopes in Medicine by Earl W. Feelin. This LibriVox recording is in the public domain. Read by Larry Ann Walden. Conclusions. In summary then, we may say that Radioisotopes play an important role in medicine. For the diagnostician, small harmless quantities of many isotopes serve as tools to aid him in gaining information about normal and abnormal life processes. The usefulness of this information depends upon his ingenuity in devising questions to be answered, apparatus to measure the results and explanations for the results. For therapeutic uses, on the other hand, the important thing to remember is that radiation damages many kinds of cells, especially while they are in the process of division reproduction. Footnote. See your body in radiation and the genetic effects of radiation, other booklets in this series, for detailed explanations of radiation effects. End footnote. Cancer cells are self-reproducing cells, but do so in an uncontrolled manner. Hence cancer cells are particularly vulnerable to radiation. This treatment requires potent sources and correspondingly increases the hazards of use. In all cases, the use of these potentially hazardous materials belongs under the supervision of the U.S. Atomic Energy Commission. Footnote. The use of radium is not under AEC control. End footnote. Licenses are issued by the commission after investigation of the training, ability and facilities possessed by prospective users of dangerous quantities. At regular intervals, courses are given to trained individuals in the techniques necessary for safe handling. And graduates of these courses are now located in laboratories all over the country. The future of this field cannot be predicted with certainty. Research in hundreds of laboratories is continuing to add to our knowledge through new apparatus, new techniques and new experiments. Necessarily, the number of new fields is becoming smaller. But most certainly, the number of cases using procedures already established is bound to increase. We foresee steady improvement and growth in all uses of radioisotopes in medicine. Appendix. Measuring instruments. Footnote. One family of measuring instruments is described in whole body counters, another booklet in this series. These are large devices that make use of the existing crystals or liquids. In footnote. The measurement of radioactivity must be accomplished indirectly, so use is made of the physical, chemical and electrical effects of radiation on materials. One commonly used effect is that of ionization. Alpha and beta particles ionize gases through which they pass, thereby making the gases electrically conductive. A family of counters uses this principle, the ionization chamber, the proportional counter, and the Geiger-Mueller counter. Certain crystals, sodium iodide being an excellent example, emit flashes of visible light when struck by ionizing radiation. These crystals are used in scintillation counters. Ionization chambers. One of a pair of electrodes is a wire located centrally within a cylinder. The other electrode is the wall of the chamber. Radiation ionizes the gas within the chamber, permitting the passage of current between the electrodes. The thickness of a window in the chamber wall determines the type of radiation it can measure. Only gamma rays will pass through a heavy metal wall. Glass windows will admit all gamma and most betas. And plastic, mylar windows are necessary to admit alpha particles. Counters of this type, when properly calibrated, will measure the total amount of radiation received by the body of the wearer. Proportional counters. This is a type of ionization chamber in which the intensity of the electrical pulse it produces is proportional to the energy of the incoming particle. This makes it possible to record alpha particles and discriminate against gamma rays. Geiger-Mueller counters. These have been widely used and are versatile in their applications. The potential difference between the electrodes in the Geiger-Mueller tube, similar to an ionization chamber, is high. A single alpha or beta particle ionizes some of the gas within the chamber. In turn, these ions strike other gas molecules, producing secondary ionization. The result is an avalanche or high-intensity pulse of electricity passing between the electrodes. These pulses can be counted electrically and recorded on a meter at rates up to several thousand per minute. Cintillation counters. Since the development of the photoelectric tube and the photomultiplier tube, a combination of photoelectric cell and amplifier, the cintillation counter has become the most popular instrument for most purposes described in this booklet. The flash of light produced when an individual ionizing particle or ray strikes a sodium iodide crystal is noted by a photoelectric cell. The intensity of the flash is a measure of the energy of the radiation, so the voltage of the output of the photomultiplier tube is a measure of the wavelength of the original gamma ray. The cintillation counter can observe up to a million counts per minute and discriminate sharply between gamma rays of different energies. With proper windows, it can be used for alpha or beta counts as well. Solid state counters. The latest development is a tiny silicon transistor type diode detector that can be made as small as a grain of sand and placed within the body with very little discomfort. Scanners. Many of the applications described in this booklet require accurate knowledge of the exact location of the radioactive source within the body. Commonly a detecting tube is used, having a collimating shield so that it accepts only that radiation that strikes it head-on. A motor-driven carrier moves the counter linearly at a slow rate. Radiation is counted, and whenever the count reaches the predetermined amount from one count to many, an electric impulse causes a synchronously moving pin to make a dot on a chart. The scanner, upon reaching the end of a line, moves down to the next line and starts over, eventually producing a complete record of the radiation sources it has passed over. End of section 5 End of Radioisotopes in Medicine by Earl W. Feelen