Iron-59

Time-lapse motion pictures of the liver of a 3-year-old girl were made with the scintillation camera 1 hour after injection of 50 microcuries of iodine-131-labeled rose bengal dye. This child was born without a bile-duct system and an artificial bile duct had been created surgically. She developed symptoms that caused concern that the duct had closed. These scans show the mass of material containing the radioactive material (small light area) moving downward and to the right, indicating that the duct was still open.

For many years, a dye known asrose bengalhas been used in testing liver function. About 10 years ago this procedure was improved by labeling the dye with ¹³¹I. When this dye is injected into a vein it goes to the liver, which removes it from the blood stream and transfers it to the intestines to be excreted. The rate of disappearance of the dye from the blood stream is therefore a measure of the 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 blood stream. 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 ¹³¹I is sometimes used for location of brain tumors. It appears that tumorsalter 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.

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The brain behaves almost uniquely among body tissues in that a “blood-brain barrier” exists, so that substances injected into the blood stream 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 “hot spots”.

Iron is a necessary constituent of red blood cells, so its radioactive form, ⁵⁹Fe, 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 once had.

On the other hand, direct measurement of absorption of iron by the digestive tract can be accomplished only by using ⁵⁹Fe. Inachlorhydriathe 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 specialcases, sometimes called “tired blood” in advertising for medicines, iron compounds are prescribed for the patient. If ⁵⁹Fe is included, its appearance in the blood stream can be monitored and the effectiveness of the medication noted.

This multiple-port scintillation counter is used for iron-kinetic studies. The tracer dose of iron-59 is administered into the arm vein and then the activities in the bone marrow, liver, and spleen are recorded simultaneously with counters positioned over these areas, and show distribution of iron-59 as a function of time. When the data are analyzed in conjunction with iron-59 content in blood, information can be obtained about sites of red blood cell production and destruction.

The phosphate ion is a normal constituent of the blood. In many kinds of tumors, phosphates seem to be present in the cancerous tissue in 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 ³²P gives off beta rays but no gammas, the counter must be placed very close to the suspected tissue, since beta particles have verylittle 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 ³²P labeled phosphate intravenously some hours before surgery. A tiny beta-sensitive probe counter then can be moved about within the operative site to indicate to the surgeon the limits of the cancerous area.

Normal blood is about 1% sodium chloride or ordinary salt. This fact makes possible the use of ²⁴Na in some measurements of the blood and other fluids. The figure illustrates this technique. A sample of ²⁴NaCl 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.

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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.

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Because of its short half-life of six hours, technetium-99m[10]is coming into use for diagnosis using scanning devices, particularly for brain tumors. It lasts such a short time it obviously cannot be kept in stock, so it is prepared by the beta decay of molybdenum-99.[11]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.

Using a “nuclear cow” to get technetium from its parent isotope. The “cow” is being fed saltwater through a tube. The saltwater drains through a high-radiation (hot) isotope. The resultant drip-off is a daughter such as technetium-99m. This new, mild isotope can be mixed with other elements and these become the day’s supply of radioisotopes for other scans. Technetium-99mdecays in 6 hours. Thus greater amounts, with less possibility of injury, can be administered and a better picture results.

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.

A technician holds an inexpensive portable X-ray unit that was developed by the Argonne National Laboratory. Compare its size with the standard X-ray machine shown at left and above.

Still more recently, ¹²⁵I 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[12]of ¹²⁵I, weighs 2 pounds, yet has adequate shielding for the operator. It is truly portable.

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.

More than 30 years ago, when deuterium (heavy hydrogen) was first discovered, heavy water (D₂O) was used for thedetermination 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 4 to 6 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 (³H) (radioactive hydrogen) has been produced in abundance. Its oxide, tritiated water (³H₂O), 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.

The total body water is determined by the dilution method using tritiated water. This technician is purifying a urine sample so that the tritium content can be determined and the total body water calculated.

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 multichannel 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 caused 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.

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 in 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 diagnosticskill. 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.

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.

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“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 6 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.

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, as was mentioned earlier, concentrates in the thyroid gland, and is converted there to protein-bound iodine that is slowly released to the blood stream. Iodine-131, in 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, ¹³²I hasa half-life of only 2.33 hours. What this means is that the same weight of radioactive ¹³²I will give a greater radiation dose than ¹³¹I would, and lose its activity rapidly enough to present much less hazard by the time the iodine is released to the blood stream. Iodine-132 is therefore often preferred for treatment of this sort.

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 1 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 blood stream, they will pass readily into brain tumors andnotmove 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.

SEQUENCE OF EVENTS IN NEUTRON CAPTURE THERAPY USING BORON-10Neutron capture treatment of a brain tumor, using the Brookhaven National Laboratory research reactor (center).

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).

(1) A lead shutter shields the patient from reactor neutrons.

(2) A compound containing the stable element boron is injected into the bloodstream; the tumor absorbs most of the boron.

(3) After 8 minutes, when the tumor is saturated, the shutter is removed and neutrons bombard the brain, splitting boron atoms so that fragments destroy tumor tissue.

(4) Twenty minutes later the shutter is closed and the treatment ends.

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.

Another disease which is peculiarly open to attack by radioisotopes ispolycythemia vera. 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 has been very effective in causing disappearance of symptoms for periods from months to years, lengthening the patient’s life considerably. The purpose of the ³²P 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 2 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 bringsappreciable concentration in the blood-forming tissues (about twice as much in blood cells as in general body cells).

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Survival ofpolycythemia verapatients after ³²P therapy.

One other pertinent fact is that these 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). Theremay be a small increase in the number of cases of leukemia among those treated with ³²P compared with the general population. The controversy arises over whether the ³²P treatmentcausedthe 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.

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.

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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 mayremain suspended or settle out upon the lining. In either case, since they are not dissolved, they do not pass through the membranes or cell walls but remain within the cavity. Through its destructive and retarding 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 nontoxic; 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 in over two-thirds of the cases treated.

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 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 1620 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 needle used for implantation of yttrium-90 pellets into the pituitary gland is shown in the top photograph. In the center X ray the needle is in place and the pellets have just been passed through it into the bone area surrounding the pituitary gland. The bottom X ray shows the needle withdrawn and the pellets within the bone.

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 in that the time of removal is not of critical importance.

Ceramic beads made of yttrium-90 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 reasoned that destruction of this gland might well slow down growth of a tumor elsewhere in the body. The trouble 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-90 oxide beads, glasslike 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.

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.

The cobalt-60 unit at the M. D. Anderson Hospital and Tumor Institute in Houston, Texas, employs a 3000-curie source. This unit has a mechanism that allows for rotation therapy about a stationary patient. Many different treatment positions are possible. This patient, shown in position for therapy, has above her chest an auxiliary diaphragm that consists of an expanded metal tray on which blocks of either tungsten or lead are placed to absorb gamma rays and thus shape the field of treatment. In this case they allow for irradiation of the portions of the neck and chest delineated by the lines visible on the patient.

Since a curie is the amount of radioactivity in a gram of radium that is in equilibrium with its decay products, a 1000-curie source is comparable to 2 pounds of pure radium. Neglecting for the moment the scarcity and enormous cost of that much radium (millions of dollars), wehave to consider that it would be large in volume and consequently difficult to apply. Radiation from such a quantity cannot be focussed; 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.

Diagram of teletherapy unit

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.

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).[13]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.[14]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 train 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 totally 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.

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-Müller 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.

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 gammas 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.

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.

These have been widely used and are versatile in their applications. The potential difference between the electrodes in the Geiger-Müller 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.

Since the development of the photoelectric tube and the photomultiplier tube (a combination of photoelectric cell and amplifier), the scintillation 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 scintillation 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.

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.

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 carriermoves 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 pen 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.

Radioactive Isotopes in Medicine and Biology, Solomon Silver, Lea & Febiger, Philadelphia, Pennsylvania 19106, 1962, 347 pp., $8.00.Atomic Medicine, Charles F. Behrens and E. Richard King (Eds.), The Williams & Wilkins Company, Baltimore, Maryland 21202, 1964, 766 pp., $18.00.The Practice of Nuclear Medicine, William H. Blahd, Franz K. Bauer, and Benedict Cassen, Charles C. Thomas, Publisher, Springfield, Illinois 62703, 1958, 432 pp., $12.50.Progress in Atomic Medicine, John H. Lawrence (Ed.), Grune & Stratton, Inc., New York 10016, 1965, volume 1, 240 pp., $9.75.Radiation Biology and Medicine, Walter D. Claus (Ed.), Addison-Wesley Publishing Company, Reading, Massachusetts 01867, 1958, 944 pp., $17.50. Part 7, Medical Uses of Atomic Radiation, pp. 471-589.Radioisotopes and Radiation, John H. Lawrence, Bernard Manowitz, and Benjamin S. Loeb, McGraw-Hill Book Company, New York 10036, 1964, 131 pp., $18.00. Chapter 1, Medical Diagnosis and Research, pp. 5-45; Chapter 2, Medical Therapy, pp. 49-62.

Radioactive Isotopes in Medicine and Biology, Solomon Silver, Lea & Febiger, Philadelphia, Pennsylvania 19106, 1962, 347 pp., $8.00.

Atomic Medicine, Charles F. Behrens and E. Richard King (Eds.), The Williams & Wilkins Company, Baltimore, Maryland 21202, 1964, 766 pp., $18.00.

The Practice of Nuclear Medicine, William H. Blahd, Franz K. Bauer, and Benedict Cassen, Charles C. Thomas, Publisher, Springfield, Illinois 62703, 1958, 432 pp., $12.50.

Progress in Atomic Medicine, John H. Lawrence (Ed.), Grune & Stratton, Inc., New York 10016, 1965, volume 1, 240 pp., $9.75.

Radiation Biology and Medicine, Walter D. Claus (Ed.), Addison-Wesley Publishing Company, Reading, Massachusetts 01867, 1958, 944 pp., $17.50. Part 7, Medical Uses of Atomic Radiation, pp. 471-589.

Radioisotopes and Radiation, John H. Lawrence, Bernard Manowitz, and Benjamin S. Loeb, McGraw-Hill Book Company, New York 10036, 1964, 131 pp., $18.00. Chapter 1, Medical Diagnosis and Research, pp. 5-45; Chapter 2, Medical Therapy, pp. 49-62.

Atoms Today and Tomorrow(revised edition), Margaret O. Hyde, McGraw-Hill Book Company, Inc., New York 10036, 1966, 160 pp., $3.25. Chapter 9, The Doctor and the Atom, pp. 79-101.Atomic Energy in Medicine, K. E. Halnan, Philosophical Library, Inc., New York 10016, 1958, 157 pp., $6.00. (Out of print but available through libraries.)Teach Yourself Atomic Physics, James M. Valentine, The Macmillan Company, New York 10011, 1961, 192 pp., $1.95. (Out of print but available through libraries.) Chapter X, Medical and Biological Uses of Radioactive Isotopes, pp. 173-184.Atoms for Peace, David O. Woodbury, Dodd, Mead & Company, New York 10016, 1965, 259 pp., $4.50. Pp. 174-191.The Atom at Work, Jacob Sacks, The Ronald Press Company, New York 10010, 1956, 341 pp., $5.50. Chapter 13, Radioactive Isotopes in Hospital and Clinic, pp. 244-264.

Atoms Today and Tomorrow(revised edition), Margaret O. Hyde, McGraw-Hill Book Company, Inc., New York 10036, 1966, 160 pp., $3.25. Chapter 9, The Doctor and the Atom, pp. 79-101.

Atomic Energy in Medicine, K. E. Halnan, Philosophical Library, Inc., New York 10016, 1958, 157 pp., $6.00. (Out of print but available through libraries.)

Teach Yourself Atomic Physics, James M. Valentine, The Macmillan Company, New York 10011, 1961, 192 pp., $1.95. (Out of print but available through libraries.) Chapter X, Medical and Biological Uses of Radioactive Isotopes, pp. 173-184.

Atoms for Peace, David O. Woodbury, Dodd, Mead & Company, New York 10016, 1965, 259 pp., $4.50. Pp. 174-191.

The Atom at Work, Jacob Sacks, The Ronald Press Company, New York 10010, 1956, 341 pp., $5.50. Chapter 13, Radioactive Isotopes in Hospital and Clinic, pp. 244-264.

Ionizing Radiation and Medicine, S. Warren,Scientific American, 201: 164 (September 1959).Nuclear Nurses Learn to Tame the Atom, W. McGaffin,Today’s Health, 37: 62 (December 1959).How Isotopes Aid Medicine in Tracking Down Your Ailments, J. Foster,Today’s Health, 42: 40 (May 1964).Nuclear Energy as a Medical Tool, G. W. Tressel,Today’s Health, 43: 50 (May 1965).

Ionizing Radiation and Medicine, S. Warren,Scientific American, 201: 164 (September 1959).

Nuclear Nurses Learn to Tame the Atom, W. McGaffin,Today’s Health, 37: 62 (December 1959).

How Isotopes Aid Medicine in Tracking Down Your Ailments, J. Foster,Today’s Health, 42: 40 (May 1964).

Nuclear Energy as a Medical Tool, G. W. Tressel,Today’s Health, 43: 50 (May 1965).

Radioisotopes in Medicine(SRIA-13), Stanford Research Institute, Clearinghouse for Federal Scientific and Technical Information, 5285 Port Royal Road, Springfield, Virginia 22151, 1959, 180 pp., $3.00.

The following reports are available from the Superintendent of Documents, U. S. Government Printing Office, Washington, D. C. 20402.

Isotopes and Radiation Technology(Fall 1963), P. S. Baker, A. F. Rupp, and Associates, Oak Ridge National Laboratory, U. S. Atomic Energy Commission, 123 pp., $0.70.Radioisotopes in Medicine(ORO-125), Gould A. Andrews, Marshall Brucer, and Elizabeth B. Anderson, 1956, 817 pp., $6.00.Applications of Radioisotopes and Radiation in the Life Sciences, Hearings before the Subcommittee on Research, Development, and Radiation of the Joint Committee on Atomic Energy, 87th Congress, 1st Session, 1961, 513 pp., $1.50; Summary Analysis of the Hearings, 23 pp., $0.15.

Isotopes and Radiation Technology(Fall 1963), P. S. Baker, A. F. Rupp, and Associates, Oak Ridge National Laboratory, U. S. Atomic Energy Commission, 123 pp., $0.70.

Radioisotopes in Medicine(ORO-125), Gould A. Andrews, Marshall Brucer, and Elizabeth B. Anderson, 1956, 817 pp., $6.00.

Applications of Radioisotopes and Radiation in the Life Sciences, Hearings before the Subcommittee on Research, Development, and Radiation of the Joint Committee on Atomic Energy, 87th Congress, 1st Session, 1961, 513 pp., $1.50; Summary Analysis of the Hearings, 23 pp., $0.15.

Available for loan without charge from the AEC Headquarters Film Library, Division of Public Information, U. S. Atomic Energy Commission, Washington, D. C. 20545 and from other AEC film libraries.

Radioisotope Applications in Medicine, 26 minutes, black and white, sound, 1964. Produced by the Educational Broadcasting Corporation under the joint direction of the U. S. Atomic Energy Commission’s Divisions of Isotopes Development and Nuclear Education and Training, and the Oak Ridge Institute of Nuclear Studies. This film traces the development of the use of radioisotopes and radiation in the field of medicine from the early work of Hevesy to the present. Descriptions of the following are given: study of cholesterol and arteriosclerosis; cobalt labeled vitamin B₁₂ used to study pernicious anemia; history of iodine radioisotopes and the thyroid; brain tumor localization; determination of body fluid volumes; red cell lifetime; and use of radioisotopes for the treatment of various diseases.Medicine, 20 minutes, sound, color, 1957. Produced by the U. S. Information Agency. Four illustrations of the use of radioactive materials in diagnosis and therapy are given: exact preoperative location of brain tumor; scanning and charting of thyroids; cancer therapy research; and the study of blood diseases and hardening of the arteries.Radiation Protection in Nuclear Medicine, 45 minutes, sound, color, 1962. Produced by the Fordel Films for the Bureau of Medicine and Surgery of the U. S. Navy. This semitechnical film demonstrates the procedures devised for naval hospitals to protect against the gamma radiation emitted from materials used in radiation therapy.

Medicine, 20 minutes, sound, color, 1957. Produced by the U. S. Information Agency. Four illustrations of the use of radioactive materials in diagnosis and therapy are given: exact preoperative location of brain tumor; scanning and charting of thyroids; cancer therapy research; and the study of blood diseases and hardening of the arteries.

Radiation Protection in Nuclear Medicine, 45 minutes, sound, color, 1962. Produced by the Fordel Films for the Bureau of Medicine and Surgery of the U. S. Navy. This semitechnical film demonstrates the procedures devised for naval hospitals to protect against the gamma radiation emitted from materials used in radiation therapy.

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