Part 7

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), we have 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.

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.

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). 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. 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.

APPENDIX

Measuring Instruments

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.

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 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.

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-Müller Counters

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.

Scintillation Counters

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.

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 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.

SUGGESTED REFERENCES

Technical Books

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.