LST 2-5

Part 2 Continued


2-6 Radiation exposure in medical applications. Exposure of patients and workers in the medical applications of ionizing radiation constitutes a clear-cut situation where there is essentially always an overall benefit to man's health and welfare to offset any small element of risk. But the medical uses of x rays are known to virtually everyone, with the result that when some radiation "hue and cry" is raised, a frequent public reaction is " we have to make those doctors (or dentists) cut out their careless exposure of patients" -- or some such statement.

It is obvious that any man-made source of radiation can always be reduced to some degree -- until it is completely eliminated. How far this is carried out depends upon a balancing of the costs of reduction against the loss of the benefits from the radiation use. To give an example: Suppose it were possible, with only some small loss of medical information, to reduce the overall medical x-ray exposure of the public. From the chart in Figure 4, it is seen that medical x rays contribute 40 millirem a year to the average exposure of individual in the public. A reduction by 10% would cut 4 mrem from that annual exposure. In the meantime the average individual is being exposed to all those other radiations for a total of about 360 millirem in a year, and can do little or nothing to reduce that overall figure. Four millirem is only about 1.1 percent of 360 millirem. In other words, a ten percent reduction in x-ray exposure will yield only a one percent saving in man's average overall dose. A ten percent reduction in the average annual exposure of the public to medical x rays would be very costly in terms of benefits and information lost, a cost that, would, of course, be passed on to the public. In considering such a step, we should not lose sight of the fact that the dose to any individual from natural radiation sources varies more than one percent from day to day.

2-7 Radiation exposure from consumer products. The use of tobacco products, in any form, contributes a substantial dose to limited areas of bronchial tissue of smokers. It is clear that smokers have an elevated rate of lung cancer, but because tobacco smoke contains carcinogens other than radioactive materials, it is not clear how much, if any, of the cancer risk can be attributed to the radiation exposure. In addition to direct radiation exposure to the lungs of the smoker, there is secondary exposure of those who inhale air containing smoke exhaled by smokers. The amount of radiation exposure from this source has not been well quantified, but the increased incidence of lung cancer due to second-hand smoke has been clearly established. It is unfortunate that tobacco in any form, a consumer product that probably makes a major contribution to the average dose to the public, is also the agent for which we have the least quantitative foundation for a radiation risk evaluation.

Because no professional group feels that it has an adequate understanding of some of the details involved, there is a reluctance among scientists to quantify exposures from smoking with the same degree of confidence as for the other radiation exposures. It is also for that reason that they do not include smoking in their summaries of population exposures to ionizing radiation. (See, e.g., NCRP-93, table 5.1). Radiation exposure from tobacco products is not included in the pie chart in Figure 4.

Since the NCRP has not included it in the summary of radiation exposures from consumer products, it will also be left out here. It should be noted that the figure of 1,300 mrem to the bronchial tissue is applied to the average exposures of the users (mainly smokers) and thus is not averaged over the entire population. It is also a dose to particular tissue, while the doses in Figure 4 are stated as effective whole body doses. The next two tables list the major sources of radiation exposure included in the "consumer products" category. Only those sources contributing an annual dose of 0.1 millirem, or more, to the average exposure of the population are included. For obvious reasons, some of the exposures show a fairly wide range of dosage contributions.

Table 2 lists the sources contributing relatively large doses to a great many people.

Table 3 covers dose contributions to many people but the dose is relatively small or is limited to a very small portion of the body.

Luminous watches and clocks, airport inspection systems, electron tubes, and fluorescent lamp starters each contribute less than 0.001 millirem (10 microsievert) and are not listed. The rounded total for the lot is from 0.15 to 1.2 millirem (0.0015 to 0.12 millisievert)

Attention should be directed to the significance of such low levels of radiation exposure. For example, changing the altitude of where you live by 100 or 200 feet involves dose changes more than 1 millirem in a year. Radon exposures in the home will vary a good many millirem, up and down, during the year or even the day. The NCRP, in considering the risks associated with such small exposures, has recommended that there be some level of exposure below which no individual need be concerned. This is called "Negligible Individual Dose" (NID). A value of 1.0 millirem (0.01 mSv) in a year has been recommended as constituting a "boundary below which the dose can be dismissed from consideration". This applies per source or per practice. Given the essentially negligible amounts of radiation from such consumer products as listed in Table 3 , as well as several of those in Table 2, there seems little cause for concern when compared with all of the much larger exposures that are essentially beyond our control.

2-8. The effects of exposure to ionizing radiation. Most of the information in this section is designed to provide the reader with a limited opportunity to examine the overall problem of radiation effects, with some kind of informed perspective. Unfortunately, much of the information on radiation effects that is easily available to the public varies between being inadequate to being incorrect by design to support some social or political position. This lack of proper information is especially ironic in view of the fact that ionizing radiation and its effects are among the most studied and best understood sciences today.

When ionizing radiation strikes the body, it randomly hits or misses millions of cells. For the cells that are not hit, the ray simply passes by and no harm is done. If a cell is hit directly, the cell may be completely killed or, somewhat less likely, just damaged. When a cell is completely killed, there is no great harm to the overall organism; the cell is dead, its debris is carried away by the blood, and a new cell is usually generated in its place. Similar actions, from various causes, are going on continuously with all the cells in the body. If any permanent harm is done by radiation exposure, it is from cells that are damaged, but not killed. It is the damaged cell, that may be regenerated as a potentially pre-cancerous cell. Over a period of many years, or decades the result may be a full-blown, malignant cancer. It is the least likely of the various results that may occur when radiation passes through a body of cells. Radiation might cause cancer; it does not necessarily do so.

It has long been known and accepted that people who are exposed to large doses of ionizing radiation stand an increased chance of developing cancer at some later time . This was clear from the experiences of the early radiologists before the adoption of modern radiation safety practices in the mid-1930's. However, beginning in about 1940 the general belief began that of all radiation effects, the one of most concern was genetic injury, and there was no level of exposure below which genetic effects would not occur. This view dominated until the mid 1960's, by which time the scientific community became convinced that the major problem lay in cancer production at doses lower than those considered carcinogenic two or three decades earlier. Animal studies supported the view that there was no low-dose cut-off value could be established for radiation carcinogenesis, but, as already noted, this has neither been proven or disproved for the human population.

Naturally, the public has been bombarded with stories about this, especially since the opening of the atomic energy era. Another fact known, but not nearly so well by the public, is that cancers can also be caused by literally thousands of other physical and biological agents that have been identified by man. Cancers are often spoken of as "occurring naturally", which is another way of saying "specific cause unknown".

In the search for the causes of cancer, animals are treated with large doses of agents suspected of being carcinogenic (capable of causing cancer). Usually the amount of the chemical dose to the animal is many times greater in proportion to the weight of the animal, than would be the dose per weight of man. This is done to speed up the development of an effect, thus shortening the testing times. Then, if an agent, given in a sufficiently large quantity to an animal, can cause cancer, it is conservatively assumed that any quantity, no matter how small, of the same agent, given to man, may also cause cancer.

This may be an acceptable method for testing toxic agents, drugs, or foods, but it is not necessarily suitable for evaluating the risks from ionizing radiation. It is known for ionizing radiation that for the same moderate dose there are differences in effect between exposures received at high dose rate for a short period of time, and those received at a low dose rate over a longer period of time. The effect may be as much as two to ten times greater for the high-dose rate, compared to the low-dose rate.

A further difficulty with this is that man is always being exposed to so many other carcinogenic agents -- agents that can cause cancer -- that it is not possible to state which, if any, of the agents might have been responsible for a cancer that is not found until many years after the exposures. If it happens to be known that a person was exposed to some man-made source of radiation, there is a natural tendency among uninformed individuals to relate a subsequent cancer to that exposure. It is almost never possible to specifically identify any of the hundreds of other possible causes. Rarely can a particular cancer be positively identified as having been caused by a particular set of circumstances. In most situations. Ionizing radiation is one of the least likely among the causes.

For such reasons as sketched above, it is important for readers to have at least a superficial idea of the carcinogenic effects of radiation, so they can exercise their own judgment of that hazard in comparison with other hazards and can develop their own sense of perspective about the overall problem. It must be remembered that the relationship with cancer is are far better understood for radiation than for most other agents.

It is important to distinguish generally between "high-dose" and "low-dose" radiation, although there is no real borderline between the two. In the context discussed here, a "high dose" is an acute radiation dose of twenty rems or more. It is in the range above about ten rems that there are extensive, and widely accepted, data relating the magnitude of the dose to the seriousness of the effect or injury, the probability of its occurrence, and the chance of death. In the low-dose range -- below 20 rem -- no clearly injurious radiation effects have been found

The significance of the "low dose" designation appears to be thoroughly misunderstood by the general public. For example, when a recent study indicated that the effects of low doses of radiation might be two or three times larger than previously estimated, there were clear elements of alarm in the reporting. What was not appreciated was the fact that the report was discussing exposure levels in the range where no direct effects on man have been observed in the first place. There have occasionally been statistical reports of effects at such low doses, but thus far the test samples have been so small that any conclusions drawn from them and have not stood up to the test of repetition and evaluation by other workers.

Low-dose effects are those that might be caused by doses of less than 20 rem (20,000 millirem), whether delivered acutely -- all at once -- or spread out over a period as long as a year. As already noted, five rem in a year has been considered, since 1956, as an acceptable exposure limit for radiation workers. It has been the "official dose limit" used by the Nuclear Regulatory Commission and must therefore be regarded as relatively safe in comparison with other industrial risks. Studies by the recognized protection bodies have shown that under most circumstances an annual dose limit of one rem can be met by the radiation industry within the ALARA principle. It may be anticipated that such a limit will be proposed.

There are substantial quantities of acceptable data showing relations between dose and effect in the high-dose region, especially in the range of 50 to 400 rem, delivered acutely. These are largely data resulting from studies of the survivors of the Japanese atomic bombing and of some relatively large groups of patients who were treated with substantial doses of x-rays.

Whether the effects of a given dose will, or will not, be discernible depends very much upon the rate at which the dose is delivered. For example, a dose of 200 rem of gamma radiation, administered to the whole body in a half hour, would cause nausea and vomiting in a few percent of the recipients; perhaps one percent of those exposed would die of the acute radiation syndrome within the next month or two. The same dose, distributed over a period of a month or more, would not even be noticed by the recipient. For larger acute doses - of the order of 350 to 400 rem - it would be expected that about half of the people exposed to this in a short time period would die within the next 30 days. By contrast, the same dose administered uniformly over a year's time could pass unnoticed by most exposed persons.

Both of the cases above are obviously extremes and, outside the realm of radiation therapy, are likely to be encountered only in nuclear warfare, something not being discussed in this book. Our discussions are concerned only with exposures from controlled sources that are likely to be encountered by the public, and should be less than 500 millirem in a year for individuals, and less than a fifth of that (100 millirem) a year for continuous exposure to the public at large. For such small exposures it is not possible to compare dose effects under various exposure conditions, for the simple reason that no one has been able to positively identify any effects.

In the absence of reliable data on low dose effects, important assumptions have to be made by radiation protection specialists. One is that there is no "cut-off" level of radiation exposure -- commonly called a threshold -- below which no carcinogenic effect will occur. However, for a few non-cancerous effects, there are indeed, well-established thresholds. For example, it is known with certainty, that high doses of radiation can cause cataracts. It is also well established that to produce cataracts requires a dose of over 500 rem to the eye.

There is another complicating factor in understanding radiation effects, and that is known as the latent effect or latency. This refers to the delay between the cause of some effect (i.e., exposure to radiation) and the eventual appearance or detection of a clinical effect. In actual circumstances, except for leukemia which may develop in as few as two years, exposure to a dose of radiation may not show any effect for as many as twenty, thirty, or more, years. In the interim, many people will die from other unrelated causes, before the latent period runs out. Because of this long latency, the individual lives many years beyond his exposure, during which time he is further exposed to hundreds or thousands of other carcinogenic agents. It is thus imaginable that because many of these insulting agents are more dangerous than radiation, if and when a cancer does show up, it could as well be due to one or more of the other agents and not the radiation.

In summary, the discussions in this section have focused on what is frequently described as "low-level" radiation. The average member of the general public receives about 0.36 rem in a year from all sources. ( see Figure 4 ). That means that the average person receives less than about a thirtieth of the upper bound dose for the low- dose region, the region for which it has not been possible to find any effects that can be specifically ascribed to ionizing radiation.

2-9 Ionizing radiation in the service of man. It is common, in the discovery of some new and important scientific principle, that there is a considerable lag between the first findings and their application to bettering the life of man. However, it might well be said that almost from the first day after the discovery of x rays possible medical applications were evident. X-rays also quickly found application in many areas of commence and industry. It is now fairly well known that x-rays and gamma rays are used extensively for the inspection of critically important components or devices, the failure of which could be costly in lives. The inspection of certain aircraft parts is an excellent example. Before 1940, most such applications used x-rays and the gamma rays from radium; since then, radiation from artificially-produced radioactive materials has been widely used.

The discovery of artificially-produced radioisotopes in the late 1930's, and their ultimate production in substantial quantities in nuclear reactors, changed the ionizing-radiation picture radically. Fortunately, there had been enough experience in the preceding forty years to warn the scientific and technical communities that the new sources of radiation, now in our hands , had to be treated with great caution and respect. In this country, the U. S. Advisory Committee on X-ray and Radium Protection had already developed the basic guidelines, as early as 1934, for protection against x-rays generated up to a million volts and for the gamma rays from radium. (See supra section 1-1). The subsequent nuclear programs started from that point. The potential uses for these new radioactive materials and their radiations, was only allowed to proceed as our knowledge developed on how to control their undesirable and dangerous aspects.

The introduction of any new device or procedure involving ionizing radiation always requires that there be a clearly defined social benefit in addition to scientific or technological gains. Decisions on these issues are rarely simple or precise. The development of the uses of ionizing radiation affords an excellent example of the recognition of this problem, its importance, and the complexity of the considerations which must be made before applications of a new technology are allowed to become widespread and possibly out of control. These issues are mentioned briefly because, in the nearly fifty years since the discovery of artificial radioactivity, its exploitation has moved at a pace much slower than that of many other new discoveries. It is partly because of such caution that many of the applications, a few of which will be mentioned below, have come slowly and carefully and at an extremely low price in terms of injuries or lives lost through misapplication. It is important that it be kept that way.

In the earlier decades of the diagnostic applications of x-rays. the most common use was to seek and identify an internal disorder suggested by external symptoms. They later came into use to detect disease in advance of any clinical symptoms. The effectiveness of the early identification of disease was demonstrated by the virtual elimination of tuberculosis. Today, an outstanding example of the importance of early diagnosis is the success of mammography programs for the early detection of breast cancer -- detections which can be accomplished in the very early stages of the disease when the chances of cure are the highest.

It has already been noted in Figure 4, that the applications of ionizing radiation in consumer products contributes only about three percent of the average exposure of the population to all kinds of ionizing radiation -- both natural and man made. The medical applications contribute about 15% of our exposure, and all of the rest -- about 82% -- is derived from natural sources, over which we have little or no control. This small fraction of human exposure to radiation from consumer products, nevertheless, should not be accepted with equanimity. It should only be accepted if it can be reasonably demonstrated that it provides an important benefit, more than offsetting any associated risk. It involves one of the basic principles of radiation control, namely , any radiation application must produce a net positive benefit.

Given below are a number of other applications of ionizing radiation, listed in a publication of the International Atomic Energy Agency (September 1979) titled "RADIATION--A FACT OF LIFE."

The IAEA report addresses radiation from radioisotopes. In contrast to X-rays, the radioisotopes can be placed in, or directed to, exactly the place where the radiation may be wanted in the body. On the other hand, x rays are generated outside the region to which the application is being made. The radiations from radioisotopes are used extensively in both medical diagnosis and the treatment of disease, and they play a role often not possible by any other agent known to man. The net benefit, in terms of health and human lives, is enormous. Radiation is a major tool in the treatment of certain kinds of cancer. Selection of the sources of radiation and their location in the body allows the treatment to be tailored to the patient. This has proven to be very effective in inhibiting the growth of tumors, or actually destroying them, without unduly damaging the healthy tissue near the tumors.

Radioisotopes play an essential role in various medical diagnostic procedures -- again taking advantage of the special characteristics of the different radiations that may be available. Injection of small amounts of radioisotopes into such body organs as the heart, lungs, brain, liver, or kidneys makes possible a special visualization of the functioning of those organs. Together with improved imaging devices and sophisticated computers, the actions and behavior of these organs can be observed and tracked. Without such radiation sources, these assessments would be very difficult if not impossible.

Radiation can be used to sterilize many medical products such as surgical dressings, sutures, catheters, and syringes. Of special value is the fact that the sterilization can be performed while the items are contained in their sealed packages; thus they can remain sterile indefinitely. The gamma rays from radioisotopes like cobalt-60 or cesium-137 are especially well adapted for such purposes. These are now normal, every-day procedures. Moreover, radiation does not introduce undesirable residues, as sterilizing chemicals or gases may. Radiation also makes it possible to treat products which are difficult to sterilize by heat or steam. Since radio-sterilization is a cold process, it can be applied to heat-sensitive materials like the plastics used in heart valves. It appears to be the only means of sterilizing a number of heat-sensitive pharmaceutical items.

Irradiation has important preservation properties for food products such as potatoes, onions, meats, and meat products. Once irradiated, they can be stored without refrigeration. There are also many industrial chemical processes which can only be carried out under radiation conditions. The use of radioisotopes to provide radiation in spots where it can be utilized has been important in a variety of environmental studies and in the analysis of some air pollution problems. Applications in the fields of agriculture and hydrology are also important and cannot be accomplished by other known means. And in outer space, radioisotopes power generators that allow information to be sent back to earth from Jupiter, Pluto, and beyond.

And so it goes, with new developments in these areas occurring almost daily, and all carried out with extremely low risk to those involved in the procedures.