Pediatric Annals

X-ray, Microwave, and Ultrasound: The Real and Unreal Hazards

Robert L Brent, MD, PhD

Abstract

The effects of high-energy radiation (x-rays, gamma rays, particulate radiation) have been more extensively studied than any other environmental hazard. In spite of the vast amount of knowledge available about ionizing radiation, the public and many physicians and scientists are ignorant of the quantitative and qualitative effects of ionizing radiation.

Our laboratory is frequently called into consultation when a woman of reproductive age is exposed to diagnostic x-ray. More often than not, physicians have provided exposed women with erroneous information without the benefit of a minimal collection of data. In a recent instance, an obstetrician who had advised a woman to have an abortion following exposure to diathermy was not knowledgeable about the physical characteristics of microwave radiation and was confused about the difference between microwave and ultrasound. The confusion about the effects and hazards of radiation stem from the fact that the term "radiation" is applied to x-ray, microwave, and ultrasound energy. Actually, these three forms of energy have quite different physical charateristics and markedly different biologic effects (Table 1).

LIMITING THE USE OF THE WORD

It would be far better, from an educational viewpoint, to limit the use of the term "radiation" to high-energy ionizing radiation. Radar, microwave, shortwave, diathermy, frequency modulation (FM) broadcast range, and radio waves are various forms of long-wavelength electromagnetic waves that have little in common with x-ray and gamma rays, at least from a biologic standpoint. The use of radiation to describe ultrasound is even more confusing and erroneous. In England, physicians refer to diagnostic ultrasound as sonography and consciously refrain from using the term radiation in conjunction with any aspect of the clinical use of ultrasound.3 It is best to limit the use of the term radiation because of the connotations it arouses in lay persons and many physicians.

IONIZING RADIATION: CAUSES OF ANXIETY

Radiation is an anxiety-producing word because of its association with the effects of ionizing radiation. These are three of the associations people make, and the effects of each:

1. The word radiation is related to the effects of the atomic bomb. In the minds of many it is impossible to separate the effects of low-dose ionizing radiation from the psychologic, physical, and radiation effects of atomic weapons. The fact that the horrendous effects of the atomic and hydrogen bomb are at one end of a continuum of radiation effects distorts in the minds of many the effects of all forms of energy labeled "radiation."

Table

1. Proceedings of the Symposium on the Late Effects of Ionizing Radiation, March 13-17, 1978, Volume I. International Atomic Energy Agency, p. 545.

2. Okada, S., et al. A review of thirty-year study of Hiroshima and Nagasaki atomic bomb survivors. /. Radiât. Res., Supplement, 1975 (published by the Japan Radiation Research Society, Chiba, Japan), pp. 1-164.

3. Brent, R. L., and Gorson, R. O. Radiation exposure in pregnancy. In Mosely, R. D., Jr., et al. (eds.): Current Problems in Radiology, Volume 2. Chicago: Year Book Medical Publishers, 1972, pp. 1-48.

4. Brent, R. L. Radiations and other physical agents. In Wilson, J. G., and Fraser, F. C (eds.): Teratology, Volume 1. New York, Plenum Press, 1977, pp. 153-223.

5. Brent, R. L. Radiation teratogenesis (paper submitted to the Teratogen Update Section of Teratology), 1980.

6. The Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Repon of the Advisory Committee on the Biological Effects of Ionizing Radiations. Third Edition. Washington, D. C Division of Medical Sciences, National Academy of Sciences, National Research Council, August 1980.

7. The Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Report of the…

The effects of high-energy radiation (x-rays, gamma rays, particulate radiation) have been more extensively studied than any other environmental hazard. In spite of the vast amount of knowledge available about ionizing radiation, the public and many physicians and scientists are ignorant of the quantitative and qualitative effects of ionizing radiation.

Our laboratory is frequently called into consultation when a woman of reproductive age is exposed to diagnostic x-ray. More often than not, physicians have provided exposed women with erroneous information without the benefit of a minimal collection of data. In a recent instance, an obstetrician who had advised a woman to have an abortion following exposure to diathermy was not knowledgeable about the physical characteristics of microwave radiation and was confused about the difference between microwave and ultrasound. The confusion about the effects and hazards of radiation stem from the fact that the term "radiation" is applied to x-ray, microwave, and ultrasound energy. Actually, these three forms of energy have quite different physical charateristics and markedly different biologic effects (Table 1).

LIMITING THE USE OF THE WORD

It would be far better, from an educational viewpoint, to limit the use of the term "radiation" to high-energy ionizing radiation. Radar, microwave, shortwave, diathermy, frequency modulation (FM) broadcast range, and radio waves are various forms of long-wavelength electromagnetic waves that have little in common with x-ray and gamma rays, at least from a biologic standpoint. The use of radiation to describe ultrasound is even more confusing and erroneous. In England, physicians refer to diagnostic ultrasound as sonography and consciously refrain from using the term radiation in conjunction with any aspect of the clinical use of ultrasound.3 It is best to limit the use of the term radiation because of the connotations it arouses in lay persons and many physicians.

IONIZING RADIATION: CAUSES OF ANXIETY

Radiation is an anxiety-producing word because of its association with the effects of ionizing radiation. These are three of the associations people make, and the effects of each:

1. The word radiation is related to the effects of the atomic bomb. In the minds of many it is impossible to separate the effects of low-dose ionizing radiation from the psychologic, physical, and radiation effects of atomic weapons. The fact that the horrendous effects of the atomic and hydrogen bomb are at one end of a continuum of radiation effects distorts in the minds of many the effects of all forms of energy labeled "radiation."

Table

TABLE 1COMPARATIVE ASPECTS OF VARIOUS FORMS OF "RADIATION"

TABLE 1

COMPARATIVE ASPECTS OF VARIOUS FORMS OF "RADIATION"

2. Populations of people that have received high exposures to radiation are known to have increased incidences of cancer. These populations include the radium-dial workers, uranium miners, patients receiving radiation therapy or isotope therapy for various diseases, and the populations receiving the higher exposures in Hiroshima and Nagasaki following the atomic-bomb detonation.1

Cancer, itself, is an anxiety-provoking term. In our culture it is a disease with which people are "afflicted" and one that is dreaded by a large segment of the population. Few persons are aware that the maximum risk for the occurrence of cancer is extremely small in populations that have been exposed to much lower doses of radiation than the uranium miners, etc.

3. The immense psychologic consequences of high-energy - radiation exposure is extremely important and cannot be ignored when one is considering the deleterious effects of radiation. A review of the 30-year study of the Hiroshima and Nagasaki atomic-bomb survivors revealed some interesting findings, as Okada and associates2 have reported:

Thirty years of study clearly showed that radiation effects are dose-dependent. A survivor aware of the magnitude of the radiation dose, can better understand the extent of his own risks. More than 90% of survivors received much less than 10 rads from the A-bombs. With this knowledge, such survivors can realize that their fears of disease from A-bomb exposure may be exaggerated and that the possibility of their developing any such disease is no greater than those of non-exposed individuals. Those who received higher doses have greater risks .... It is worth noting that, thus far, the life expectancy of the exposed population under study and of their offspring is at least equal to that for the population in the rest of Japan. Thirty years of study has not eradicated radiation damage but has yielded reasonable estimates of the risks involved.

Although the results of the 30-year study of atomic-bomb survivors have been widely disseminated in Japan, the heavy psychologic burden of radiation exposure is still apparent. "The parents of a man wishing to marry a woman known to be a survivor, or the daughter of a survivor, may object to or even attempt to prevent their marriage," Okada et al.2 report, "fearing the woman's health may be adversely affected, or that their children might have genetic defects."

Granted that the risks from high-energy radiation exposure may be exaggerated or not well understood, there is no question that such radiation can damage tissue and produce long-term effects. If the quantitative data pertaining to radiation effects are understood, these phenomena can be placed into better perspective. Radiation risks should be compared with spontaneous risks and the risks produced by exposure to other environmental hazards.

HARMFUL EFFECTS OF HIGH-ENERGY RADIATION

The effects of high-energy radiation can be divided into two categories: threshold (nonstochastic) and nonthreshold (stochastic). Threshold effects include many of the symptoms seen in acute radiation syndrome, such as gastroenteritis, death, severe hematopoietic suppression, skin erythema, epilation, glossitis, weight loss, and septicemia. Most of these symptoms will never occur in a human population receiving less than 25 rem (25,000 mrem), and none of these signs and symptoms would be seen in persons receiving below 10 rem.

Other threshold effects include some having long-term consequences, such as telangiectasia, shortening of the life span, aplastic anemia, and cataracts. Although there may not be total agreement on the connection between congenital malformations and high-energy radiation, most scientists believe that increased incidence of such anomalies will not occur in offspring exposed to 10 rem or less during embryonic development.3*5

The two main nonthreshold effects are the induction of cancer and mutations, but the likelihood of such effects in a given person at a given time is assumed to be stochastic. This means that even at very low exposures and low exposure rates some risk for occurrence can be predicted. Because of the dire consequences of these effects, we assume maximum risks at low exposure and low exposure rates by linearly extrapolating risks from populations receiving high exposures. In some instances, we know that a linear projection is grossly conservative - if not in error - but it is the most conservative risk estimate that can be made and, therefore, the one offering the population the most protection if the estimate is to be used to establish maximum permissible levels.

For those interested in the epidemiologic data related to radiation-induced malignancies, there is a thorough discussion in the reports of the Advisory Committee on the Biological Effects of Ionizing Radiations (BEIR).6,7 The estimates for malignancy induction following high exposure to radiation are shown in Table 2.

In order to translate these estimated risks into accurate predictions of actual disease, a population exposed to acute radiation must be followed for a lengthy period. This was done over a 25-year period in Japan following the detonation of the atomic bomb in 1945. The statistics indicate there was a risk of between 50 and 78 deaths per million exposed persons for every rem of exposure received.

It must be understood that this is a maximum estimate, and it is very likely that low-dose and low-dose-rate radiation has less of a carcinogenic effect because of the tissues' ability to repair and recover from some radiation effects. Furthermore, when one is dealing with exposures at the levels of diagnostic radiation and below, even the maximum estimate of malignancy induction results in cancer incidences that are such a small fraction of the spontaneous incidence that the rise in incidence cannot be observed unless one is dealing with very large exposed and nonexposed populations.

Table

TABLE 2MALIGNANCY INDUCTION FOLLOWING HIGH EXPOSURE AND HIGH-EXPOSURE RATE TO IONIZING RADIATION

TABLE 2

MALIGNANCY INDUCTION FOLLOWING HIGH EXPOSURE AND HIGH-EXPOSURE RATE TO IONIZING RADIATION

The genetic risks from radiation have been clearly demonstrated in experimental animals but have been much more difficult to demonstrate in human populations.6 Even the large Japanese population exposed in Hiroshima and Nagasaki has not exhibited any clearly demonstrable radiation-induced genetic effects. Furthermore, there are experiments demonstrating a definite dose-rate phenomenon that indicate that there is an an exponential reduction in the genetic effects of radiation as the dose rate is reduced. The risk of chronic low-dose radiation is maximally only between 0.005 and 0.05, the risk of spontaneous mutations per rad. This can be translated to mean that the radiation exposure necessary to double the spontaneous mutation rate is between 20 and 200 rad. Since the Japanese population exhibited no significant genetic effects at exposures well above 1 rad, it can be understood why it will be difficult to demonstrate an increase in genetic effects in populations exposed to radiation in the diagnostic range.

The recent nuclear accident at Three Mile Island, Pa., resulted in a great deal of hysteria and fear. But, as Table 3 indicates, if one uses the maximal risks provided in the BEIR Report, the fact that 2,000,000 people may have received from 1 to 2 mrem of radiation from Three Mile Island will result in a virtually undiscernible increase in radiation-induced disease.

Table

TABLE 3RADIATION-INDUCED DISEASE THAT CAN BE ANTICIPATED FROM THREE MILE ISLAND

TABLE 3

RADIATION-INDUCED DISEASE THAT CAN BE ANTICIPATED FROM THREE MILE ISLAND

RISKS FROM DIAGNOSTIC RADIATION

From the clinician's viewpoint and the patient's viewpoint, the risks of cancer and mutations from diagnostic radiation are exceedingly small and are at least thousands of times less probable than the risk of the spontaneous occurrence of cancer and mutations. Therefore, necessary diagnostic x-ray procedures have a very high benefit-to-risk ratio. On the other hand, unnecessary radiation exposure in the diagnostic range is unacceptable, since no benefit exists.

From the population's standpoint, radiation exposure should be kept as low as possible, in spite of the very small risk, because, if a population of 200,000,000 is exposed to an additional rem of radiation, this exposure can be translated into a maximum incidence of additional cancers over a 30-year period that may reach several thousand. These several thousand cases could be imperceptible because, during the same period, 30,000,000 to 50,000,000 people would have developed some type of malignancy.

It is important that the pediatrician, in addition to being aware of these maximum risks of radiation exposure, be able to answer patients' questions about radiation hazards. More frequently than not, I find that physicians are not aware of radiation risks and exaggerate the risks to themselves and their patients. This is a disservice, and can produce unwarranted anxiety and fear without justification.

MICROWAVE AND ULTRASOUND

The physical and biologic characteristics of microwave and ultrasound energy are indicated in Table 1. Since they do not produce ionization, these two forms of energy have far different characteristics from x-rays or gamma rays.

Maximum permissible levels for occupational and medical exposures have been suggested for both microwave and ultrasound. Persons working near FM radio or radar stations or near microwave ovens are not exposed to levels above the permissible maximum.

Microwave ovens. A microwave oven generates 2,450-MHz microwaves. This wavelength can produce hyperthermia above the 25-mwatt level, with a penetration of several centimeters. There is no way to receive exposure from a microwave oven, however, without bypassing several safety interlocks, and it is very easy to shield microwaves since a proper screen or even thin metal-foil shield is 100 percent effective in preventing radiation from reaching a person.

Theoretically, if a microwave oven had a door leak,, one could expose himself by placing a part of his body in direct contact with the area. In this way, it is conceivable that after several hours the person might receive a measurable exposure. On the other hand, since electromagnetic waves dissipate at a rate related to the square root of the distance, it is obvious that a leaking microwave oven would have no consequences several meters away - unless it interfered with some sensitive electronic device that was responsive to that wavelength of electromagnetic radiation.

Radar, microwave, radio waves, FM, and diathermy are all electromagnetic waves ranging in frequency from 27.5 MHz (diathermy - 27,500 vibrations/second) to 104-105 MHz (microwave communications). Diathermy electromagnetic waves have great penetration and can readily heat a human torso. Microwaves of 2,450 MHz or 915 MHz have less penetration but can also produce significant hyperthermia. Microwaves with frequencies above 10,000 MHz have minimal penetration but could produce significant hyperthermia at the skin level if the energy was high enough.

Although a nonthermal effect has not been clearly demonstrated for these forms of electromagnetic irradiation, the matter of nonthermal effects is still being investigated. The organs most vulnerable to the thermal effects of microwave radiation are the eye and the developing embryo, because these structures have the least capacity to dissipate heat. There is no indication that these forms of electromagnetic energy have the capacity to produce mutations or malignancy. The clinician can therefore reassure his patients that microwave ovens, properly handled, are reasonably safe. Since food and utensils can get quite hot with microwave exposure, one must take the usual precautions in removing hot materials from the oven.

Ultrasound. The use of diagnostic ultrasound has increased dramatically in the past 10 years.* Although studies dealing with the effects of ultrasound are still under way, this form of energy appears to be relatively safe.7 Since ultrasound does not produce tissue ionization, the use of diagnostic ultrasound should reduce the necessity for many x-ray procedures.

The use of ultrasound for fetal monitoring and fetal diagnosis is rapidly expanding. No epidemiologic studies up to the present time indicate that diagnostic ultrasound results in any measurable or significant biologic effects. Of course, studies on the biologic effects of ultrasound are continuing. Furthermore, epidemiologic studies of infants who are exposed in utero are continuing. There are some data on the biologic effects of ultrasound concerning deoxyribonucleic acid repair, cytogenetic alterations, and teratogenesis. Results at the present time indicate that low exposure to ultrasound presents little or no risk.

CONCLUSION

The word radiation evokes emotional responses in both patients and professionals that frequently polarize their views as pro or con. All forms of energy present risks. In the area of high-energy radiation, we probably have a better comprehension of the maximum risks allowable than we have for any other environmental hazard. There are no radiation hazards connected with diagnostic ultrasonography, and as for necessary diagnostic roentgenography, the benefits far outweigh the minimal risks that do exist.

REFERENCES

1. Proceedings of the Symposium on the Late Effects of Ionizing Radiation, March 13-17, 1978, Volume I. International Atomic Energy Agency, p. 545.

2. Okada, S., et al. A review of thirty-year study of Hiroshima and Nagasaki atomic bomb survivors. /. Radiât. Res., Supplement, 1975 (published by the Japan Radiation Research Society, Chiba, Japan), pp. 1-164.

3. Brent, R. L., and Gorson, R. O. Radiation exposure in pregnancy. In Mosely, R. D., Jr., et al. (eds.): Current Problems in Radiology, Volume 2. Chicago: Year Book Medical Publishers, 1972, pp. 1-48.

4. Brent, R. L. Radiations and other physical agents. In Wilson, J. G., and Fraser, F. C (eds.): Teratology, Volume 1. New York, Plenum Press, 1977, pp. 153-223.

5. Brent, R. L. Radiation teratogenesis (paper submitted to the Teratogen Update Section of Teratology), 1980.

6. The Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Repon of the Advisory Committee on the Biological Effects of Ionizing Radiations. Third Edition. Washington, D. C Division of Medical Sciences, National Academy of Sciences, National Research Council, August 1980.

7. The Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Report of the Advisory Committee on the Biological Effects of Ionizing Radiations, Second Edition. Washington, D. C: Division of Medical Sciences, National Academy of Sciences, National Research Council, November, 1972, p. 216.

8. Report of the Consensus Development Conference on Antenatal Diagnosis. Bethesda, Md.: National Institute of Child Health and Human Development, National Institutes of Health, Publication 79-1973, April, 1979, p. 199.

TABLE 1

COMPARATIVE ASPECTS OF VARIOUS FORMS OF "RADIATION"

TABLE 2

MALIGNANCY INDUCTION FOLLOWING HIGH EXPOSURE AND HIGH-EXPOSURE RATE TO IONIZING RADIATION

TABLE 3

RADIATION-INDUCED DISEASE THAT CAN BE ANTICIPATED FROM THREE MILE ISLAND

10.3928/0090-4481-19801201-07

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