E.A. Riedesel, DVM, DACVR
Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA, USA
The diagnostic use of x-ray irradiation, if done without regard to proper radiation safety practices, has the potential to cause significant deleterious biologic effects. The major effects are the induction of fatal cancers, genetic effects passed on to children, and developmental defects in the fetus when irradiated in utero. The major sources of radiation exposure to the veterinary worker are the primary x-ray beam, scattered radiation from the patient, and leakage radiation from the x-ray tube housing. The principal means of radiation protection are time, distance, and shielding: keep the time of exposure to radiation as short as possible; increase your distance from the source of radiation; place shielding material between you and the source of radiation. Annual radiation dose limits for occupational workers, set by state health codes and the United States Nuclear Regulatory Commission, are established to “limit the probability of occurrence of random carcinogenic and genetic effects” and to “prevent completely the occurrence of deterministic effects.” In addition to these limiting dose values, it is expected that the working conditions are such that the radiation dosage is kept As Low As Reasonably Achievable. If strict adherence is given to the safe practice of radiography, the probability of radiation-induced injury is negligible.
Not long after the discovery of x-rays in 1895, their use in evaluating anatomic changes caused by disease processes was well recognized. The first paper on veterinary radiography was written in 1896. At the same time, the damaging effects of large radiation doses were becoming known. In 1896, 23 cases of radiodermatitis were reported. In review articles of 1911 and 1914, 54 cancer deaths attributable to radiation and 198 radiation-induced malignancies were reported. The first American death attributed to radiation exposure occurred in 1904. The cause for the morbidity and mortality associated with radiation was the complete absence of any protection for the radiation worker, be they the scientist, physician, or assistant. Very long exposure times were needed to adequately expose the “plates,” so patients were exposed to high doses. The first official action taken to investigate methods to reduce radiation exposure occurred in England in 1921.
Knowledge of the effects of excessive human radiation exposure comes from the study of the health of early radiologists, survivors of the atomic bombs and nuclear power plant accidents, radium dial watch painters, people treated with radiation for ankylosing spondylitis, and numerous other groups. Data from experimental exposure of animals have also been extrapolated to the human. The radiation dose and dose rate to these groups range from acute high dose exposure to chronic lower dose exposure. Because of the knowledge gained from the studies, it is obvious that radiation exposure entails some risk of harmful effects. The changes in radiation practice that have occurred in the medical and veterinary profession have focused on the reduction of radiation exposure delivered to the patient and that received by the radiation worker. In the veterinary profession, we use x-rays primarily to aid us in making diagnoses and to sequentially evaluate the response to treatments. By the nature of our patients, we are much more likely to need manual restraint than our human counterparts. We thus are chronically exposed to radiation of low doses.
Methods of Injury
X-rays, being a form of ionizing radiation, cause cellular damage by altering critical molecular structures, principally DNA. The alteration in the DNA can be sublethal and be repaired, causing no acute or long-term effect, or it can be sublethal and upon repair induce a mutation in the cell that leads to the development of cancer months to years later. The radiation damage can also be lethal. If only a small cluster of non-dividing cells are killed, there may be no recognizable effect. However, if the cells destroyed are critical to maintaining that cell line, then the absence of this cell line will be expressed. The most significant concern generated for the chronic low-dose exposure is the late effects.
There are two classes of late effects as related to exposure dose: stochastic and deterministic (non-stochastic) effects. Stochastic effects are random effects for which any dose, however small, carries with it a probability of producing the effect. The effect will either occur or not occur. The probability that the effect will occur increases as the cumulative radiation dose increases. However, the severity of the effect is not related to the dose. That is, the effect will not be more severe if the dose is higher. The stochastic effects are the genetic effects and carcinogenesis.
Results of Injury
The deterministic effects are somatic effects that increase in severity with increasing dose in affected individuals. The effects are caused by damage to an increasing number of cells and amounts of tissue. These effects are basically degenerative, and the best-known examples are cataracts, organ atrophy, and tissue fibrosis. The deterministic effects have a threshold dose below which the effect will not occur. Above the threshold dose, the severity of the effect increases with increasing radiation dose.
A major question is how much radiation exposure is acceptable or safe? That’s the question to which there is no definite answer. Ample scientific evidence indicates that any dose of radiation poses some possibility of causing a damaging effect. If the damaging effect involves a non-critical cell/organ system, then it may never be realized by the individual. Mathematic interpolation of data indicates that for the stochastic effects, there is no known threshold dose below which there is no risk of the effect occurring. This forms the basis for the establishment of the radiation protection standards. The exposure dose levels set by these standards are intended to make the environment of the radiation workplace such that the stochastic effects are never likely to be a problem. The risk of the stochastic effects still exists, but if exposure dose is kept as low as possible, then the risk is reduced. At the same time, the dose limits are set below the threshold for the deterministic effects. It must be kept in mind, however, that the radiation dose equivalent is cumulative.
The NCRP dose limits are all subject to the concept of ALARA—as low as reasonably achievable. Ideally occupational exposure would be zero. Since that is not possible, the facilities and equipment should be designed and used so that exposure to personnel is minimal. No unnecessary exposures should be allowed.
The Keys to Radiation Exposure Reduction
The intensity of radiation exposure is dramatically reduced as the distance from the source is increased. This follows the inverse-square law: the exposure rate from an x-ray source is inversely proportional to the square of the distance from the source. This means that if the distance from the x-ray tube is doubled, the exposure is reduced to 1/4 of the initial dose rate. This forms the basis for the recommendation that manual restraint of animal patients be done as infrequently as possible. The use of mechanical restraining devices and/or chemical restraint is recommended to keep humans away from the primary beam and should be the preferred choice.
Assuming that the radiation is leaving the x-ray tube at a constant rate, the total dose equivalent received depends on the length of time exposed. Thus, the amount of radiation received can be controlled by the time of exposure. Using exposure times as short as possible will reduce the radiation exposure received.
Even though radiation interacts with any type of material and is reduced in amount by these interactions, certain materials are more efficient in absorbing radiation. These are the best materials to be used for shielding. The purpose of shielding is to attenuate the x-ray beam so that either none or extremely small amounts of radiation will reach the person or area being shielded. For the x-ray energies of diagnostic radiology, lead has been the shielding material of choice. Lead aprons and gloves should be worn by anyone assisting in the x-ray procedures. Gloves should be worn by anyone who is manually restraining the patient or holding the x-ray cassette. Preferably the x-ray cassette is not being held by hand but rather is placed within a cassette holder that has a rod or arm to position the holder’s hands farther away from the primary beam. Gloves should be worn even if a cassette holder is used. The lead aprons and gloves are intended to protect the individual from scattered radiation and not the useful or primary x-ray beam. Aprons and gloves of 0.5-mm lead equivalency are recommended.
Shielding of the x-ray facility’s walls, ceiling, and floor must also be considered when there is any likelihood of access to the adjacent areas by either employees or the general public. Such shielding can be in the form of lead or various thicknesses of other construction materials to achieve a specified lead equivalency.
Rotation of personnel who assist in radiography. It is advisable, if possible, to have several assistants trained to make radiographs. This then serves to divide the radiation exposure among individuals so that no one individual will bear the entire burden of cumulative exposure.
Plan of Radiographic Procedure
Plan your radiographic procedures. Radiation exposure can be significantly reduced if good technical quality is accomplished the first time. This is improved by the establishment of technique charts and careful positioning of the patient. Each time a retake is done, radiation exposure is increased.
It is not uncommon for veterinarians to purchase older, used x-ray machines. In general, older x-ray machines have an increased risk of poor and unsafe performance. It is suggested that any newly acquired older x-ray machine be inspected by a radiation physicist for radiation output and leakage potential. Generally, the state Department of Public Health inspects veterinary radiation facilities and should be contacted for inspection services.
With older machines, it is usually necessary to use a long exposure time to achieve the necessary film density. Radiation exposure time and, thus, radiation exposure dose can be reduced by using the rare earth film screen systems instead of the older calcium tungstate systems. The rare earth systems require less radiation exposure to achieve the needed film blackening.
Most radiation exposure received by those participating in making radiographs comes from scattered radiation from the patient. Anything that can be done to reduce the amount of scattered radiation produced will reduce the exposure to the personnel. A very efficient method of reducing scatter radiation is to reduce the surface area that is exposed. This is accomplished by using x-ray beam-limiting devices. These can be in the form of exchangeable cones or cylinders or best by an adjustable collimator. The cones and cylinders must be changed when a change in field size is desired. This is often not done, and instead the largest one is affixed to the tube. The adjustable collimator allows for an infinite number of rectangular to square-sized fields to be easily “dialed” in. Most adjustable collimators come with a visible light that corresponds with the primary x-ray beam. As light is illuminated onto the patient, you are able to selectively expose only the area that needs to be evaluated. This results in a reduction of scattered radiation to the operators and improves image quality as well.
The use of radiation dosimeters is strongly recommended for any person who participates in any routine manner in the making of radiographic exposures. The primary use of the dosimeters is to have a record of exposure dose. This record shows whether exposures are below the regulated dose limits. Most of the radiation safety codes indicate that a film dosimeter be worn if the individual has the potential to receive 1/10th of the annual limit. Without any monitoring, there is little way of knowing that potential. For many people, the fear of the unknown is great. An accurate documentation of the exposure received can go a long way to allay such fears. The dosimeters can help detect a leakage of radiation from a machine that may otherwise go unnoticed. Having personnel exposure records may be essential in defending a potential violation investigation. Periodic review of these exposure records can help to revise radiation practices to further reduce personnel exposure, i.e., adhering to the ALARA component of the regulations.
Many veterinary practices do not hire assistants that are trained in the use of x-rays. Thus, it is your responsibility to train these individuals as to the safe use of x-rays. It is important to discuss with them the potential deleterious effects of exposure to ionizing radiation and how this exposure can be minimized. The use of high school or junior high school students, under the age of 18, to assist in making radiographs (i.e., in the actual setting in which the exposure is made) is strongly discouraged.
What About the Employee of Child-Bearing Age or the Pregnant Employee?
There are two major issues to this concern. First, the hazard of radiation exposure to the employees of child-bearing age carries a risk of genetic effects; however, this has been estimated to be very minimal. At a low radiation dose rate, the testes are much more radiosensitive than the ovaries. The differences between the sexes are so pronounced that for practical purposes, at a low dose rate, almost all of the radiation-induced genetic burden in a population is carried by the males. To avoid radiation exposure to the gonads, proper use of shielding (i.e., wearing aprons) must be followed. When proper lead aprons are worn, the exposure to the gonadal area should be negligible.
The second issue is the pregnant employee. The most radiosensitive tissues or organs are those that have a high mitotic rate. Such is the case of the embryo and the fetus during the majority of development. The effects of radiation on the unborn child are of great concern. The classic effects are:
1. Death induced by relatively small doses before or immediately after implantation of the embryo (0 to 9 days). This results in spontaneous abortion, and the pregnancy may go unrecognized. If exposure during this time does not cause embryonic death, the embryo usually develops into a normal fetus with no residual effects.
2. During the period of organogenesis (10 days to 6 weeks), the radiation effects will usually be those of malformations. The central nervous system is particularly sensitive in the human embryo since the CNS is developing slowly throughout this period of organ development. Some temporary intrauterine growth retardation can be seen, but there is usually recovery from this. If the radiation dose is high enough, death can occur, but it will usually occur in the neonatal period.
3. Irradiation during the fetal period (6 weeks to term) can result in permanent growth retardation. There is a low risk of general malformations but a persistent high risk of CNS malformations. Death can be caused by irradiation during this phase, but the dose required gradually approaches the adult dose in the late fetal stages.
4. In utero irradiation results in an increased risk of development of fatal and non-fatal childhood cancers. Risk estimates indicate an overall risk of a 50% increase over the natural incidence. The risk appears to be greater if the irradiation occurred during the first trimester.
The safest procedure for the employee actively trying to become pregnant or who is pregnant would be to avoid any and all exposure to ionizing radiation. This may be possible by reassignment of duties. Exposure can be greatly decreased by rotation of employees who assist in radiography. Neither of these may be possible. In such instances, it is recommended that the woman be provided with a lead apron that wraps totally around the body and thus protects from inadvertent exposure to the trunk from any direction. Two personnel monitoring devices should be worn: one at the collar level and one at the waist level under the apron. The latter dosimeter is used to estimate any radiation dose to the fetus as a consequence of irradiation of the mother. Since the embryo/fetus is considered to be a member of the general public, it is restricted to a maximum permissible dose of 5 mSv for the entire 9-month gestation period and a monthly dose not in excess of 0.5 mSv.
The x-ray imaging is an invaluable tool to veterinary diagnostics. Equipment costs are affordable by any veterinary practice. Through some unfortunate early exposures to high levels of radiation, we know of serious consequences of the misuse of radiation. Federal and state standards have been set to ensure that the benefits derived from the use of x-rays are not outweighed by the risks of its use. The key to the safe use of x-rays is to put into practice as many of the safety recommendations as possible. Even though annual dose-limit standards have been established, it is imperative to practice the use of ionizing radiation in such a manner as to keep the exposure to yourself and your employees “as low as reasonably achievable.” Blatant disregard for radiation safety is intolerable!
1. Bushong, S. 1993. Radiologic Science for Technologists. Physics, Biology, and Protection. Mosby, St. Louis, MO. Pp. 521–669.
2. Hall, E. 1988. Radiobiology for the Radiologist. (3rd ed.) J.B. Lippincott Company, Philadelphia, PA.
3. Kurklis, L. 1993. Radiation safety: what you can’t see can hurt you. Topics Vet Med. 4:28–29.
4. Lavin, L. 1994. Radiography in Veterinary Technology. W.B. Saunders Co., Philadelphia, PA. Pp 21–33.
5. Morgan, J. 1993. Techniques of Veterinary Radiography. (5th ed.) Iowa State University Press, Ames, IA. Pp. 84–97.
6. National Council on Radiation Protection. 1970. Radiation Protection in Veterinary Medicine. Bethesda, MD. Report No. 36.
7. National Council on Radiation Protection. 1989. Radiation Protection for Medical and Allied Health Personnel. Bethesda, MD. Report No. 105.
8. Wolbarst, A. 1993. Physics of Radiology. Appleton & Lange, Norwalk, CT. Pp. 263–291.
9. Wootton, R. 1993. Radiation Protection of Patient. University of Cambridge Press, New York, NY.
10. Wrigley, R. and Borak, T. 1993. The effect of kVp on the dose equivalent received from scattered radiation by radiography personnel. Vet Radiol. 24:181–185.