Radiology Continuing Education Series
Course 5 of 6
Radiation Safety-Importance and Procedures
Course 1 – Physics of Radiology
Course 2 – Choosing the Appropriate Exposure Factors
Course 3 – Recording the Image
Course 4 – Poor Quality Films-Causes and Corrections
Course 5 – Radiation Safety-Importance and Procedures
Course 6 – Pros and Cons of Digital Radiography-CR vs. DR
Radiation Safety – Importance and Procedures:
The diagnostic use of x-ray if done without regard to proper radiation safety practices has the potential to cause significant harmful biological effects. The major harmful effects are to induce fetal cancers, produce genetic effects passed on to children, and to cause developmental defects in the fetus that is irradiated in utero. Major sources of radiation exposure in the veterinary workplace are the primary x-ray beam, the scattered radiation from the patient, and leakage radiation from the x-ray tube itself. The primary 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 and place x-ray shielding material between you and the source. Annual Radiation dose limits have been set for occupational workers. These are set by state health codes and the United States Nuclear regulatory Commission with the intent that the annual allowed exposure “limits the probability of occurrence of random carcinogenic and genetic effects” and to will “prevent completely the occurrence of threshold dose effects” In addition to these regulated limiting dose values, it is expected that the working conditions be such that radiation dosage is kept As Low As Reasonably Achievable (ALARA). If strict adherence is made to the safety practice of radiography, the probability to radiation induced Injury is negligible.
X-rays being a form of ionizing radiation, cause cellular damage by altering critical molecular structures, principally DNA.
The alteration in DNA can be:
1. Sublethal and be repaired causing no long term effect
2. Sublethal and upon repair induce a mutation in the cell that leads to the development of cancer to be manifested months to years later.
3. Lethal to the cell. If only a small cluster of non-dividing cells is killed there may be no
recognizable effect. If the cells killed are critical to maintaining that cell line, then the absence
of this cell line will be expressed.
The radiation effects of greatest concern to the veterinary worker are the chronic low dose effects. The veterinary worker participating in diagnostic radiography will be exposed intermittently to low doses of radiation. These low doses are unlikely to result in any acutely detectable effects. However there is a possibility of late occurring effects. There are 2 classes of late effects. Random (stochastic) and deterministic (non-stochastic or threshold) effects. Random effects are those for which any ionizing dose however small, carries with it a probability of producing the effect. The effect will either occur or nor occur. The probability that the effect will occur increases as the cumulative radiation dose increases. However the severity of the effects is not related to the dose. That is, the effect will not be more severe if the cumulative dose is higher. The random effects are the genetic effects and tumor induction.
The deterministic effects are cellular / organ effects that increase in severity with increasing accumulated exposure dose. The effects are caused by damage to an increasing number of cells and amounts of tissue. There effects are basically degenerative to the cells / organ. The best-known examples are cataract formation, organ atrophy, and tissue fibrosis. The deterministic effects have a threshold does below which the effect will not occur. Above the threshold dose the severity of the effect increases with increasing radiation dose.
So how much radiation exposure is acceptably safe? That’s a question to which there is not an entirely accurate answer. There is much scientific information to indicate that any dose of radiation poses some possibility of causing a damaging effect. Mathematical interpolation of data indicates that for the random effects, there is no known threshold dose below which there is no risk of the effect occurring. This knowledge forms the basis for the establishment of the radiation protection standards. The exposure dose levels set by these standards are intended to make the work environment such that the random effects are never likely to be a problem. The risk of the random effects still exists but if the 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.
The occupational exposure limits recommended by the National Council on Radiation Protection (NCRP) and adopted by federal and state health codes are as follows:
a. Annual Effective Dose Equivalent – Whole body ———————————–50 mSv*
b. Lens of the eye ————————————————————————–150 mSv
c. Dose Equivalent to all other body regions ——————————————-500 mSv
[red bone marrow, breast, lung, thyroid, bone surfaces, gonads, skin and extremities]
d.Cumulative Dose Equivalent —————————————————–10 mSv x age
Education and Training (under 18 years of age):
a. Annual Effective Dose Equivalent – Whole body ————————————–1 mSv
b. Dose equivalent to the lens of the eye, skin, extremities —————————-50 mSv
Embryo & Fetus:
a. Total for 9 months of gestation ———————————————————–5 mSv
b. Monthly during gestation —————————————————————0.5 mSv
a. Continuous exposure —————————————————————–1 mSv
b. Infrequent exposure ——————————————————————5 mSv
Sv = Sievert, 1 mSv = 10 – 3 Sv, 1 Sv = 100 rem
The NCRP dose limits are all subject to the concept or ALARA. 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 tolerated.
What are the keys to radiation exposure reduction?
A. Distance: The intensity of radiation exposure is dramatically reduced as the distance from the source of radiation is increased. Radiation intensity follows the Inverse Square Law – the exposure intensity varies inversely with the square of the distance from the source. This says that if you double your distance from the x-ray tube you reduce your exposure to ¼ of the emission intensity. Thus it is important to move yourself as far from the x-ray tube as possible. This forms the basis for recommending that manual restraint of animal patients be done as infrequently as possible.
Use mechanical restraining devices and / or chemical restraint as frequently as possible. If your patient is anesthetized there is in most cases no reason to handhold any part of the animal for positioning.
B. Time: 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 duration of exposure. Using exposure times as short as possible will reduce the radiation exposure received.
Rotation of personnel: Who assists in the radiography also reduces may one individual’s time of radiation exposure. If possible, have several assistants trained to produce the radiographs. This serves to divide the radiation exposure among individuals so that no one person will bear the entire burden of cumulative exposure.
Plan the radiographic procedures: Radiation exposure can be significantly reduced if good technical quality is accomplished the first time. Each time a retake exposure is done, radiation exposure is increased. Careful positioning of the patient the first time and having a successful technique chart should reduce the number of retake exposures. Do it right the first time by paying attention to details.
Supplemental radiography devices: In large animal radiography, the x-ray cassette must be held adjacent to the animal’s body. It is preferable not to do this by directly hand holding the cassette. Simple cassette holders with long handles can be made or purchased. These allow you to position the cassette and keep your hands an increased distance from the primary and scattered x-ray beam. Lead shielded gloves should still be worn when using cassette holders.
In radiography of small animal patients, it is more likely that you want to use your hands to position the patient. This is most necessary in the unsedated / unanesthetized patient. When hand held positioning is needed, your hands should never be placed within the primary beam, even if they are within leaded gloves. Roller gauze, tape, sandbags, thin rope ties, and x-ray transparent tools such as plastic salad tongs should be used.
C. Shielding: Radiation interacts with any type of material and is reduced by some amount by these interactions. However 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 low energy radiation reach the person or area being shielded. For the x-ray energies of diagnostic radiology, lead has been the shielding material of choice. If during the x-ray procedure you cannot step out of the room or behind a shielding barrier a leaded apron should be worn. In small animal practices, anyone who is manually holding the patient during the exposure should wear a lead apron and lead gloves.
Anyone who must be in the room to monitor an anesthetized/sedated patient should wear a lead apron. The same is true for large animal radiography. Likewise if you must hold the x-ray cassette aprons and gloves should be worn. Aprons and gloves are not cost prohibitive and thus easily fall into the ALARA category for achieving reduced exposure to radiation.
X-ray room shielding also needs to be addressed. The walls, ceiling and floor must be shielded when there is any likelihood of access to the adjacent areas by employees or the general public. The shielding can be in the form of added lead panels or by using an appropriate thickness of other construction materials to achieve the required lead equivalency. Consultation with a health physicist and state code requirements should be done when building or remodeling a radiography area in the practice.
D. Equipment: It is not uncommon for veterinarians to purchase older used x-ray machines. In general older x-ray machines have an increased risk of performing poorly and unsafely. It is suggested that any newly acquired older x-ray machine be inspected by a radiation physicist for radiation output and leakage potential. The State Department of Public Health inspects veterinary practices using radiography on a rotating basis. The frequency of inspection varies by state codes.
Rare earth film-screen systems are strongly recommended in all practices. Any reduction in radiation exposure needed to blacken the film results in reduction of exposure to personnel.
Most radiation exposure received by personnel comes from the scattered radiation produced by the patient. Anything that can be done to reduce the amount of scatter produced will reduce the exposure to the personnel. A very efficient method of reducing the scatter radiation is to reduce the surface area of the patient irradiated by the use of collimation. The light beam collimators are extremely useful in the regard as they cast a light over the area of primary beam exposure. Neither gloved nor ungloved hands should be placed in this lighted area.
E. Personnel exposure monitoring: Radiation dosimeters are recommended for any person who participates in any routine manner in the making or radiographic exposures. The primary usefulness of the dosimeter is to have a record of your exposure dose. This record shows that the manner in which you carry out your radiation practice is in fact resulting in exposures well below the regulated dose limits. Most of the codes require that a dosimeter be worn if an occupationally exposed individual has the potential to receive 1/10th of the annual whole body effective dose equivalent (5 mSv or 5 rem). However without any monitoring, there is little way of knowing that potential. For many, the fear of the unknown is great. An accurate documentation of the exposure received can go a long way to allay such fears. Periodic review of the exposure records can help you reassess and revise your radiation practice to further reduce personnel exposures.
High school or junior high/middle school students frequently assist in veterinary practices. While this is an excellent recruiting tool for the veterinary technician or veterinary professions, it is recommended that these students not be directly involved in positioning patients or the x-ray machine during the taking of radiographs. The regulated dose limits for students under 18 years was established for the controlled training of radiology technologists and allied health profession technicians who use radiography in their profession.
The employee of childbearing age or pregnant employee:
There are two major issues for these employees. First, the hazard of radiation exposure to the employee of childbearing age carries a risk of genetic effects. However this had been estimated to be very minimal. At low radiation exposures the testes are much more radiosensitive than the ovary. The differences between the sexes are so pronounced that for all practical purposes almost all of the radiation induced genetic burden in a population is carried by the males, when low radiation exposure doses are involved. To avoid radiation exposure to the gonads proper use of shielding (i.e., wearing leaded aprons) must be followed. When proper lead aprons are worn the exposure to the gonad 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 activity of the embryo and the fetus during the majority of its 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 (0-9 days). This results in spontaneous abortion and the pregnancy generally goes unrecognized. If the exposure during this time does not cause embryonic death, the embryo usually develops into a normal fetus with no residual or late developing effects.
2. During the period of organ genesis (10 days to 6 weeks), the radiation effects are those of malformation. The central nervous system is particularly sensitive throughout this period of organ development. 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 likely occur in the neonatal period versus induced abortion.
3. Irradiation during the fetal period (6 weeks to term) can result in permanent growth retardation. There is a low risk of general malformation 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 fatal and non-fatal childhood cancers. Risk estimates indicate an overall risk of a 50% increase over the natural occurrence. The risk of childhood cancer induction 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. Her 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 issued 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 monthly dose not in excess of 0.5 mSv.
In summary the key to the safe use of x-rays is to put into routine everyday practice as many of the safety recommendations as possible. Even though annual dose limits 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 As Low As Reasonably Achievable.
References available upon request.