Radiation Protection For Radiologic Technologists.
According to the International Commission on Radiologic Protection (ICRP), "the primary aim of radiologic protection is to provide an appropriate standard of protection for man without unduly limiting the beneficial practices giving rise to radiation exposure."[1,2] Radiation protection is of particular importance to radiologic technologists because recent information from the Committee on the Biological Effects of Ionizing Radiation (BEIR) suggests a greater risk of radiation injury (3 to 4 times higher) than previously estimated. Data accumulated from Hiroshima and Nagasaki atom bomb survivors contributed to the ICRP's recommendation for a reduction in annual dose limit for radiation workers from 50 mSv to 20 mSv.
Uncertainty regarding the potential effects of exposure to the low levels of radiation generally used in radiology prompted the radiologic community to increase its focus on quality control as an important method to reduce the effective radiation dose. Studies are under way to determine the effects of exposure to low-level radiation. In addition, the introduction of new imaging technologies such as magnetic resonance (MR) brings new concerns to light, including the bio-effects of exposure to magnetic fields and radio-frequency waves.
Efforts to reduce dose and protect both patients and health care workers are important for several reasons:
* Evidence suggests that radiation is harmful not only to the exposed individual, but to future offspring as well.
* Diagnostic radiation is the primary source of artificial radiation exposure.
* No dose of radiation is considered safe.
* While the majority of radiologic procedures use low doses of radiation, some procedures deliver high doses.
* The radiologic technologist is responsible for the total amount of radiation delivered to a patient. Therefore, a comprehensive understanding of radiation protection is essential.
The Biologic Effects of Radiation
The bioeffects of radiation can be grouped into 2 categories: somatic effects and genetic effects. Somatic effects are the direct results to the exposed individual; genetic effects are the consequences to future offspring. Somatic effects can be either stochastic (mutational) or nonstochastic (cell-killing). Nonstochastic effects include burns, epilation (hair loss) and damage to the immune system and lens of the eye. Also known as deterministic effects, nonstochastic effects are associated with a threshold dose and increase in severity as the dose increases. Stochastic effects are random effects due to low-dose damage to single cells. If stochastic effects later are followed by clone formation or fertilization, the mutants are not recognized by the immune system and cancer or leukemia can result. Stochastic effects occur randomly and there is no threshold dose.[4,5]
Radiation is deposited randomly within a cell and damage to cells related to irradiation is sporadic. The effects are not necessarily permanent and injuries often heal. Change or damage due to radiation is often indistinguishable from other types of trauma. Biologic changes due to radiation damage can occur within hours (such as death after a nuclear explosion) or years later (as in the case of subsequent generations affected by damaged germ cells). Recent data suggest that the immune system plays a complex role in the etiology of cancer and leukemia, and that, all in all, survivors of the atomic bombs showed surprisingly few harmful effects. These findings suggest that a re-evaluation of the effects of low-level radiation may be in order.[4,5]
Damage to DNA can be direct or indirect. Indirect damage is injury to other parts of the cell, such as water. Free radicals can be formed due to the interaction between radiation and water. The most damaging of these is believed to be OH-, which is implicated in two thirds of cases of damage to mammalian cells. The initial impact of radiation occurs within 10 to 18 seconds; however, overt symptoms can take days, months or years to manifest.
Studies show that the critical site for radiation-induced cell death is the nucleus, implicating DNA as the most vulnerable part of the cell. DNA consists of a double helix composed of sugar-base pairs. There are 4 base pairs: adenine, guanine, cytosine and thiamine. These bases are paired with complementary sugars: cytosine with guanine and adenosine with thymine. The order of these base pairs determines the genetic code. Irradiation can result in breaks in these pairs, causing cell death or mutations. Strand breaks can be single-strand breaks (SSB) or double-strand breaks (DSB). Although both types of strand breaks can be repaired, SSB are less damaging because the opposite portion of the strand dictates the base. Cell death is more common with DSB because in double-strand breaks entire segments of DNA can be lost, resulting in cell death.[6,7] (See Fig. 1.)
[Figure 1 ILLUSTRATION OMITTED]
Because DNA is the building block of chromosomes, damage to DNA can result in chromosomal mutations. These mutations can be the result of duplication, deletions, translocations or inversions of genetic material. Each of these is associated with particular malignancies. For example, Burkitt's lymphoma, acute promyelocytic leukemia and ovarian cancer are associated with translocation. Deletions are associated with small cell lung cancer, neuroblastoma, retinoblastoma and Wilms tumor. Breast cancer is associated with gene amplification.[6,7]
Mechanism of Cell Death
Irradiated cells can die because of reproductive failure. This occurs when the irradiated cell undergoes mitosis (cellular division). Oncogenes are mutated genes believed to be involved in the transformation of a normal cell to a malignant cell. Normally the antagonists, proto-oncogenes, are present in mammalian cells and regulate cell growth. It is believed that radiation can liberate destructive oncogenes through chromosomal changes such as point mutations, deletions and translocations.[6,7]
Suppressor genes, believed to suppress the development of tumors, may be affected by malignant changes caused by chromosomal alterations. Changes to DNA can result in mutations that eliminate or alter the activity of the suppressor genes.[6,7]
Syndromes Related to Irradiation The primary effect of acute whole-body exposure to high doses of radiation is death within days or weeks of exposure, depending on the dose (the higher the dose, the sooner death occurs). Death is associated with a particular set of symptoms known as "total body syndrome." Listed in order of severity, from highest to lowest dose, are 3 distinct radiation-induced syndromes: cerebrovascular syndrome, gastrointestinal syndrome and hematopoietic (bone marrow) syndrome.
Dose-related syndrome data are based on studies of mammals. Humans are the most sensitive mammals to radiation, and human thresholds are generally at the lower end of the spectrum. However, organ failure occurs in increasing dose order in all mammals. The first organ system affected is bone marrow, followed by GI syndrome and finally the CNS syndrome.
* Bone marrow syndrome. The bone marrow or hematopoietic syndrome occurs in mammals with acute exposures of 2 to 10 Gy. Death occurs weeks after exposure and is the result of the death and depletion of stem cells in bone marrow and other blood-forming organs. In this syndrome, the higher the dose, the quicker death will occur. The human threshold dose for this syndrome is [is less than]1 Gy. In most mammals, deaths begin to occur approximately 10 days post-exposure, peak at 2 weeks and are completed within 30 days. In humans, deaths begin later, peak at 30 days and continue up to 60 days postexposure.
* Gastrointestinal syndrome. This syndrome is associated with acute whole-body exposure in mammals to doses between 10 and 100 Gy. The time of death does not appear to be related to dose. Death is the result of damage to and depletion of stem cells in the gastrointestinal mucosa. Death caused by GI syndrome is faster than that associated with bone marrow syndrome because cells in the gastrointestinal tract have a shorter life span than bone marrow cells. In humans, the threshold for this syndrome is 6 Gy. Without medical intervention, these individuals will die within 3 to 4 days after exposure. To date, no human has survived a dose greater than 10 Gy, regardless of medical treatment.
* Central nervous system syndrome. With doses greater than 100 Gy, there is a direct correlation between dose and time of death. Death caused by CNS syndrome results from the failure of both the central nervous and cardiovascular systems. It is considered a "superlethal" dose, in that there is no possibility of survival. Death results from a buildup of pressure inside the skull, which literally explodes.
The "lethal dose 50," abbreviated [LD.sub.50], is defined as the dose at which 50% of the population will die. In humans, [LD.sub.50] for radiation exposure is speculative. Based on data from Hiroshima and Nagasaki, the human [LD.sub.50] is estimated at 3 to 4 Gy. As mentioned previously, the threshold for bone marrow syndrome is [is less than] 1 Gy. In the Chernobyl disaster, approximately 200 employees were exposed to doses [is greater than] 1 Gy and developed hematopoietic syndrome. Of these, 35 employees experienced severe bone marrow failure and 13 died. The remainder survived with medical support.
Radiation Exposure In Utero
Most of the data on fetal exposure to radiation is extrapolated from animal studies. Due to the high rate of mitosis occurring in the developing embryo, it is hypothesized that the fetus is highly susceptible to the effects of radiation. These effects are divided into 3 categories: lethal effects, congenital malformations and growth disturbances. The categories are referred to as the "classic triad of embryologic syndromes."
* Lethal. These effects take place after implantation of the fertilized ovum, or any time during intrauterine development. Death can occur before or at the time of birth.
* Congenital malformations. These effects are expressed after birth and are the result of exposure during organogenesis. Specific deformities can be traced to intrauterine exposure at particular times.
* Growth disturbances. These effects can occur either before or after birth. They may not be associated with any other deformities.
The type of effect is based on the dose and timing of gestational exposure, and radiation effects can be traced to a particular period of gestation. Gestation is divided into 3 periods:
* Preimplantation, which is the time between fertilization and implantation.
* Organogenesis, which is the period of major organ formation.
* Fetal stage, which is the growth period of newly formed organs.
Exposure during preimplantation results in a high incidence of death caused by rapid rates of mitotic cell division. Extremely low doses (0.1 Gy) are fatal to mice embryos. Embryos that survive have few abnormalities, with the exception of exencephaly (ie, protrusion of the brain outside the skull).
Because organ formation occurs on certain days, radiation exposure during organogenesis can result in organ-specific abnormalities. The most common abnormalities are seen in the brain and skeletal systems and in behavior. While the incidence of prenatal deaths decreases with exposure during organogenesis, the number of deaths at birth increases.
Exposure during the fetal stage is associated with a decreased incidence of obvious abnormalities or deaths. High doses are required to cause death or gross malformations. Exposure is more commonly associated with later problems such as cancer and other diseases.
Again, it should be reiterated that these data are extrapolated from nonhuman studies. Human data on the effects of in utero exposure are derived primarily from survivors of the atomic bomb blasts on the cities of Hiroshima and Nagasaki at the end of World War II. The most common abnormalities observed in offspring born to survivors were microencephaly and mental retardation.[10-14] The most severe effects on brain development were observed in fetuses exposed between 8 and 15 weeks and 16 to 25 weeks after conception. Microcephaly is the most common side effect associated with in utero exposure after the first trimester. A clear dose-response relationship was observed with doses from 0.1 Gy to 0.19 Gy. Mental retardation is believed to be associated with an exposure of 0.1 Gy to 0.4 Gy. It is estimated that exposure to 1 Gy results in a 30-point drop in IQ.[10,11]
Latent effects of radiation are known as stochastic effects. These include cancer and hereditary defects that will be passed on to future generations. Stochastic effects are not dose related and occur randomly. Some examples are the high incidence of cancers in survivors of the Hiroshima and Nagasaki bomb blasts, an increased incidence of leukemia in patients irradiated for ankylosing spondylitis, the increased incidence of leukemia in radiologists and an increased incidence of thyroid cancer in children who were irradiated for ringworm or an enlarged thymus.
The Atomic Bomb Casualty Commission (ABCC), predecessor of the Radiation Effects Research Foundation (RERF), was established in 1947 to conduct long-term, comprehensive epidemiological and genetic studies of the survivors of the atomic bombs. Today this study still depends on the voluntary cooperation of several tens of thousands of survivors of the bombings of Hiroshima and Nagasaki. An in-depth follow-up study of mortality in the study population of 120 000 people, including bomb survivors and controls, has continued since 1950. The study of tumor incidence was initiated through linkage with a tumor registry system in Hiroshima and Nagasaki in 1958. In the same year, biennial medical examinations of 20 000 individuals began. Follow-up studies also have been conducted on in utero-exposed people and first-generation offspring of the survivors.
Based on these studies spanning half a century, leukemia and cancers associated with A-bomb radiation are known to be higher among bomb survivors than among the unexposed. Among A-bomb survivors, radiation cataracts, hyperparathyroidism, delayed growth and development and chromosomal aberrations also occur more often. However, to date no evidence exists of genetic effects in the children of A-bomb survivors.
There are no data available to establish the absolute risks of cancers associated with radiation exposure. It is also not known whether the effects are cumulative. With the exception of radiation-induced leukemia, risk is unknown. Models to estimate risk have been unsuccessful because they tend to either overestimate or underestimate the risk associated with low doses.
Leukemia is the most common cancer associated with radiation exposure. Radiation exposure between the ages of 7 and 12 results in an increased incidence of acute lymphatic leukemia. Adults exposed to radiation are at an increased risk of developing acute and chronic myeloid leukemia. The incidence of leukemia in survivors of the Hiroshima and Nagasaki atomic bomb blasts was 3 to 5 times higher than in the general population.[16,17]
Data regarding radiation-induced breast cancer are derived from Japanese survivors and a group of Canadian women exposed to diagnostic and therapeutic radiation. Doses in these cases ranged from 0.04 Gy to 0.2 Gy. In the Canadian study, the incidence of breast cancer was clearly dose related. This study surveyed tuberculosis patients who underwent multiple fluoroscopic examinations of the chest, mastitis patients given radiotherapy and survivors of the atomic bombs.
The data suggest that the risk is greatest for those exposed as adolescents, although there is risk with exposure at any age. There was a linear dose-response relationship in each group of patients. Fractionation did not seem to reduce risk, nor did the time since exposure (even after 45 years of observation). The interval between exposure and the clinical appearance of radiogenic breast cancer may be mediated by hormonal or other age-related factors, but is unrelated to dose. Age-specific absolute risk estimates for all studies are remarkably similar. The best estimate of risk among American women exposed after age 20 is 6.6 excess cancers per 100 000 women.
The increased incidence of lung cancer in patients exposed to radiation is known from the Japanese atomic bomb survivors and from miners exposed to radon. However, the absolute risk is difficult to determine because lung cancer is associated with other factors such as cigarette smoking.
Other cancers associated with radiation exposure are thyroid cancer and osteosarcoma. While other organs may be vulnerable, the breast, lungs and thyroid seem to be the most susceptible. In examining records of the Japanese survivors, the majority of early cancer cases were leukemia; however, an increasing incidence of solid tumors has occurred over time and those cases now outnumber leukemia cases.
Radiation-induced cancers are believed to be the result of damage to or loss of "checkpoint" genes (ie, genes that control cellular proliferation), the liberation of oncogenes or the loss of suppressor genes. Cancer can be related to one or more of these effects.
Hereditary Effects of Radiation
When radiation affects germ cells, mutations can occur that can be transmitted to future offspring. Mutation can occur at the gene level or the chromosome level. Each chromosome consists of 2 genes, so mutations at the gene level may not appear for several generations. Mutations in chromosomes can result in more profound and immediate changes. This is particularly true with genetic material in which even point mutations can result in severe abnormalities.
Once again, it should be noted that the majority of radiation-induced genetic changes are based on non-human studies. The available human-based information is derived from studies of Japanese atomic bomb survivors and their offspring. Evidence indicates that most mutations are harmful, that any dose of radiation can cause genetic changes and that there is a linear dose response (ie, the higher the dose, the higher the number of mutations). Although mouse data are used to extrapolate the risk of human exposure to radiation, humans seem to be less sensitive to the genetic effects of radiation than other mammals.[4,5,22]
Sources of Radiation Exposure
Radiation sources can be natural or artificial. (See Fig. 2.) The majority of exposure (82%) is due to natural sources; however, there are numerous artificial sources of radiation that should be minimized if possible. Of artificial radiation sources, medical exposure is the most important.
Radon gas is a major source of radiation exposure to the general public. Radon is a byproduct of the natural decay of uranium 238. It is an odorless, colorless gas that can be removed with proper ventilation. Radon arises from naturally occurring radionuclides in granite, sandstone, limestone and wood. Granite contributes twice as much as stone, which contributes twice as much as sandstone and wood. Radon is present in varying concentrations in all soils.
Radon enters buildings from soil, water, natural gas and building materials. It is a problem because its decay particles can be breathed into the bronchial epithelium and result in an increased incidence of lung cancer. These short-lived breakdown products, termed "radon daughters," include alpha-emitting solids that may be deposited in the lungs. Evidence clearly links lung cancer risk in miners with high exposure to radon daughters. The level of risk associated with the much lower but chronic doses received in buildings is difficult to establish. By some extrapolations, radon daughters may be responsible for a significant number of lung cancer deaths. The existence or extent of synergy with smoking is unresolved. Local conditions can cause high levels of radon in some buildings, and measures that reduce indoor radon are potentially valuable.
Other natural sources of radiation include cosmic radiation, earth sources and internal sources. Cosmic radiation includes solar and galactic radiation. Exposure to cosmic radiation increases with altitude, doubling with each 2 km above sea level. Earth radiation includes radiation from the air, terrestrial radiation, radiation from buildings, radionuclides in food and drink and endogenous radiation (ie, radiation from within the human body). Radionuclides that occur naturally in food and drink include P-40, strontium 89 and 90 and cesium 137, among others. Luckey demonstrated that foods irradiated in the sterilization process do not become radioactive.
Exposure to artificial radiation can be occupational, medical or public. Occupational exposure includes all exposures that occur during or because of employment. For example, radiologic technologists may be exposed to radiation during x-ray examinations and fluoroscopy. Medical exposures are incurred because of medical diagnosis or treatment. Public exposures are due to other factors, such as nuclear fallout and consumer products.
Sources of artificial radiation include industry, nuclear fallout and medical procedures. Examples of industrial radiation include exposure from mining, nuclear power plants and consumer products. Fallout is the radiation produced because of nuclear testing and chemical explosions in nuclear plants. As an example, radiation released from the accident at Chernobyl was greater than that related to both atomic bomb explosions. Nuclear power plants also emit radioactive wastes as a byproduct of production. Accidents account for the release of additional radioactive particles and gases into the atmosphere.
Consumer products also use radioactive materials, including smoke alarms, illuminated watches and computer monitors. When used up to 7 hours per day, this can result in an annual radiation dose of up to 0.3 mGy. In children, radiation exposure to the eye from computers may be more problematic than gonadal exposure.
Of all sources of artificial radiation, however, medical radiation is the greatest, accounting for 11% of the total. As such, it is important that radiation protection recommendations be strictly adhered to and that medical exposure to radiation benefits the patient.
Medical exposure occurs in one of the following ways: as primary radiation that passes through the patient, as leakage (ie, radiation that escapes from the protective housing of the x-ray tube) or as internal or external scattered radiation (ie, scattered by the patient due to Compton interactions). Of these, leakage and scatter radiation are particularly important to the radiologic technologist. Leakage and scatter radiation exposure occur when radiation protection controls, such as standing inside the control booth and wearing a lead apron, are not followed.
Sources of Radiation from Medical Imaging
Potential imaging sources of radiation exposure include radiography, fluoroscopy, mammography, CT, nuclear medicine, ultrasound and MR. There are 2 types of medical radiation: ionizing and nonionizing. Ionizing radiation is used in radiography, fluoroscopy, mammography, CT, nuclear medicine and radiation therapy, whereas nonionizing radiation is used in sonography and MR. The source of radiation exposure varies among imaging techniques. For example, nuclear medicine uses radionuclides, whereas radiation therapy uses x-rays, neutrons, photons and heavy ions to irradiate tumor cells. In MR, radiowaves are the source of radiation exposure.
* Conventional radiography. The x-ray tube is the main source of ionizing radiation in radiography. The x-ray tube consists of 2 electrodes (cathode and anode) that are placed opposite one another in a vacuum in a glass container. X-rays are produced when electrons in the filament of the cathode are accelerated to high speeds and jump across the vacuum, hitting the target of the anode. A radiation beam of both long and short wavelength x-rays is emitted from the tube. While the short (high-energy) x-rays are able to penetrate the patient's body, long wavelength x-rays generally are absorbed by the patient, thus increasing the total dose. Researchers continue to seek ways of eliminating these low-energy photons.
There are several factors involved in determining the amount of radiation exposure received by a patient during radiography. Some of these factors can be controlled by the radiologic technologist, including kilovoltage (kVp), distance, beam area, grids and screens. Other factors are not within the technologist's control, such as wave form and the thickness and density of the patient and tabletop. A major factor affecting exposure is the size of the x-ray beam delivered to the patient. The radiologic technologist can reduce radiation exposure by collimating the beam to the area of investigation.
Technique factors (eg, kVp, milliamperes [mA] and exposure time) are important in reducing exposure. Density is determined by length of exposure and mAs. The amount of kVp used determines contrast. All of these affect the dose to the patient. Kilovoltage affects beam penetration, while amperes affect the amount of radiation hitting the patient. Techniques using high kVp with low mAs result in less radiation exposure to the patient because more radiation is transmitted through the patient rather than absorbed by the patient's tissues. On the contrary, low kVp and high mAs result in more exposure because more radiation is absorbed.
* Fluoroscopy. Fluoroscopy is similar to radiography although exposure can last for several minutes to allow the radiologist to interpret the television image. During the procedure, the patient also is exposed to spot imaging. After the procedure, overhead radiographs are taken. Compared with conventional radiography, in which one small area or areas are examined, fluoroscopy requires that the beam travel over a larger area of the patient. In summary, there are 3 important considerations in fluoroscopy: the beam may cover a wider area of the body, longer exposure times are required due to the nature of the exam and additional radiation is incurred as a result of spot films and overhead images.
* Mammography. Due to the soft tissue of the breast, breast imaging has different physical properties. The image is produced using low kVp with a molybdenum target. As such, mAs is higher. This makes radiation protection extremely important because of the high dose used and also because the breast is sensitive to radiation.
* Computed tomography. CT uses a mathematical formula to construct 3-D computer images based on single x-ray images. In CT, a highly collimated beam of x-ray radiation passes through very thin slices of the body. The x-ray tube rotates around the patient's body to image cross-sectional slices. Instead of film, these x-rays fall on special detectors. The dose, which depends on the type of examination, is affected by kVp, mAs, filtration, collimation and detector efficiency.
* Magnetic resonance imaging. MR involves exposure of the patient to 3 fields: a static magnetic field, a pulsed radiofrequency electromagnetic field and transient magnetic field gradients. The patient is placed in a strong static magnetic field, which causes magnetic nuclei to align with this field. The patient then is exposed to radiofrequency waves. When the radiofrequency waves are shut off, the body emits radiofrequency waves of the same frequency. These radio waves are collected by a receiver and digitized for computer processing, which results in an internal image. The third field, the transient magnetic field gradient, is an external magnetic field designed to torque the static magnetic field for spatial excitation. Possible biologic effects of MR are due to static magnetic fields, transient magnetic field gradients and exposure to radiofrequency waves.
Principles of Radiation Protection
Every framework for radiation protection is based on 3 basic principles as outlined by Bushong in 1993:9
* Keep exposure time to a minimum. This is particularly important because dose is directly related to time. If time is tripled, dose is tripled.
* A protective shield must be placed between the source of radiation and exposed individuals. This flexible shield usually is made of lead-equivalent material and is placed over the patient's reproductive organs.
* The distance between the patient and the source of radiation is to be kept as large as feasible. As distance increases, exposure decreases, according to the inverse square law. If the distance is doubled, the dosage is reduced 4-fold.
Bushong noted that several technical and procedural factors can influence radiation exposure. Perhaps the foremost and simplest to use is correct patient positioning. Positioning is important to minimize or eliminate the need for additional images. Technique also is important, including the following:
* Optimizing kVp and mAs to reduce dose and achieve acceptable images.
* Using appropriate film-screen combinations.
* Assuring that a minimal number of views are taken.
* Protecting the patient's reproductive organs and other vital organs not under investigation.
* Assuring proper collimation of the x-ray beam.
* Observing proper protection procedures, such as use of a lead-equivalent apron and standing in a protective control booth during imaging.
* Ensuring that the patient understands instructions regarding breathing and motion.
* Checking that the patient is free of foreign objects and immobilized, if necessary.
* Observing the patient throughout the procedure.
As discussed previously, radiation effects can be either stochastic or deterministic. Examples of stochastic effects are cancer and genetic defects. Stochastic effects are not dose dependent. There is a possibility of their occurrence with each exposure. Deterministic effects include cataracts, hemopoietic changes and problems with fertility. The severity of deterministic effects increases with increasing radiation dose.
It is possible that any dose of radiation may have a stochastic effect and as such, no dose of radiation is considered safe. Therefore, it is imperative that the benefits of any clinical use of radiation outweigh the risks. It is part of the radiologic technologist's responsibility to explain the risks and benefits of the radiologic examination to the patient. In many cases, the potential benefits greatly outweigh risks. For example, CT of the spine in a patient with a suspected cervical fracture is essential to determine the exact nature of the injury and avoid possible paralysis. In this case, exposure to radiation is far less dangerous than misdiagnosis of the injury.
However, not all cases are as clear. In some cases, potential benefit may be small or negligible, and the associated risk may be greater than the benefit. For example, breast cancer screening in young women (ie, younger than 35) without symptoms is considered low benefit and high risk.
It is important that medical exposure to radiation be used appropriately and at as low a dose as possible. According to the NCRP, the goal of radiation protection is "to prevent the occurrence of serious radiation-induced conditions (acute and chronic deterministic effects) in exposed persons and to reduce stochastic effects in exposed persons to a degree that is acceptable in relation to the benefits to the individual and to society from the activities that generate such exposure."
There are 3 general principles of radiation protection, also known as the first triad of radiation protection: justification of a procedure, optimization of protection and radiation dose limits. (See Fig. 3.)
[Figure 3 ILLUSTRATION OMITTED]
* Justification with net benefit. When new imaging techniques or radiation therapies are introduced, it is important that the benefits and risks be compared with existing procedures. For example, the risks and benefits of CT and MR vs traditional x-ray procedures must be evaluated in terms of radiation exposure and clinical efficacy.
* Optimization/ALARA. The idea of trying to keep the radiation dose as low as possible dates back to 1954. Data collected by scientists involved in the Manhattan Project encouraged the NCRP to recommend that radiation levels be kept "as low as reasonably achievable," hence the acronym ALARA. This concept now is commonly referred to as optimization of radiation protection.
* Dose limits. Dose limitation deals with the concept of radiation dose received by an individual on an annual basis. These limits are intended to prevent deterministic effects and to decrease the possibility of stochastic effects. Dose limits have been established for occupational, medical and public exposures.
Dose limitation is achieved in part through the proper use of radiation, which leads to the second triad of radiation protection: time, shielding and distance. (See Fig. 4.) The goal of the second triad is to keep the radiation dose delivered to the patient as low as possible.
[Figure 4 ILLUSTRATION OMITTED]
* Time. Exposure is a function of the exposure rate and time. In other words, if the exposure time is short, the total dose delivered is small. It is essential that the radiologic technologist minimize exposure times.
Distance. The distance from the source of radiation to the patient is important. Exposure to radiation follows the inverse square law, which states that radiation exposure decreases inversely as a square of the distance. As the distance increases, the intensity decreases. Every time the distance is doubled, the exposure is decreased by a factor of 4. If the distance is tripled, exposure decreases by a factor of 9, and so on.
Maximizing distance from the source of radiation is important not only for the patient, but for the radiologic technologist as well. Another limitation is source-to-image receptor distance (SID). To obtain optimal images, the appropriate SID must be observed. If the correct SID is not used, a second exposure may be required, increasing the patient's radiation exposure. It is important that the technologist move as far away as possible during a radiologic procedure to minimize exposure to scatter radiation. However, there are limitations to this distance, such as instances in which the technologist must remain near the patient to carry out other requirements. In these cases, the last corner of the triad -- shielding -- comes into play.
* Shielding. Certain substances, such as lead, concrete and plastic, attenuate or reduce the intensity of x-rays. These materials can be placed between the radiation source and the patient to reduce exposure. Types of individual shields include lead-equivalent aprons; thyroid, gonadal and eye shields; shields used to protect health care personnel and x-ray tube shields.
Lead aprons or gonadal shields protect areas of the body that are not under investigation from exposure to radiation. (See Fig. 5.) Personnel shielding involves the use of protective devices to shield the health care professional. These include lead aprons, gloves, thyroid shields and eye shields. (See Fig. 6.) In addition, portable lead-plastic shields also may be used to protect health care workers from scatter radiation during procedures such as angiography or fluoroscopy in which personnel must remain near the patient to carry out the procedure. (See Fig. 7.)
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In addition to shields worn on the body, other types of shields also are important, such as room shielding and x-ray tube shielding. Radiographic examination rooms are lined with lead to protect others from scatter radiation. Windows in the control booth are made of a transparent lead-plastic, permitting the examiner to watch the procedure from the control booth. (See Fig. 8.) Portable lead-plastic shields also can be used to minimize exposure to scatter radiation for the technologist and for members of the patient's family who wish to remain in the room during the examination. (See Fig. 9.) The x-ray tube housing is lined with lead because x-rays scatter in all directions.
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An important component of radiation protection is radiation detection and measurement. Radiation dosimeters are used to measure radiation. There are 4 methods used to measure radiation: ionization, the photographic effect, luminescence and scintillation. In clinical radiology, ionization and scintillation are used to measure radiation output. Personnel dosimetry is used to monitor the cumulative exposure to occupationally exposed personnel. Monitoring is performed with personnel dosimeters such as pocket dosimeters, film badges and thermoluminescent dosimeters. The dose is measured at B-month intervals, referred to as a time-integrated dose. The dose is extrapolated as representative of a total-body dose for that period of time. Dosimeters have a dose limit of 0.1 mSv to 0.2 mSv (10 to 20 mRem).
A pocket dosimeter has an ionization chamber, an eye piece, a transparent scale, a hollow charging rod and both a fixed and moveable fiber. A standard charge is administered to the charging rod using a dosimeter charger. This charge causes the moveable and fixed fibers to repel each other. However, as exposure to radiation increases, the fibers lose the ability to repel and move closer to each other. Using the eyepiece, the moveable fiber can be seen through the transparent scale and radiation exposure can be gauged. (See Fig. 10.)
[Figure 10 ILLUSTRATION OMITTED]
Although film badges have been in use for approximately 60 years, they largely have been replaced by pocket dosimeters. While film badges are relatively inexpensive and easy to use, they have limitations. Film badges can detect radiation levels [greater than] 0.1 mSv, but are inadequate for recording lower levels of radiation. Furthermore, film badges are subject to fogging, which is caused by high temperatures or exposure to light. As such, film badges have a life span of only about 4 weeks. In addition, the film in these badges must be chemically processed and compared to a standard test film to determine the level of exposure.
Thermoluminescent dosimeters (TLDs) are based on the theory of thermoluminescence (ie, that certain materials are known to emit light when heated). (See Fig. 11.) One thermoluminescent material is lithium fluoride (LIF). When an LIF crystal is exposed to radiation, some electrons become trapped in higher energy levels. When the crystal is heated, these electrons return to their normal energy levels, giving off light in the process. This light, which can be measured, is proportional to radiation dose. The TLD can measure exposures as low as 5 mR, whereas pocket dosimeters can measure exposures as low as 10 mR to 200 mR. The film badge cannot measure exposures less than 10 mR. The primary disadvantage of the TLD is cost.
[Figure 11 ILLUSTRATION OMITTED]
To be accurate, the personnel dosimeter must be worn appropriately. In the case of standard x-rays, where a lead apron is not worn and the examiner remains in the control room, the dosimeter should be worn on the trunk at waist level facing anteriorly or on the upper chest at collar level. This location provides a good estimate of total-body exposure and partial exposures to other regions of the body. In examinations in which the technologist must remain close to the patient (eg, fluoroscopy or angiography) a lead apron is worn. In such cases, 1 or 2 dosimeters may be used. One is worn beneath the apron; this is believed to give an estimate of the amount of radiation exposure to internal organs. Another may be worn outside the apron on the collar to estimate dose to the head, neck and thyroid.
In addition to personnel dosimetry, radiology facilities must be surveyed to check the status of radiation safety, to ensure equipment is working properly and the department is a safe environment for patients and personnel. This survey consists of 5 stages: investigation, inspection, measurement, evaluation and recommendation. Quality assurance is an important component of this survey. It is essential that quality control procedures be established to assure that all equipment and procedures meet quality standards.
Establishing and maintaining a successful radiation protection program requires cooperation from several different departments within an organization. This includes administration, which is in charge of budgeting, policies and procedures; the radiation safety committee, which includes a radiation safety officer; and other personnel involved in radiologic procedures. Guidelines for establishing and maintaining radiation protection programs have been developed by several radiation protection agencies such as the ICRP, NCRP and RPB.
Radiation protection would be pointless without meticulous record keeping. Accurate records are important for several reasons. These include documentation for patients and occupationally exposed personnel, establishing compliance with regulatory requirements, protection against liability, epidemiologic research and the creation of evaluation mechanisms.
Recommended Dose Limits
As we continue to gain a better understanding of the potential effects of radiation on the human body, dose limits have decreased to meet ALARA standards for protecting the patient from deterministic and stochastic effects. There are different dose limits for different categories of individuals, including occupationally exposed workers, adult patients, pediatric patients, pregnant women and fetuses.
Almost immediately after the discovery of x-rays, it became apparent that high doses of radiation caused injury to the skin. A variety of terminology was used to describe what were at the time considered "acceptable doses" of radiation. The first term, used from 1900 to 1930, was "skin erythema dose," which described the dose that was strong enough to cause skin reddening (600 rem or 6 Sv). The ICRP was formed in 1928, and soon thereafter the term "tolerance dose" was used to describe a dose below which no radiation damage was presumed to occur.
In the mid 1950s, the term "maximum permissible dose" (MPD) was introduced. Like its predecessor, this term was used to describe the highest dose that would not injure the patient. It was believed that doses below the (MPD) would result in no deterministic or stochastic effects, while doses at this level were associated with minimal risks that were acceptable in light of the perceived benefits. The formula used to calculate MPD was MPD = 5(N - 18) rads, where N = age in years. It was assumed that individuals 18 years or younger would not be trained to perform radiologic examinations. Thus, someone 40 years of age could be exposed to 22 rem. This formula has been replaced by a cumulative dose equivalent of 10 mSv (1 rem) x age in years.
The term "dose equivalent limit," also called the "effective dose equivalent," was introduced in the early 1970s and replaced MPD. Dose equivalent limit was replaced in 1990 by "effective dose," which remains in use. This term is used to relate the dose to a particular part of the body to the entire body. New data from survivors of the Hiroshima and Nagasaki bombings indicate that the radiation risk was 3 to 4 times greater than previously indicated. Consequently, the ICRP dose limit for occupationally exposed individuals was reduced from 50 mSv to 20 mSv (5 rem to 2 rem). In addition to total-body dose limits, there are also organ-specific dose limits and dose limits for the general public.
The effective dose limit for the entire body is 20 mSv per year or 100 mSv averaged over 5 years. However, in no case should dose exceed 50 mSv in a single year. Exposure to different regions of the body is defined separately. The effective dose refers to exposure to the entire body. The ICRP recommends that pregnant women receive no more than 2 mSv during their pregnancy to minimize radiation risk to the fetus. The effective dose for the public is considerably lower.
Data from numerous studies indicate that recommended dose limits for occupationally exposed individuals are being met. In a large Canadian study using data from 1978 to 1988, effective dose was investigated for radiologic technologists. In the study, no one received the maximum annual dose of 50 mSv and the vast majority (79%) received a dose equivalent of 0.2 mSv, while 13% received an annual dose equivalent between 0.2 and 0.5 mSv. In a subsequent study investigating occupational exposure over an 11-year period for radiologic technologists, the mean annual exposure decreased during the latter portion of the study, possibly indicating the effectiveness of radiation protection procedures, such as proper shielding, distance and dose limits, advocated as part of the ALARA principle.
Radiation protection is integral to the practice of radiology. Excessive exposure to radiation is known to increase the risk of diseases such as cancer and can result in stochastic effects that may not appear for years after exposure. It is the responsibility of radiologic technologists to follow ALARA standards to minimize potential radiation risk to their patients, coworkers and themselves. At present, due to the possibility of stochastic effects, there are no known safe doses of radiation. It is therefore essential that radiographic procedures be performed only when the risk of the procedure is outweighed by the perceived benefit. The radiologic technologist can achieve these goals through careful collimation, proper shielding, using the lowest dose of radiation possible without compromising the image, minimizing exposure time and minimizing patient movement to achieve optimal images.
Tremendous strides have been made in the field of radiation protection, including improved dosimetry and imaging techniques, shielding materials that can be conformed to fit a particular patient and mobile lead shields. These developments have enhanced the profession's ability to meet ALARA principles.
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Julliana Newman, B.A., ELS, is assistant clinical professor in the Department of Continuing Education in the College of Pharmacy at the University of New Mexico in Albuquerque. Ms. Newman has written on a wide range of topics for radiologic technologists, pharmacists, physicians and nurses. She is the winner of several regional, national and international writing awards. Ms. Newman lives in Albuquerque with her husband and 3 children.
Reprint requests may be sent to the American Society of Radiologic Technologists, Communications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.
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