Models of Radiation Exposure:
Implications for the Chiropractic Practice
Part 1: Models of Radiation Exposure–Implications for the Chiropractic Practice
In the past few years, patient the linear-no threshold model of radiation exposure has come under scrutiny. New research has indicated evidence for a linear quadratic threshold, ask no threshold, or hormetic model. Because of this new research, national regulatory organizations are conducting major research projects to further explore these models. These models will be explored in detail. Even with new models, certain chiropractic x-ray series can approach the limits of a safe range. A comparison of common x-ray procedures is included as well as a discussion on the varying doses between x-ray systems used in the field. Further, recommendations for decreasing dose are discussed.
Before exploring what we know about radiation exposure, a definition of terms is necessary. A dose of 1 rem is equivalent to a dose of 1 roentgen of X- or gamma radiation, which is equivalent to a dose of 1 rad from X-, gamma, or beta radiation.1 In the metric system, 1 Gray (Gy) equals 100 rad (or 1 cGy = 1 rad) and 1 Sievert (Sv) equals 100 rem (1cSv = 1 rem). A dose of 1 Sv equals a dose of 1Gy. Even though the metric system is the scientific preference, since most U.S. government regulations are in rems and mrems, these units will be used throughout the paper.
Recent research has caused the scientific community to reassess what we know about low levels of radiation exposure. In 1999, the U.S. Department of Energy initiated a 10 year, 200 million dollar research program to assess the health risks of low dose radiation. One of the major goals of the project is to discover the existence of thresholds and at what levels they may occur.2 The Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation, publishers of the BEIR reports, has also initiated a new study to more accurately assess the risk of low dose radiation with their BEIR VII report currently underway.3Any new radiation protection standards will likely be based on the results of these projects.
We know that ionizing radiation damages DNA. Most of the time, cells are able to repair the damage or breaks in a strand. If the cell repairs the DNA improperly, then a mutation has occurred. If the mutated cell survives and is able to reproduce, its progenitors may become malignant. While significantly more DNA damage is caused by natural metabolic processes during the day, ionizing radiation is unique in that it is able to cause breaks in both strands of DNA, greatly increasing the risk for permanent damage.4
High doses of radiation can cause burns, gastrointestinal disturbances, cancer, and death5. The Linear Low Dose Theory of radiation exposure assumes that the damage done to an individual is directly proportional to the amount of radiation exposed. There is also an assumption that the amount of radiation damage one gets is accumulated over a lifetime.
Theodore Rockwell likens this with the idea that since 100 tablets of aspirin will harm someone, then ingesting on aspirin a week for 100 weeks will cause the same harm.6 While laws governing radiation exposure are based on the Linear Low dose theory most of the scientific community has declared it too simple and inaccurate. The Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation (publishers of the BEIR reports), the United States Department of Energy, as well as the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) are all either advocating or exploring other models of radiation exposure including the linear quadratic model, the threshold model, and the hormesis model.7,8,9
The linear quadratic model, an extension of the linear model, suggests cancer occurrences at low doses have a direct and linear relationship to the dose; at high levels the incidence of carcinogenesis rises because there is more of a chance for a double strand break of the DNA.10 The Threshold model states that below a certain limit there is no detrimental effect on the organism. The Hormetic or U-shape response theory is based on evidence that at low level of radiation, there is actually a beneficial effect on the organism and a harmful affect comes happens at larger doses.11
The linear quadratic model, like the linear model, assumes that since there is destruction of DNA at all levels of radiation, even the smallest doses are a potential cancer risk. It must be stated that even proponents of this theory state that there is a negligible risk below a certain level.12 Proponents of the threshold model point to evidence that health effects from radiation exposures have only been found at over 10 rem.13,14,15,16 One of the new understandings of cell metabolism and repair that is cited is that oxidation of DNA is a regular and natural occurrence. Some estimate that a mammal cell will undergo one million DNA injuries from non-radiation oxidative sources per day. In contrast, the average natural background radiation to U.S. citizens of 360 mrem per year will likely only cause DNA injury in a cell twice per year. These injuries are quickly and efficiently repaired through natural processes.17 Proponents of the Linear Quadratic theory argue that ionizing radiation causes double strand breaks that are much harder to heal. However, the body will usually repair even this double strand break with a half time of 0.5 to 2 hours.18 In fact, exposure to ionizing radiation activates many DNA repair, and tumor suppressor genes.19 This allows the body to either repair mutated DNA or destroy the cells that were unable to be repaired. Low levels of radiation will also stimulate the destruction of free radicals for several days after the exposure, and stimulate T-cell activity for several weeks (these cells attack premalignant and malignant cells). These mechanisms are seen in exposures of 0.1-0.5 Gy (10-50 rem) but are not seen in exposures greater than 0.5 Gy (50 rem). In addition cell apoptosis or programmed cell death is also seen as a way to cleanly eliminate damaged cells. Apoptosis is also seen in higher radiation dose exposures.20 In reality two opposite effects on an organism is occurring simultaneously with low levels of radiation. One is a damaging, cell destroying, and mutagenic effect from DNA exposure and the other is a stimulatory effect to combat damage from all oxidative processes. The big question is how these processes are interacting with one another. The idea that the anti-oxidative, anti-cancerous, stimulatory effect is more pronounced is called hormesis.
The radiological hormesis model states that low levels of radiation of radiation exposure cause an overall stimulatory effect to an organism’s immune system and leads to less cancer and longer life. This hypothesis was accepted in the scientific community in the early and mid 1900’s when it was demonstrated in over 100 published experiments on plants, microbes, insects and animals as well as clinical data.21 There are many reasons that radiation hormesis was marginalized during the 1940’s and onward,* included are the difficulty in designing experiments, lack of a proper definition of the concept or an understanding of the biological explanation of its mechanism of action. Finally, organized criticism from the scientific community managed to hinder funding and prevent research from being published. It was not until the mid 1980’s with the discovery of the concept of adaptive response to radiation that the model of radiation hormesis was given attention in the scientific community.22 Since then there have been several journal issues dedicated to the issue of hormesis as well as two books.23,24,25,26,27,28,29
There is considerable evidence to suggest a hormetic effect takes place. In the controversial shipyard worker study, non-nuclear workers had a similar overall death rate than the general population, in contrast to nuclear workers who had a significantly less mortality rate. The only difference, though not significant, was in the case of leukemia and other hematopoietic diseases that were slightly higher in the workers with a lifetime cumulative dose greater than 0.5 rem.30
In one of Pollycove’s literature reviews, he cites several studies that support the hormesis model. In one Canadian breast cancer study, researchers tracked death from breast cancer among 32710 women that underwent several fluoroscopy studies between 1930 and 1951. Results showed that breast cancer reduced dramatically for patients with a cumulative exposure of 150 mGy (15 rem) and was still reduced in patients with a cumulative exposure of 250 mGy (25 rem). In another study, cancer mortality in Ural villages is compared following exposure from an explosion at the Soviet Mayak plutonium processing facility in 1957. The 7,852 villagers were tracked based on inhaled plutonium exposures of 496 mGy (49.6 rem), 120 mGy (12.0 rem), and 40 mGy (4 rem) groups. Compared with unexposed villages, mortality from cancer tumors was reduced by 28%, 39%, and 27% for each group.31
Mine et al analyzed a 100,000 person database of Nagasaki A-bomb survivors. They revealed that mortality for men was slightly less or the same as control population with doses below 200 cGy (200 rem). Women were only slightly lower in the 1-49 cGy range (1-49 rem), while being the same or slightly higher for the 50-199 cGy (50-199 rem) range. While cancer mortality was higher in the 50-199 rem groups, death from non-cancerous causes was reduced.32
Van Wyngaarden and Pauwels in their literature review cite a 1983 Canadian study that showed that workers at nuclear power plants had a 58% lower cancer mortality rate than the national average. They also cite a Chinese study comparing two areas with differing background radiation and cancer mortality. One group of 74,000 lived in an area of high background radiation of 2.28 mGy/year (228 mrem/yr), the other group of 78,000 lived in an area of lower background radiation of .95 mGy/yr (95 mrem/yr). While the cancer mortality was insignificant for general populations, the 40-70 year old age bracket had a significant decrease in cancer mortality for the higher background radiation group.33
Luckey cites several studies stating radiologists practicing after the 1930’s, when safeguards were implemented, had a longer life span for other physicians. He states that they received doses between 5 and 50 mSv/yr (.5-5 rem/yr).34 In addition, he cites studies comparing residents of Kerala, India to other provinces. He states that Kerala residents receive 4-13 mGy/yr (.4-1.3 rem/yr) of cosmic and terrestrial radiation. This region has the lowest infant mortality and the longest lifespan than any other region of India.35
Whether the threshold or hormetic model is more accurate is less important than defining the actual threshold or point above which causes more damage than benefit. The above data would indicate that a lifetime accumulative dose of 200 rem would still be safe. However, different tissues are affected differently from ionizing radiation. The BEIR V report looks into detail at many tissues, they conclude that while there was an increased incidence in leukemia in Hiroshima and Nagasaki, it was not significant below 0.4 Gy (40 rem). In the Bikini atoll islands, there was a significant increase in thyroid disease and cancer in doses above 0.3 Gy (30 rem). The pediatric brain has been observed to be susceptible to tumor induction from doses between 0.15-0.4 Gy (15-40 rem). An increase in bladder cancer was observed among British patients with a mean dose of 0.31 Gy (31 rem). In addition, there is an increased chance of leukemia for children who were exposed to x-rays in utero at 5-50 mGy (0.5-5 rem).36
Doody et al conducted a retrospective study on 5573 women with scoliosis who received a mean breast radiation dose of 10.8 cGy (10.8 rem) from an average of 25 full spine radiographs. Increases were seen in individuals who received most of their exams just before and during puberty. Increases were even seen in the 1-9 cGy (1-9 rem) group. They found a 70% increase in mortality from breast cancer, based on comparisons with the general U.S. population of non-scoliotic white women. Reasons why this contradicts the previously mentioned study of Canadian women and fluoroscopy include the fact that reproductive history was not taken into account. The authors state that nulliparity, a risk factor for increased breast cancer, was more common among women with scoliosis. In addition, scoliosis is a disease process and in itself may have an increased risk factor for breast cancer. No comparison was done with other cancers or mortalities.37,38
Clearly, ionizing radiation has a destructive effect on a cellular as well as an epidemiological level. In addition, there is also a survival response on the part of the organism to combat this injury. This survival response is extremely efficient at repairing DNA damage to the organism. Because of this repair mechanism, the idea of lifetime accumulative dose being equal to an acute exposure is invalid. Further, there is evidence that at low levels there is probably an overall beneficial effect on the organism. However, there are increases with certain diseases at levels below 50 rem. While the U.S. Nuclear Regulatory Commission annual limits for adults working in the nuclear industry are 5 rems (5000mrems),39 the Occupational Safety and Health Administration (OSHA) states that radiation dose for workers shall be no more than 3 rems per 3 months, with an annual dose not to exceed the formula 5(N-18) rems, where “N” equals the workers age in years at their last birthday.40 These regulations are based on the ALARA principles and the idea that there is no safe level of radiation exposure. Based on the presented material, a safe exposure level should be set at a conservative average of 5 rem per year with no more than a 10 rem exposure for a given year. Doses above 15 rem per year should only occur in emergency or life threatening situations. Even at 15 rem per year, cancer or an early death is not assumed, only the increased possibility of cancer or early death. There are many people with exposures in excess of 200 rem that lived long and cancer free lives. Considerations should be made for fetuses, immunocompromised individuals, children, and radiosensitive tissue. In part two, implications for the aforementioned facts will be looked at for chiropractic practice.
Radiation from medical procedures is the largest man made source of radiation exposure to the world’s population. These medical procedures are going to continue to increase in use.41 One CT exam is twenty times the effective dose as the average radiation exposure to U.S. citizens. While CT and fluoroscopy are among the highest dose per exam, some chiropractic procedures can rival them in yearly dose. Unfortunately, there is a huge discrepancy in doses between x-ray units. Fortunately, this means that there is plenty of room to decrease dose in most systems. In part one of this paper, models of dose responses to humans were explored, including that of the hormetic model. The hormetic model states that at low levels, the natural repair response to damage from ionizing radiation actually is stronger than that of the radiation; as a result the organism experiences an overall beneficial effect from the dose. Finally, part one concluded a safe x-ray dose as being at or below 5 rem per year average, not to exceed 10 rem in a single year. This paper will try to quantify these values in comparison to both both common x-ray exposure, and other common radiation exposures. Because of the great variance of dose between x-ray units, a discussion on the state of x-ray in the United States as well as recommendations to reduce x-ray exposure will be included.
Ionizing radiation is a reality of life on earth. The average exposure to background ionizing radiation for Americans is 300-360 mrem. This can vary greatly depending on the where a person is living. Exposure for people living on the Atlantic coast are 55mrem, while people living in northeastern Washington state are exposed to 1200 mrem per year. Most of this radiation exposure is from the sun and radon exposures.42 A round trip cross-country airline flight will expose a person to 2-5 additional mrem.43 Total average yearly dose for smokers is 1300 mrem per year.44
Table 1 compares common radiological procedures. The International Commission on Radiation Protection (ICRP) is currently drafting a policy paper to provide reference levels for diagnostic procedures. This paper is based on data collected from other governmental, national, and international organizations. Data combined for the ICRP section of Table 1 includes the Institute of Physical Sciences in Medicine, International Atomic Energy Agency, European Commission, American Association for Physicists in Medicine, and National Radiological Protection Board. Only 3rd quartile values or values from the 75 percentile were used. Values were very consistent between studies, however, not all studies performed the same exam. When there was only one study with exam performed, that value was used, when values were not matched, the value represents an average of studies done.45 The Nationwide Evaluation of X-ray Trends (NEXT) is a U.S. based national study that periodically measures the x-ray exposures on a standardized phantom on randomly selected x-ray units in the country. More than 300 x-ray units throughout the country were tested during the 1995 abdominal and lumbosacral spine study. Based on these findings, regulations have been incorporated in several states. Findings in this column represent 75 percentile values.46 Kereiakes and Rosenstein published the Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-ray in 1980. The values are averages based on a study by the FDA, published in 1970. Although the material is quite dated, the values are comparable to the other third quartile and 75 percentile studies, making it useful for views not included in the more recent studies.47 With an increase in radiology use in medicine, it is possible that a patient will receive exposures from additional studies from other health care practitioners.
For a comparison with other medical procedures, Table 2 has been included. Data from Handbook of Radiation Doses and ISRP also comes from sources not mentioned previously. This table is included to show the risk of exposure with other exams.
These figures are for single views. Most x-ray series average three views. A three view lumbar series would have an exposure of 4.5 rem. A 5-view lumbar series would have an exposure of 6.5 rem. NEXT study lumbar AP exam, 75 percentile, is 0.5 rem meaning that 75% of the x-ray units delivered a dose of 0.5 rem or less on standardized phantoms. Unfortunately, this means that 25% of the x-ray systems were delivering more than this amount. In fact, the one facility in the lumbar NEXT study recorded a 2 rem exposure. In contrast, 25% of the facilities delivered an exposure of .25 rem. If a similar reduction was made for the whole series, a 5 view lumbar series would be 3.25 rem, well under the 10 rem maximum per year. Four percent of the facilities surveyed were able to deliver exposures less than .1 rem; this is fully one fifth of the exposure from the 75th percentile. If a similar exposure was able to be made with the full series the exposure would only be 1.3 rem. This would be inconsequential in regards to patient total dose. 48,49 In reality this is one of the only series that chiropractors in the general practice should worry. Other procedures for concern are full spine and upper cervical pre- and post x-rays. Lateral full spine dosages have not been studied. It would be prudent to use the lateral lumbar as an estimate, giving a series dose of around 4 rem. In addition, the dose given for an AP full spine exposure of 0.280 rem is probably low compared to an actual 75th percentile. It is likely that it would be closer the AP Lumbar exposure of 0.5 to 0.9 rem. Follow up at six months would give a cumulative dose of 9 rem. The initial pre and post upper cervical series (5 total views) would give a dose of 2.2 rem, additional views at .5 rem a piece would allow 3-7 total post series (depending on whether additional vertex views are used) to stay around 5 rem. However, with simple procedures to limit x-ray exposure, these exposures could easily be reduced in half or more. To find out dose on a particular x-ray system, contact a health physicist or contact Nuclear Associates of Carle Place, NY.50 They have a portable ion chamber that is reusable (with charger) and would give rough estimates for any radiographic series of x-rays. A 5 rem unit and charger sell for less than $300.**
Joel Gray’s article Optimize X-ray Systems to Minimize Radiation Dose has excellent and inexpensive procedures to minimize dose. First, by increasing the half-value layer in the tube from the FDA minimum of 2.3 mm of aluminum to 3.2 mm of aluminum, radiation exposure to the patient can be reduced 35%. This will have no effect on film density or contrast. Increasing kVp from 70 to 80 or from 60 to 70 and adjusting mAs can reduce the patient exposure by 50% with minor changes to contrast. An increase from 60 to 80 kVp can reduce the patients’ exposure by 75%. Using matching film, screens, developer, and processor from the same company can reduce exposure up to 50%.51 Further, the Conference of Radiation Control Program Directors recommends increasing film speed to at least 400 speed and updating technique charts. They also recommend checking to see if the grid is installed properly and that the proper film type is used.52 Guebert’s Essentials for Diagnostic Imaging is a good reference for improving film quality and reducing exposure. Some major considerations are light leakage (to cassette, processor and darkroom), dirty screens, and low power coming from outside electrical transformer.53 Clearly with little effort, major reductions to exposure can be accomplished.
In conclusion, it is evident that new models of radiation exposure are needed to accurately define the risk of radiation exposure. However, even with new models of exposure, certain common chiropractic radiological procedures can be considered a risk with a projected safe level of 10 rem maximum per year. This is especially true with the wide variable of exposure between x-ray systems used in the field. Finally, with a small effort to quality control, significant reductions in exposures can be made to yield an effective zero harm for radiological procedures in chiropractic.
X-RAY EXPOSURE FOR COMMON EXAMS (REM)
|View||ISRP (averaged)54||NEXT Studies5556||Handbook of Radiation Doses (FDA)57|
|Skull PA or AP (Including Nasium, Vertex or Base Posterior Views)||
|Full Spine AP||
|Lumbar AP or PA||
X-RAY EXPOSURE FOR OTHER EXAMS (REM)
|CT Head Routine||
|CT Chest Routine||
|CT Abdomen Routine||3.5|
|CT Pelvis Routine||
|CT Vertebral Trauma||
|Fluoroscopy Normal Mode||
|Fluoroscopy High Level Mode||
|Urinary Tract (with or without contrast)||1||
|Radiation Therapy (neoplasm) ***||
Calabrese E, Baldwin L, Radiation Hormesis: the Demise of a Legitimate Hypothesis Human and Experimental Toxicology 2000; 19: 76-84.
*** Total doses are divided up into many treatments over 5 to 6 weeks.
1 Occupational Safety and Health Administration, Ionizing Radiation 1910.1096, Regulations (Standards – 29 CFR, 1996; 1910.1096(a)(7).
3 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR VII), Health Effects of Exposure to Low Levels of Ionizing Radiation, Time for Reassessment?, Washington: National Academy Press, 1998; 2-4.
4 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR VII), Health Effects of Exposure to Low Levels of Ionizing Radiation, Time for Reassessment?, Washington: National Academy Press, 1998; 14-16.
5 Guebert G, Essentials of Diagnostic Imaging, St. Louis: Mosby, 165-168.
6 Rockwell T, Our Radiation Protection Policy is a Hazard to Public Health, The Scientist, 1997; 11: 9.
7 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR VII), Health Effects of Exposure to Low Levels of Ionizing Radiation, Time for Reassessment?, Washington: National Academy Press, 1998; 33-41.
9 United Nations Scientific Committee on the Effects of Atomic Radiation, Annex D. Medical Radiation Exposure, UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes. New York: United Nations, 2000; 2.
10 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation, Health Effects of Exposure to Low Levels of Ionizing Radiation: BEIR V, Washington: National Academy Press, 1990; 21.
11 Luckey T, Radiation Hormesis, Boca Raton: CRC Press, 1991; 98.
12 National Council on Radiation Protection and Measurements, The Application of ALRA for Occupational Exposures: NCRP Statement No. 8, Bethesda: NCRP, 1999; 2.
13 U.S. Department of Energy, DOE Handbook: Radiological Worker Training, Washington, DOE: 1998; 32.
15 American Nuclear Society, Health Effects of Low-Level Radiation: Position Statement 41, La Grange Park: 1999; 2.
16 U.S. Environmental Protection Agency, Estimating Radiogenic Cancer Risks Addendum: Uncertainty Analysis, Washington, EPA: 1999, 21.
17 Feinendegen L, Pollycove M, Biological Responses to Low Doses of Ionizing Radiation: Detriment Versus Hormesis Part 1, Journal of Nuclear Medicine, 2001; 42 : 22N.
18 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR VII), Health Effects of Exposure to Low Levels of Ionizing Radiation, Time for Reassessment?, Washington: National Academy Press, 1998; 17-18.
19 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR VII), Health Effects of Exposure to Low Levels of Ionizing Radiation, Time for Reassessment?, Washington: National Academy Press, 1998; 21-22.
20 Feinendegen L, Pollycove M, Biological Responses to Low Doses of Ionizing Radiation: Detriment Versus Hormesis Part 1, Journal of Nuclear Medicine, 2001; 42 : 24N.
21 Calabrese E, Baldwin L, Radiation Hormesis: Its Historical Foundations as a Biological Hypothesis Human and Experimental Toxicology 2000; 19: 68-75.
22 Calabrese E, Baldwin L, Radiation Hormesis: the Demise of a Legitimate Hypothesis, Human and Experimental Toxicology 2000; 19: 76-84
23 Human and Experimental Toxicology, 2001; 20(6).
24 Critical Reviews in Toxicology, 2001; 31(4-5).
25 Human and Experimental Toxicology, 2001; 20(3).
26 Human and Experimental Toxicology, 2001; 19(1).
27 Journal of Applied Toxicology, 2000; 20(2).
28 Health Physics, 1987; 52(5).
29 Luckey T, Radiation Hormesis, Boca Raton: CRC Press, 1991.
30 Matanoski G, Health Effects of Low-level Radiation in Shipyard Workers, Washington, DOE; 1991; 334-335.
31 Pollycove M, Biological Responses to Low Doses of Ionizing Radiation: Detriment Versus Hormesis, Journal of Nuclear Medicine 2001, 42 : 28N-29N.
32 Mine M, et al. Beneficial effect of A-bomb Radiation. International Journal of Radiation Biology 1990: 56 ; 1036-1038.
33 Van Wyngaarden K, Pauwels E, Hormesis: Are Low Doses of Ionizing Radiation Harmful or Beneficial? European Journal of Nuclear Medicine 1995; 22: 482.
34 Luckey T, Radiation Hormesis, Boca Raton: CRC Press, 1991; 183-184.
>35 Luckey T, Radiation Hormesis, Boca Raton: CRC Press, 1991; 14, 182-183.
36 Committee on Health Effects of Exposure to Low Levels of Ionizing Radiation, Health Effects of Exposure to Low Levels of Ionizing Radiation: BEIR V, Washington: National Academy Press, 1990; 242, 290, 311, 352-354.
37 Doody M, et al, Breast Cancer Mortality After Diagnostic Radiography. Spine 2000; 25: 2052-2063.
38 Pollycove M, Biological Responses to Low Doses of Ionizing Radiation: Detriment Versus Hormesis, Journal of Nuclear Medicine 2001, 42 : 28N.
39 U.S. Nuclear Regulatory Commission,10 CFR Subpart C–Occupational Dose Limits for Adults, NRC Regulations (10 CFR), 1995; §20.1201.
40 Occupational Safety and Health Administration, Ionizing Radiation 1910.1096, Regulations (Standards – 29 CFR, 1996; 1910.1096(b)(2).
41 United Nations Scientific Committee on the Effects of Atomic Radiation, Annex D. Medical Radiation Exposure, UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes. New York: United Nations, 2000; 295.
42 U.S. Department of Energy, Office of Civilian Radioactive Waste Management, Facts About Radiation –2000:2-3. DOE publication DOE/YMP-0403.
43 U.S. Department of Energy, Office of Civilian Radioactive Waste Management, Facts About Radiation –2000:2-3. DOE publication DOE/YMP-0403.
44 U.S. Department of Energy, DOE Handbook: Radiological Worker Training, Washington, DOE: 1998; 22.
46 Conference of Radiation Control Program Directors, Next 1995 Abdomen and LS Spine X-Ray Data, 1995. Pamphlet.
47 Kereiakes J, Rosenstein M, Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Ray, Boca Raton: CRC Press, 1980; 193.
48 Gray J, Lower Radiation Exposure Improves Patient Safety. Diagnostic Imaging; 20 : 61-64.
49 Conference of Radiation Control Program Directors, Next 1995 Abdomen and LS Spine X-Ray Data, 1995. Pamphlet.
50 Guebert G, Essentials of Diagnostic Imaging, St. Louis: Mosby, 56.
51 Gray J, Optimize X-ray Systems to Minimize Radiation Dose. Diagnostic Imaging; 20 : 61-64.
52 Conference of Radiation Control Program Directors, Methods to Reduce Exposure, Pamphlet, 1988. Online www.crcpd.org/PDF/7-88_QA.pdf.
53 Guebert G, Essentials of Diagnostic Imaging, St. Louis: Mosby, 165-168.
55 Conference of Radiation Control Program Directors, Next 1994 P/A Chest X-Ray Data, 1994. Pamphlet.
56 Conference of Radiation Control Program Directors, Next 1995 Abdomen and LS Spine X-Ray Data, 1995. Pamphlet.
57 Kereiakes J, Rosenstein M, Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Ray, Boca Raton: CRC Press, 1980; 208-209.
59 Smith A, Shah G A, Kron T, Variation of patient dose in head CT. British Journal of Radiology 1998;71:1298.
60 Conference of Radiation Control Program Directors, Next 1995 Abdomen and LS Spine X-Ray Data, 1995. Pamphlet.
61 Kereiakes J, Rosenstein M, Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Ray, Boca Raton: CRC Press, 1980; 208-209.
62 Conference of Radiation Control Program Directors, Next 1992 Mammography X-ray Data, 1992. Pamphlet.
63 Kereiakes J, Rosenstein M, Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Ray, Boca Raton: CRC Press, 1980; 208-209.
64 Mettler F, Upton A, Medical Effects of Ionizing Radiation 2nd ed., Philadelphia, W. B. Saunders: 1995; 49.