| | Plain Film Radiography, Pregnancy, and Therapeutic Abortion RevisitedReceived 24 June 2004; received in revised form 5 May 2005 The potential harmful effects versus clinical benefit from plain film radiography remain a significant concern and topic of debate among all health professions. Topping the list of concerns are the radiation of the developing embryo in a pregnant patient and the maximum permissible occupational dose of a pregnant employee. This topic was addressed earlier by Joseph W. Howe and Terry R. Yochum in 1985. The authors felt that it would be beneficial to revisit this topic. Many women are not aware of pregnancy early in the first trimester. Occasionally, a patient or employee will have exposure to ionizing radiation only to later discover her gravidity. Moderate doses of ionizing radiation can be harmful to the developing embryo. However, the extent of damage depends on the dose of radiation received, the exposure rate, and the gestational age of the developing embryo. Despite strict adherence to the 10-day rule, a pregnant patient will inadvertently receive exposure to low-dose ionizing radiation via the primary beam. Female employees routinely involved in performing and processing plain film radiography may be exposed to secondary or scatter radiation. This review will discuss the biological effects of low-dose ionizing radiation and pregnancy in plain film radiography. A discussion of radiation protection and radiation control measures will follow to assist the physician in counseling pregnant patients exposed to radiation and the rights and responsibilities of both the employer and employee concerning occupational exposure to low-dose ionizing radiation. Radiation and Pregnancy  Radiation effects on the fetus are divided into 2 subcategories: deterministic and stochastic effects. Deterministic effects cause direct or indirect damage to the fetal cells and follow the linear threshold model; dose relationship models are summarized in Table 1. Epidemiological data and animal studies suggest that the most common deterministic effects are death, mental retardation, and microcephaly.1 The frequency of deterministic effects has a risk coefficient of 40% per Sv (100 rem).1 On the other hand, stochastic effects generally cause direct damage to fetal cells and follow a nonlinear no-threshold model. Stochastic effects are entirely random, and although the probability of an effect increases with dose, exposure to a single x-ray photon could cause an effect. The results of stochastic effects are usually malignant disease or death, and the probability of a stochastic effect increases at a rate of 5% per Sv (100 rem).1 As with all radiation effects, fetal radiation effects depend on the dose and the fetal gestational age (Table 2). For the purpose of our discussion, the fetus has 3 gestational ages: 0 to 9 days is the preimplantation age, 10 days to 6 weeks is the organogenesis age, and 6 weeks to term is the fetal age. The fetus responds differently to radiation at each gestational age. During the preimplantation age, the fetus is at the greatest risk for death or failure of implantation and spontaneous abortion. At this age, fetal death is primarily a stochastic effect. Radiation will either kill the fetus or it will not; it is an all-or-nothing response. Furthermore, if the radiation exposure does not kill the fetus, congenital abnormalities are unlikely because the embryonic cells are highly undifferentiated and are still capable of undergoing mitotic divisions. Animal experimentation demonstrates that fetal death is rare, and at a dose of 100 mSv (10 rad [0.1 Gy]), there is a 0.1% chance of spontaneous abortion.2 After 9 days of age, the fetus responds to radiation much like the adult human. It is important to note that there is a 25% to 50% normal incidence of spontaneous abortions in humans, and therefore, at doses less than 100 mSv (10 rad [0.1 Gy]), spontaneous abortion secondary to radiation is immeasurable.3 After 9 days of gestational age, deterministic effects to the fetus predominate. Radiation during organogenesis particularly early on in the process results in growth retardation. The growth retardation is usually not permanent, and the fetus will recover. Permanent growth retardation occurs most commonly during the fetal age. Based on data from atomic bomb survivors, fetuses exposed to 250 mSv (25 rad [0.25 Gy]) resulted in permanent growth effects that when followed to 17 years were 2.25 cm shorter in height, 3.0 kg lighter in weight, and a 1.1 cm smaller head diameter than children who received a dose of less than 250 mSv (25 rad [0.25 Gy]).4 Low-dose, low–linear energy transfer (LET) radiation exposure during organogenesis based on animal experimentation will likely increase the incidence of congenital anomalies by 1%, although the natural occurrence of congenital anomalies in the unexposed population is 5%. A natural incidence of 5% complicates the measurement of anomalies attributable to low-dose, low-LET radiation exposure to the point of almost negating low-dose, low-LET radiation as a causative factor. The most common reported congenital anomaly in atomic bomb survivors was microcephaly and occurred from doses ranging from 100 mSv (10 rad [0.1 Gy]) to 200 mSv (20 rad [0.2 Gy]) at a mean gestational age of 8 weeks.5 Although less common than microcephaly, mental retardation was also observed in children of atomic bomb survivors. Offspring from exposed mothers had poor scholastic performance and did poorer on IQ tests. Retrospective data revealed that the poor performance on the IQ test was a function of large doses and high exposure rates. Examiners identified a loss of 30 IQ points per Sv (100 rad [0.1 Gy]) of radiation.6 Therefore, the incidence of mental retardation is apparently linear, and most experts agree on a risk coefficient of 40% per 100 mSv (10 rad [0.1 Gy]) to 200 mSv (20 rad [0.2 Gy]). The risk is greater at 8 to 15 weeks of gestational age and decreases to approximately 10% per 100 mSv (10 rad [0.1 Gy]) to 200 mSv (20 rad [0.2 Gy]) during weeks 16 to 25 of gestational age. Thus, as the fetus matures, the incidence of mental retardation drops by a factor of 4.7 The natural incidence of mental retardation is reported at 6%, and in utero radiation likely increases the incidence by approximately 0.5% and is 3 times less likely than growth retardation.8 Malignant disease in the fetus and an association with low-dose ionizing radiation from diagnostic imaging were first reported circa 1950. Since then, epidemiological studies have identified no-threshold dose or linear relationship with the incidence of malignant disease caused by fetal exposure.9 Therefore, malignant disease is stochastic and seems more probable during the fetal period because cell division has become more specialized and differentiated, and repair of chromosomal mutations is less likely. In the younger fetus, death and spontaneous abortion are the more likely results. Current evidence suggests that a minimum dose of 10 mSv (1 rad [0.01 Gy]) to the fetus during the last trimester of pregnancy increases the risk of childhood leukemia by 40% and dose of 10 mSv (1 rad [0.01 Gy]) at any stage of pregnancy increases the risk of childhood leukemia by a multiple of 1.5.10 Above doses of 10 mSv (1 rad [0.01 Gy]), the risk coefficient increases 6% per Sv (100 rad [1 Gy]) however, the clinical significance of this figure is controversial because there is no direct evidence that identifies low-dose ionizing radiation as a sole causative agent of fetal malignant disease.11 The association of low-dose ionizing radiation from diagnostic imaging with fetal malignant disease is largely being abandoned by the scientific community. Patient Dose and Estimated Fetal Dose in Plain Film Radiography We know that harmful effects of ionizing radiation can occur at any dose; most experts agree upon a threshold of 100 mSv (10 rad [0.1 Gy]).12 The next logical question is, “What dose is plain film radiography delivering to the fetus?” Many factors ultimately determine what the final dose will be and are discussed in the next section on controlling dose. Dose in diagnostic radiography is proportional to the tube current, the length (time) of the exposure, and the square of the kilovolt peak. This is a measured value and is equivalent to the entrance skin exposure (ESE). There is considerable variation among radiographic systems; guidance levels of ESEs are available. A guidance level is a value that is typically derived from a population dose survey and represents the third quartile in the range of doses observed.13 One determinant of dose is patient thickness. As the thickness of the body part increases, the amount of radiation (ESE) incident on the patient increases, because thicker patients require a beam with more penetrating power to produce a diagnostic image. Although in thicker patients the ESE may be higher, the dose absorbed by the internal organs is ultimately lower because of attenuation and scatter in the surrounding soft tissues. Organ-absorbed dose can be calculated from the measured ESE by using a conversion factor known as the F factor. Each tissue type has its own conversion factor, and the conversion factor also differs by the amount of kilovolt peak used. The fetus can be considered an internal organ, and for practical purposes, the organ dose to the fetus is generally 0.15 times the ESE.14 Table 3, Table 4 give the ESE guidance levels and estimated dose to the fetus in the primary beam for a range of body thicknesses. Even in patients measuring upward 34 cm, the organ-absorbed dose by the fetus is in the range of 5.0 to 6.0 mSv when the fetus is in the primary beam. This is well less than the 100 mSv at which biological risk occurs to the fetus. When the fetus is not in the primary beam, the total organ-absorbed dose to the fetus is equivalent to the daily background dose received by the average American (approximately 10 mSv).14 Table 5 summarizes the serial dose of multiple examinations from the more common radiographic examinations likely performed in a chiropractic office.17 Radiation Protection and Radiation Control Measures Pregnant Patients The International Commission on Radiological Protection recommends that fetal dose incidental to diagnostic irradiation be assessed, and the need for x-ray examinations of pregnant or possibly pregnant patients where the uterus might receive radiation be judged in each individual case. The National Council on Radiation Protection in the United States affirms that the risk of fetal abnormality from doses of 50 mSv (5 rad [0.05 Gy]) or less is “…considered negligible when compared with other risks of pregnancy.” Bushong2 suggests that in accidental uterine irradiation in the early weeks of pregnancy, careful estimation of uterine dose be done and advice be given. Leppin suggests that if the absorbed dose is less than 20 mSv (2 rad [0.02 Gy]), unqualified reassurance may be given to the patient.12 He suggests further that if the dose is greater than 20 mSv (2 rad [0.02 Gy]), more accurate calculation of dose would be wise; where the dose may be 100 mSv (10 rad [0.1 Gy]) or more, a medical physicist should perform measurements with a phantom. His advice is that the patient should be given information that12 “…should include estimates of the risks with due regard to other considerations such as age and the desire of the patient to have a child.” He goes on to say that12 “…only with a dose to the uterus greater than 200 mSv (20 rad [0.2 Gy]) should the person providing advice suggest termination (therapeutic abortion) in the discussion.” In light of the material cited in the previous sections, the authors concur with Leppin. The clinician should take maximum precautions, including administering a pregnancy test, to identify potentially pregnant patients before exposing the uterus to radiation. However, there should be no hesitation to use radiography, which may include the uterus in the primary beam, if there is eminent need for diagnostic information. For examinations above the diaphragm and below the hips, abdominal shielding should be used. For examinations that place the fetus in the primary beam and will result in a fetal dose greater than 10 mSv (1 rad [0.01 Gy]) but less than 50 mSv (5 rad [0.05 Gy]), alternate imaging such as ultrasound or magnetic resonance imaging should be considered. If radiography is absolutely necessary, the clinician should document the medical necessity. The patient should be informed that the examination is needed and that the risks are minimal. Lastly, the patient should have the final decision to proceed with the examination. The patient should be required to sign an informed consent form. In the unlikely event that the examination will result in a fetal dose greater than 50 mSv (5 rad [0.05 Gy]), a formal calculation should be made by a qualified radiologist or medical physicist. The patient should be formally counseled as to the risks. This should be documented in the chart, and if the patient decides to proceed with the examination, an informed consent form signed by the patient is required. When potential and confirmed pregnant patients are encountered in practice, the use of radiographic examinations shall be considered on an individual basis. The benefit should always outweigh the risk to the patient and fetus. The risks should always be discussed with the patient. Lastly, termination of the pregnancy is rarely justified after radiation exposure to the fetus from plain film diagnostic radiography. Pregnant Employees Several international, national, and state organizations make recommendations regarding radiation protection: the International Commission on Radiological Protection, the International Commission on Radiological Units and Measurements, the National Committee on Radiological Protection and Measurements, the Nuclear Regulatory Commission, the US Department of Energy, and individual states coordinate their activities through the Conference of Radiation Control Program Directors. In the United States, the foremost organizations are the National Committee on Radiological Protection and Measurements, the US Department of Energy, and the individual state. Individuals whose occupation involves radiation in the United States have a whole-body dose limit of 50 mSv/y (5 rad/y [0.05 Gy/y]).2 This occupational dose limit does not include radiation from natural background sources and medical procedures. However, the International Commission on Radiological Protection considers the fetus to be a member of the public. In the United States, a member of the public has a whole-body dose limit of 1 mSv/y (0.1 rad/y [0.001 Gy/y]) excluding medical procedures and background radiation. The US Department of Energy has set the dose limit for the fetus of a pregnant patient, from conception to term, at 5 mSv (0.5 rad/y [0.05 Gy/y]).15 Holding the limit at 1 mSv (0.1 rad/y [0.001 Gy/y]) for fetal dose would discriminate against a woman of reproductive age the right to gain employment as a radiation worker. Title 10, Code of Federal Regulations, Evaluation and Control of Fetal Exposure,15 published by the US Department of Energy, is the implementation guide of federal occupational radiation protection measures for fetal exposure. Title 10 stipulates the responsibilities of both the employer and employee. Title 10 states that (1) the fetal dose, from conception to term, shall not exceed 5 mSv (0.5 rad [0.005 Gy]); (2) the fetus shall not exceed a dose of 0.5 mSv/mo (0.05 rad/mo [0.0005 Gy/mo]); (3) pregnant radiation workers are required to wear a dosimeter on the abdomen to ensure that fetal dose limits are not exceeded; (4) a pregnant employee must declare in writing that she is pregnant and that she must declare in writing that the pregnancy has been terminated, and (5) only when the pregnancy has been officially declared in writing is the employer responsible for providing alternate work assignments, lead apron, or other protective measures. The authors advise clinicians with radiography systems in their offices to obtain a copy of Title 10 and adhere to the stated federal guidelines contained therein. An overly cautious or under cautious radiation protection program may inadvertently be discriminatory and may violate the constitutional rights of the pregnant worker.15 The US Supreme Court ruled in “United Automobile Workers v. Johnson Controls, Inc” (USLW, 1991) that15 “…decisions about the welfare of future children must be left to the parents who conceive, bear, support, and raise them rather than to the employers who hire those parents.” Radiation Control Measures There are several ways to implement convenient and inexpensive methods of reducing radiation dose to patients. One is careful collimation to the area of clinical interest. This has a 2-fold result. Unnecessary patient exposure is drastically reduced, and by reducing the area of the x-ray beam, scatter radiation is reduced, resulting in an image with much improved contrast. A second recommendation is to use filtration above the minimum legislative guidelines (the authors recommend 4.0 vs 2.5 mm of aluminum). Added filtration removes unwanted low-energy x-rays that contribute to the patient dose and but nothing to the image quality.16 Use rare earth screens with spectral matched film. Using faster screen-film combinations reduces the time of the exposure which reduces the patient dose. Use gonad and lead apron shielding of body parts not in the primary beam. The use of careful patient positioning reduces the number of retakes. Retakes constitute unwanted and unnecessary radiation. Clinicians should have a quality assurance program for film processing. When processing conditions degrade, image quality decreases. Poor image quality can lead to a modification of radiographic technique which directly affects patient dose. If these parameters are consistently used, the risk to patients and to their progeny will be minimal from plain film diagnostic radiography. Conclusion The potential harmful effects versus clinical benefits of plain film radiography remain a significant concern and topic of debate in the chiropractic profession. Moderate doses of ionizing radiation can be harmful to the developing embryo. However, the extent of damage depends on the dose of radiation received, the exposure rate, and the gestational age of the developing embryo. Proper patient selection and the determination of the most appropriate examinations using the proper number of films consistent with the diagnostic objectives cannot be overstressed. If strict radiation protection and radiation control measures are adhered to, the risk to patients and employees and to their progeny will be minimal from plain film diagnostic radiography. Therapeutic abortion should not be considered for a pregnant patient simply because she has been radiographed through the abdomen. References 1. 1Kitchin KT. Defining, explaining and understanding hormesis. 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12. 12Howe JW, Yochum TR. X-ray, pregnancy and therapeutic abortion: a current perspective. J Am Chiropr Assoc. 1985;22:76–80. 13. 13Parry RA, Glaze SA, Archer BR. The AAPM/RSNA physics tutorial for residents: typical patient radiation doses in diagnostic radiology. Radiographics. 1999;19:1289–1302. MEDLINE 14. 14El-Khoury GY, Madsen MT, Blake ME, Yankowitz J. A new pregnancy policy for a new era. AJR Am J Roentgenol. 2003;181:335–340. 15. 15Implementation guide for use with Title 10, Code of Federal Regulations, part 835 occupational radiation protection: evaluation and control of and fetal exposure. Washington (DC): United States Department of Energy; 1994;. 16. 16Behrman RH. The impact of increased Al filtration on x-ray tube loading and image quality in diagnostic radiology. Med Phys. 2003;30:69–78. MEDLINE |
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17. 17Rosenstein M. Handbook of selected tissue doses for projections common in diagnostic radiology: United States—1988. Rockville (Md): Centers for Devices and Radiological Health; 1988;. a Department of Radiology, Logan College of Chiropractic, Chesterfield, Mo b Department of Radiology, Logan College of Chiropractic, Chesterfield, Mo c Rocky Mountain Chiropractic Radiology Center, Denver, Colo d Department of Radiology, Logan College of Chiropractic, Chesterfield, Mo  Submit requests for reprints to: William M. Ursprung, DC, Department of Radiology, Logan College of Chiropractic, PO Box 1065, 1851 Schoettler Rd, Chesterfield, Mo 63006-1065.
Sources of support: No external funds were provided for this research. PII: S0161-4754(05)00354-4 doi:10.1016/j.jmpt.2005.11.012 © 2006 National University of Health Sciences. Published by Elsevier Inc. All rights reserved. | |
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