 Advertisement | ||||||||
 | ||||||||
|
||||||||
 |
 |
 |
 |
 |
 |
 |
||
|
| Review |
Motherisk Program, Department of Pediatrics (Ratnapalan, Koren), Hospital for Sick Children, Toronto, Ont.; and the Israel Poison Information Center (Bentur), Rambam Health Care Campus, The Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel
 |
Abstract |
|---|
 Top Abstract Radiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
Case 2: A 40-year-old woman arrives at the emergency department with acute pleuritic chest pain and shortness of breath. The patient is 15 weeks' pregnant. To rule out a pulmonary embolus, should you perform a ventilation-perfusion scan or computed tomography (CT) angiography?
Case 3: A 37-year-old woman who is 20-weeks pregnant reports persistent pain in her right upper thigh that is exacerbated after jogging. The pain is localized and has no radicular properties. A physical examination shows localized tenderness on the right hip joint without any abnormal neurological findings. You suspect hip bursitis but want to order a radiograph to rule out osteoarthritis with degenerative changes. The patient is nervous about the possible effects of the radiation on her baby; how would you counsel her?
Many women are exposed to radiation from diagnostic imaging procedures before they know they are pregnant or because it is necessary during a known pregnancy. These patients often question the potential effects of the radiation on the developing fetus, and they may perceive radiation as being very harmful.1–4 A realistic and informed approach to counselling these patients can minimize the anxiety felt by both patients and health care providers.
Humans are exposed to both background and man-made sources of radiation. For the purpose of this review, "radiation" refers to ionizing radiation (e.g., x-rays, 
-rays, radionuclides) and not to other forms of radiation (e.g., long-wavelength electromagnetic waves such as radar, microwaves, diathermy and FM radio waves).
Ionizing radiation in the form of x-rays and -rays are short-wavelength electromagnetic rays. Low-energy photons in x-rays and high-energy photons in -rays can alter the normal structure of a living cell both directly and indirectly. The direct mechanism involves disruption of the atom's structure to produce an ionized compound and a free electron. The indirect mechanism involves radiolysis of water and generation of free radicals.5
Ionizing radiation can cause two types of effects.5 First, loss of tissue function (deterministic effect) can occur. This type of injury has tissue-specific thresholds and may involve various repair and compensatory mechanisms. If the radiation dose is fractionated, there is greater repair and proliferation, hence there is greater tolerance of the tissue to the radiation. Second, damage can occur from a single random modification in a cell component (e.g., DNA) (stochastic effect). There is no dose threshold for stochastic effects.
Since invention of the x-ray in 1895, ionizing radiation has been harnessed for diagnostic and therapeutic purposes. With the atomic bombings in World War II, the world became aware of the serious potential carcinogenic, teratogenic and mutagenic effects of ionizing radiation. Despite the increase in concern about the health effects of ionizing radiation, the medical use of x-rays has continued to grow. In 1980, the number of radiographs performed in the United States was 225 million, including about 80 million fertile men and women.5 In 2006, the estimated total number of radiographs in the US was about 330 million.6 The fetus is exposed to unavoidable (background) radiation from cosmic rays, terrestrial radiation from ground and building and naturally occurring radioisotopes that are inhaled or ingested. The total fetal dose from background radiation sources is 0.1 rad or less during the entire pregnancy.5
|
Radiation effects |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
The dose of the ionizing radiation required to cause specific developmental effects depends on the stage of gestation (Table 1). When assessing radiation exposure during pregnancy, it is important to tally the cumulative dose delivered to the patient. Although a single procedure is usually not associated with reproductive risk, this may not be the case for multiple procedures (e.g., radiographs received by trauma patients).
|
|
Dose limits |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
The US National Council on Radiation Protection states that the risk of induced miscarriages or major congenital malformations in embryos or fetuses exposed to doses of 5 rads or less is negligible compared to the spontaneous risk among nonexposed women.8 Spontaneous risk includes a 15% chance of having a spontaneous abortion, 3% risk for major malformation and 4% risk of fetal growth restriction.5,7,9,10 The Radiation Safety Committee of the US Center for Disease Control and Prevention recommends that unborn babies of laboratory workers should not be exposed to more than 0.5 rad cumulatively from all sources of radiation during the entire gestational period.8 Typically, occupational radiation exposure is measured by tags or dosimeter badge. At present, no such devices are required for pregnant women undergoing diagnostic radiation.
|
Estimated exposure |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
|
Direct exposure of a fetus to radiation occurs when the fetus is located within the field being imaged. Indirect exposure is due to scattered radiation from maternal tissues. The fetal dose depends on the radiation dose delivered and the distance between the fetus and the area being imaged. Radiation exposure dose is inversely related to the distance (to the power of two) from the radiation source. A lead shield may reduce indirect exposure, but internal scatter in the mother will allow some radiation to reach the fetus.12 If thinly layered bismuth radioprotective latex or leaded garments are used to shield the mother's radiosensitive organs, the radiation dose that reaches the fetus can be cut by 50%.13
Fetal doses resulting from radiological examination of the mother's skull, head, neck, chest and extremities are extremely low (< 0.01 rad) because of the relatively low maternal radiation dose, beam direction and distance between the primary field and the fetus.1 A recent study claiming an association between dental radiography in pregnancy and low birth weight14 has been criticized for lack of biological plausibility.15 A publication by the International Commission of Radiological Protection reviewed experimental data about the in utero effect of radiation in animals and humans. They concluded that the risk of induction of malformation at low doses can be discounted. Data about the induction of severe mental retardation after irradiation from atomic bombs during the most sensitive prenatal period support a dose threshold of 6–31 rad between 8 and 15 weeks and 25–28 rad between 16 and 25 weeks.16 The data about loss of intelligence quotient after the bombing of Hiroshima and Nagasaki of 20–30 points per 100 rad exposure are more difficult to interpret. However, even in the absence of a true dose threshold, any effect on intelligence quotient at low doses would be undetectable.
The International Commission of Radiological Protection reported that the risk of induction of childhood solid tumours is similar to that of leukemia and that the risk of cancer in later life is similar to that following irradiation during childhood. In contrast, there are studies that suggest an increase in childhood cancer after in utero exposure to 1 rad. The excess absolute risk coefficient at this level of exposure is 6% per 100 rad.5 The British Oxford Survey of Childhood Cancer estimated the risk of cancer to be 0.022 per 100 rad. This is in agreement with the estimate of the Life Span Study from Japan that included survivors of the atomic bomb (0.028 per 100 rad).5
The American College of Obstetricians and Gynecologists has advised practitioners that, although there is no evidence that magnetic resonance imaging (MRI) is associated with adverse fetal effects, it should be avoided during the first trimester. However, because the fetal radiation dose is minimal, MRI should not be delayed if it is considered critical for the diagnosis of a serious maternal condition.
|
Perceived risk of radiation |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
Similar trends have been reported in Canada. In a recent Canadian survey, 400 family physicians and 100 obstetricians were asked about their perception of the fetal risks associated with abdominal radiographs and CT scans during early pregnancy and whether they would recommend a therapeutic abortion following such exposure.3 Of the respondents, 40% of family physicians perceived the teratogenic risk associated with abdominal radiographs to be above baseline (
5%), and 61% estimated the risk associated with CT scans to be 5% or greater. Of obstetricians, 11% estimated the risk associated with radiographs to be 5% or greater, and 34% estimated the risk associated with CT scans to be 5% or greater. Among family physicians 1% would recommended an abortion if the patient had received a radiograph, and 6% would recommend an abortion after a CT scans. None of the obstetricians reported that they would recommend an abortion after a radiograph, but 5% would recommended an abortion after a CT scan during early pregnancy.3
The high perception by physicians of teratogenic risk associated with radiation could lead to unnecessary anxiety for pregnant women who have been inadvertently exposed and who seek counselling. It could also lead to delays in needed care for pregnant women. In one prospective study, 6 women (10% of participants) exposed to low-dose diagnostic radiation during pregnancy chose to terminate the pregnancy,4 stating that it was because of anxiety often caused by physician's advice. Educational interventions about radiation exposure should be considered to facilitate accurate risk estimation by physicians.
|
The cases revisited |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
Case 2: It is critical that a pulmonary embolism be ruled out. A ventilation-perfusion scan would expose the fetus to radiation as low as 0.06–0.1 rad, which is well below the potentially teratogenic dose (5 rad). A regular chest CT scan would expose the fetus to 0.45 rad, which is also well below the range of fetal risk. However, CT angiography is associated with higher doses and involves administration of a contrast agent. As her physician, your decision should be based on the individual operator and the diagnosis sensitivity and specificity of the unit.
Case 3: You advise your patient that a radiograph of the hip is associated with a fetal radiation dose of up to 0.37 rad. This is far below the fetal radiation dose considered as safe. Although this dose is not associated with reproductive risk, this procedure is not clearly indicated and will not affect treatment. The characteristics of your patient's pain suggest exercise-induced bursitis. Rest and analgesics are the treatment of choice, even if the diagnosis is osteoarthritis. At this time, the radiograph can be postponed while the patient is observed for any change.
|
Conclusion |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
|
Footnotes |
|---|
Competing interests: None declared.
|
REFERENCES |
|---|
|
TopAbstractRadiation effectsDose limitsEstimated exposurePerceived risk of radiationThe cases revisitedConclusionREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  Corrections Can. Med. Assoc. J., April 28, 2009; 180(9): 952 - 952. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | CURRENT ISSUE | PAST ISSUES | COLLECTIONS | HELP | SEARCH |
|
|||||
|
Copyright 1995-2008, Canadian Medical Association. All rights reserved. ISSN 1488-2329
(e) 0820-3946 (p)
All editorial matter in CMAJ represents the opinions of the authors and not necessarily those of the Canadian Medical Association. |
|||||
