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EDUCATION EXHIBIT |
Department of Diagnostic Imaging and Nuclear Medicine, Graduate School of Medicine, Kyoto University, Kyoto, Japan
I am grateful to have the opportunity to comment on the excellent article by McCollough et al (1) in this issue of RadioGraphics. The authors present a comprehensive review of the effects of radiation exposure on the conceptus (embryo and fetus), which has been an important issue for radiologists, obstetricians, and others involved in medical imaging. As a specialist in gynecologic imaging, I frequently have been asked about the risks of radiation exposure to pregnant women, and this review will be very informative for any reader of this journal. The article provides a practical guide for radiologic examinations in pregnant patients.
Many medical imaging modalities depend on ionizing radiation: Radiography, fluoroscopy, computed tomography (CT), and nuclear medicine all involve radiation exposure. The recent increase in the frequency of CT use over that for other modalities is cause for great concern. CT already accounted for the major part of the effective dose to patients from diagnostic radiologic examinations (2). In addition, the development of multidetector CT scanners, with their excellent performance, has opened up a new field in diagnostic imaging and inevitably will lead to greater CT use (3).
Exposure to ionizing radiation is known to cause various adverse effects, including those on the conceptus. The adverse effects of radiation on the conceptus may be divided into two categories: those associated with a dose threshold (eg, prenatal death, growth retardation, and mental retardation) and those not associated with a dose threshold (eg, carcinogenesis). For pregnant women, imaging without the use of ionizing radiation (eg, with ultrasonography [US] and magnetic resonance [MR]) is considered the safest option. However, the diagnostic result of US is sometimes inconclusive, and access to MR imaging may be limited. In such a case, another modality must be used that can provide comprehensive information for decision making.
It is important to note that not all pregnant women who undergo diagnostic imaging examinations that involve exposure to ionizing radiation are aware of their pregnancy. The attending physician or obstetrician may be the first medical professional to provide information to the patient about the teratogenic risk associated with radiation, and the risk perceived by such physicians may be greater than the actual risk (4). All physicians who are involved in the care of pregnant women therefore need to obtain solid information about radiation risks to the conceptus.
In our clinical practice, the use of lead shielding for radiologic examinations of pregnant patients, whenever feasible, is mandated to avoid any radiation exposure to the conceptus. This practice is supported by evidence from experimental and epidemiologic studies that failed to indicate a threshold dose below which radiation exposure does not cause cancer (5), as shown in table 1 of the article by McCollough et al (1). The linear extrapolation without a dose threshold that is used to extrapolate cancer risks to very low dose levels has been the subject of much debate. Low-dose radiation–induced cancer in humans depends on several variables, and it is not possible to correct for most of these variables in any epidemiologic study (6). Despite these factors, a significant increase in the occurrence of malignancy was found even after a radiation dose of 2.0–2.5 mGy to human fetuses, with a relative cancer incidence of 1.25 (7). The increased risks are almost self-evident, but it is not clear what evidence should be referred to when radiation exposure is considered inevitable.
With a vantage point that differs from those of the authors of previously mentioned publications, McCollough et al present information collected by individuals who actually perform the examinations that result in radiation exposure to the patient. They have collected estimates of the radiation doses incurred with a specific type of examination in a particular body area. The doses, especially outside the imaging area, were found to be small. My colleagues and I recently have been concerned about the increased radiation doses needed to obtain adequate images in large patients, but the article by McCollough et al indicates that the radiation dose to internal organs increases by only 25% when the source dose increases by a factor of two.
I am especially interested in the authors’ description of their development of a practice policy and guidelines for radiation exposure of pregnant patients. A more general set of guidelines formulated at the national or international level may be desirable; however, wide variations in scanner performance and in access to imaging modalities are obstacles to establishing such uniform guidelines. A policy that is evidence based and tailored to the individual institution might be more practical. For example, the authors present in this context a description of the imaging of urolithiasis: The recommended algorithm relies primarily on the use of US, and CT is applied only if the US examination has an inconclusive result. CT is acceptable, as the article makes clear; but if resources for and access to MR imaging are not restricted at one’s institution, I would think that MR imaging would have advantages over US and CT for the detection of urolithiasis in pregnant patients (8).
Finally, I would like to emphasize the importance of educating radiologists, other medical workers, and patients about radiation risks; obtaining scientific evidence of radiation risks and performing evidence-based analyses of the benefit and risk of particular radiologic examinations in pregnant women; and reducing radiation doses (as may be done, for example, with new technologies such as automatic exposure control). As the authors stated, the imaging of pregnant women is a challenging task that requires comprehensive knowledge. I believe that this educational review article provides a good basis for radiologists in practice.
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