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Risks From Ionizing Radiation in Pregnancy
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The
dramatic increase in the number of CT examinations performed has led to
considerable discussion as to whether the radiation exposure due to CT
is a reason for concern. Annually, there are now about 65 million CT
examinations performed in the United States, including about 700,000
for children under the age of 5 years. Even though the number of CT
examinations is only one tenth of all the x-ray based procedures, CT
contributes more than two-thirds of the total medically related
radiation exposure to patients and results in a higher dose than many
nuclear imaging procedures. For example, the adult exposure from a CT
of the abdomen 200-250 times greater than from a chest x-ray. However,
the exposure from the latter is similar to that of a 7 hour airplane
ride.
Since ionizing radiation is a potential hazard to
the developing fetus, avoiding unnecessary radiation exposure to
pregnant women is a standard practice in radiology. Women are routinely
asked about the date of their last menstrual period and whether they
are likely to be pregnant. In pregnancy, ultrasound and MRI (without
contrast administration) are alternate imaging methods that have no
known risks to the fetus. In addition, most nuclear imaging methods are
permissible during pregnancy, with the exception of those that use
isotopes of iodine, because of the vulnerability of the fetal thyroid
gland, and some rarely used isotopes that have particularly long
radioactive and biological half-lives, such as selenium and iron.
However, a situation may arise in which a physician advises
radiation-based examinations, such as x-rays or CT, in pregnant
patients after evaluating the risks of radiation exposure to the fetus
together with the health risks to the mother. In addition, a woman may
be exposed to ionizing radiation when she is unaware of her pregnancy.
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Exposure to radiation is measured in rem or seiverts, Sv (100 rem = 1
Sv). However, the absorbed dose in rad or gray, Gy (100 rad = 1 Gy)
includes estimated sensitivity factors of specific organs, is more
relevant to the degree of risk.
Natural background exposure at sea level is about 300 mrad per year.
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Estimating Risk
Although exposure to ionizing radiation can cause considerable anxiety,
the risks appear to be quite small. The radiation dose associated with
CT examinations is far below the levels at which cell death will occur.
However, the statistical risk of radiation-induced mutations, which
could lead to cancer or birth defects, is debatable. The estimates of
radiation-induced cancer, congenital malformations, and mutations are
derived mostly from the survivors of high radiation doses from atomic
explosions in Hiroshima and Nagasaki. These analyses derive risk
estimates by linear extrapolation to low levels of radiation exposure.
This method is chosen, not because it is most likely to be correct, but
because it is the most conservative. It should be pointed out that
there is no direct evidence that the exposures experienced in medical
diagnostic imaging causes cancer or birth defects.
Risk of Childhood Cancer
If the incidence of cancer resulting from exposure to high doses is
extrapolated in a linear fashion, then there appears to be a small but
finite risk of cancer from CT scanning. On the basis of these analyses,
the estimated lifetime risk of a 1-year old child developing cancer is
0.18% for an abdominal CT and 0.07% for a head CT. In comparison,
approximately 23% of the population will develop cancer at some point
in their lives, which means that the increased risk due to CT scanning
is very small.
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| Additional Risk of Congenital Malformation and Childhood Cancer Due to Ionizing Radiation |
| Absorbed Dose to Conceptus above Natural Background, mrad, |
Probability that Child will have NO Malformation % |
Probability that Child will NOT Develop Cancer up to Age 12, %
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| 0 |
97% |
99.7% |
| 100 |
97% |
99.7% |
| 250 |
97% |
99.7% |
| 500 |
97% |
99.7% |
| 1,000 |
97% |
99.6% |
| 5,000 |
97% |
99.4% |
| 10,000 |
<or= to 97% |
99.1% |
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Risk of Fetal Malformation
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In
a developing fetus, DNA damage caused by ionizing radiation has the
potential, not only to cause cancer, but also teratogenesis or
mutagenesis. In the pre-implantation embryo, there is no measurable
risk of malformation, regardless of the amount of radiation exposure
and the greatest concern is death of the embryo. At this stage, if an
embryo is exposed to 10,000 mrad, the risk of death is 2%. Between 3-10
weeks of pregnancy, the threshold for the detection of an increase in
birth defects (brain damage or malformation of the lips, mid-face,
teeth, or external genitalia) is 5,000-25,000 mrads, which is
significantly greater than that delivered with diagnostic medical
imaging. After ten weeks, risk of congenital malformations decreases.
At this stage of development, there is a risk of microcephaly, but the
threshold of detection is at a radiation dose greater
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than
12,000 mrads. After 17 weeks, the greatest risk is of mental
retardation or growth restriction but only at doses that are likely to
cause symptoms of radiation poisoning in the mother.
Based
on these analyses, pregnant women exposed to less than 5,000 mrad have
similar pregnancy outcomes to those who have received only background
radiation That is, they will experience the normal rate of risk of
spontaneous abortion (about 15%), major malformation (about 3%), and
intrauterine growth retardation (about 4%). As most radiology
diagnostic examinations expose the fetus to less than 5,000 mrad of
radiation dose, there is no significant increase in major malformations
in pregnant women inadvertently exposed to ionizing radiation from
diagnostic imaging.
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| Maternal and Fetal Exposure to Ionizing Radiation from Diagnostic Imaging |
Source of Exposure
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Adult Radiation Exposure |
Fetal Radiation Exposure or Absorbed Dose |
Adverse Health Effect (Fetus) |
Background Radiation,
Sea level
Elevation, 5,000 feet
(Denver, CO)
Seven hour airplane flight
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300 mrad/yr
1,000 mrad/yr
5 mrad
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300 mrad/yr
1,000 mrad/yr
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None
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Medical Exposure
Radiography, Chest, Head, Dental
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4 mrad |
< 1 mrad
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CT, Head
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200 mrad |
< 10 mrad
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| CT, Chest |
800-1,000 mrad |
< 10 mrad |
Radiography, Upper GI Tract
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300 mrad
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4 mrad
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Radiography, Pelvis or Hip
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240 mrad
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240 mrad
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V:Q Scan
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100 mrad
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50 mrad
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Bilateral Venogram
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500-1,820 mrad
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630 mrad
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131I Sodium Iodide (1 mCi)
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470 mrad
(whole body)
780,000 mrad (thyroid)
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1,000 mrad (whole body, early fetus) |
After
10 weeks gestation, when the fetal thyroid has developed, the radiation
dose to the fetal thyroid is much higher than that of the mother’s and
can cause damage to the developing thyroid. |
CT, Abdomen or Pelvis
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1,000 mrad |
2,000 mrad
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Increases risk of leukemia by factor of 1.5 – 2 (to 1 in 2,800)
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Accidental Exposure
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5,000 – 25,000 mrads
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Threshold for detection of increase in congenital malformations in embryonic period (3-10 weeks LMP)
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Conclusion
In conclusion, any woman who gets exposed to x-rays when she is unaware
of her pregnancy must be reassured and counseled appropriately. In
addition, maternal health risks must dictate the need for
radiation-based medical imaging as exposure to ionizing radiation
associated with most radiological examinations is substantially lower
than that required to produce adverse effects. Nevertheless, it is
still advisable to avoid such exposure where possible, by using
ultrasound or MRI when equivalent information can be obtained, and to
reduce radiation exposure by, for example, using an abdominal shield
for chest CT and x-ray examinations.
References
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Further Information
For further questions on radiation exposure, please contact Dr. Mannudeep Kalra
, MGH Department of Radiology, at 617-726-3937.
This article provided useful information about the appropriate use of imaging studies:
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U.S. Nuclear Regulatory Commission Fact Sheet on Biological Effects of Radiation Source: National Council on Radiation Protection (NCRP) Report 93, 1987.
Center for Disease Control. Possible Health Effects of Radiation Exposure on Unborn Babies
International Commission on Radiological Protection (2003) ICRP Publication 90: Biological Effects after Prenatal Irradiation (Embryo and Fetus)
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Ratnalapan S, Bona, N, and Koren, G. (2003) MotherRisk Update: Ionizing radiation during pregnancy Canadian Family Physician 49: 873-874
Brenner, D, Elliston, C, Hall, E and Berdon, W.(2001) Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 176: 289-96
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