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Volume 2 Issue 2 - February 2006
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Risks From Ionizing Radiation in Pregnancy
 

Estimating Risk
Risk of Childhood Cancer
Risk of Fetal Malformation
Conclusion
Further Information
References

T
he 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.
 

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.


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.


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, %
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%


Risk of Fetal Malformation
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
  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.

Maternal and Fetal Exposure to Ionizing Radiation from Diagnostic Imaging
Source of Exposure
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
300 mrad/yr
1,000 mrad/yr

5 mrad
300 mrad/yr
1,000 mrad/yr




None

Medical Exposure
Radiography, Chest, Head, Dental
4 mrad < 1 mrad
CT, Head
200 mrad < 10 mrad
CT, Chest 800-1,000 mrad < 10 mrad
Radiography, Upper GI Tract
300 mrad
4 mrad
Radiography, Pelvis or Hip
240 mrad
240 mrad
V:Q Scan
100 mrad
50 mrad
Bilateral Venogram
500-1,820 mrad
630 mrad
131I Sodium Iodide (1 mCi)
470 mrad
(whole body)
780,000 mrad (thyroid)
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
1,000 mrad 2,000 mrad
Increases risk of leukemia by factor of 1.5 – 2 (to 1 in 2,800)
Accidental Exposure
  5,000 – 25,000 mrads
Threshold for detection of increase in congenital malformations in embryonic period (3-10 weeks LMP)

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
 


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)

  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