Answer to Question #411 Submitted to "Ask the Experts"

Q

What are the side effects of having a VQ scan and CT of the chest in the first two weeks of pregnancy? Can you quote the chances of risk and any supporting articles and studies.

A

Let me first give estimated doses to the baby from the radiation studies, then briefly mention the dose necessary to cause biological effects, and finally list some references. First, the unit of radiation dose I'm going to use is the millirad (mrad). It's simply a way to quantify a certain amount of radiation. From the references below, I've obtained the following information. A VQ scan involves the injection of a radiopharmaceutical called ^99mTc MAA (technetium macroaggregated albumin). Early in pregnancy, the dose to the baby from this radiopharmaceutical has been calculated to be 10 mrad to the baby for each millicurie (mCi) of activity administered to the mother. A typical administration for this exam is 4 mCi so the baby could receive an estimated dose of 40 mrad (4 mCi×10 mrad/mCi). The VQ scan also involves an administration of ¹³³Xe (xenon) gas by inhalation. Early in pregnancy, the dose to the baby from this radiopharmaceutical has been calculated to be 0.8 mrad to the baby for each mCi administered to the mother. A typical administration for this exam is 30 mCi so the baby might receive an estimated dose of 24 mrad (30 mCi ×0.8 mrad/mCi). For the VQ scan then, the baby could receive an estimated total dose of 64 mrad. The dose to the baby from the CT scan is a bit more difficult to estimate because a chest CT scan could be done anywhere in the chest region. If the scan was done farther away from the baby, closer to the heart, the baby would receive little if any exposure. If the scan was done in an area closer to the liver (or what we might think of as the bottom of the chest), the baby might have received up to 200 mrad. Thus, the total estimated dose the baby could have received from both of these exams would be 264 mrad assuming the highest dose from the chest CT. The fetal exposure allowable level for an occupationally exposed individual (for instance, an x-ray technologist) is 500 mrem (occupational doses are listed in radiation units of millirem; for our purposes, consider a mrad and a mrem the same). But what does that 500 mrem really mean? It's an amount of exposure considered "safe" because it is set at a level that is 10 times less than the lowest amount of radiation known to cause fetal effects, about 5000 mrem. The National Council on Radiation Protection and Measurements and the American College of Obstetricians and Gynecologists suggest that the possibility for harm to the baby from radiation exposure is very small. Fortunately, many standard diagnostic x-ray exams and nuclear medicine studies have radiation doses below 5000 mrem. The potential for harm, however, does increase for some specialty x-ray tests of the abdomen and pelvis, and for therapeutic amounts of radioactive drugs used in nuclear medicine. Since the estimated dose to your baby is below 300 mrad, you should have no cause for concern. References

• Wagner LK, Lester RG, Saldana LR. Exposure of the pregnant patient to diagnostic radiations. Medical Physics Publishing: Madison WI; 1997.
   
• US Nuclear Regulatory Commission. Regulatory Guide 8.13. Instruction concerning prenatal radiation exposure
   
• National Council on Radiation Protection and Measurements. Report 54. Medical radiation exposure of pregnant and potentially pregnant women; 1977.
   
• NCRP Report 100. Exposure of the U.S. Population from diagnostic medical radiation; 1988.
   
• NCRP Report 124. Sources and magnitude of occupational and public exposures from nuclear medicine procedures; 1996.
   
• NCRP Report 128. Radionuclide exposure of the embryo/fetus; 1998.
   
• Russell JR, Stabin MG, Sparks RB. Placental transfer of radiopharmaceuticals and dosimetry in pregnancy. Health Physics Journal 73(5):747-755; 1997.
   
• Russell JR, Stabin MG, Sparks RB, Watson E. Radiation absorbed dose to the embryo/fetus from radiopharmaceuticals. Health Physics Journal 73(5):756-769; 1997.

Kelly Classic
Certified Medical Physicist