Answer to Question #217 Submitted to "Ask the Experts"

Q

A pregnant woman has had a CT (spiraled) abdomen x ray. What effective dose is received by the uterus and what is the risk?

A

In order for you to calculate dose in this case, it is very important to determine the actual beam time and the location of the fetus with respect to the beam. It is also important to try to obtain information about beam parameters with respect to:

• voltage (kVp)
• current (mAs)
• filtration (mm aluminum)
• dose rate in beam (from direct measurements)

With this information, you can calculate the entrance skin exposure and the degree of attenuation within the patient to arrive at a reasonably accurate fetal dose. Assuming that only one-third of the incident radiation penetrates to the fetus may not be accurate, depending on the parameters noted above. Suggested references for making these calculations are:

• Exposure of the Pregnant Patient to Diagnostic Radiations (Wagner, Lester, and Saldana; Medical Physics Publishing, 1997)
   
• Data tables in the Health Physics and Radiological Health Handbook (Edited by Bernard Shleien, Lester A. Slaback, Jr., and Brian Kent Birky; Williams and Wilkins, 1998)
   
• Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Ray (Kereiakes and Rosenstein, CRC Press, 1980)

Current medical advice, according to Wagner, Lester, and Saldana, is to not consider therapeutic abortions for fetal doses of less than 15 rad unless other risk factors are present. They do not recommend therapeutic abortion in any case where dose is less than 5 rad or for any dose of less than 15 rad if the fetus is past the 15th week of pregnancy. At doses above 15 rad, abortion may be recommended, but not always, and they provide no additional information. If the calculated dose is greater than 5 rad AND other risk factors are present, I would strongly recommend the woman contact a physician specializing in such matters before she makes any irrevocable decisions.

Andrew Karam, CHP