Answer to Question #4256 Submitted to "Ask the Experts"

I am a radiation worker in a hospital. I recently discovered that I am five to six weeks pregnant. Unfortunately, while checking on a patient undergoing a contrast CT (computed tomography) of the abdomen, I was exposed to radiation (at least 1 slice was performed). How do I calculate the radiation dose to my embryo? (I was standing at the head of the patient, very close to the CT machine). Is it true that an embryo <6 weeks gestational age is radio-insensitive?

There is a paper published in the European Radiology by Theumann, et al. (1999, Vol. 9, p. 1919), that looked at radiation doses to radiologists in the room during CT (computed tomography) procedures.  In their study, they put radiation badges on a radiologist during several patient CT procedures. The radiologist was standing about 45-60 cm (1.5-2 feet) from the CT gantry.  The radiation dose to the radiologist's chest area ranged between 5 and 11 microgray (0.5 to 1.1 mrad).

If the dose, then, is roughly 5 to 11 microgray to the radiologist for an entire procedure, the dose you may have received from one slice would be less.  Even if we assume you or your embryo received somewhere between 5 and 11 microgray, that level of exposure is insignificant—it is equivalent to the radiation exposure you receive from natural background for one day.

As for the embryo being radiation insensitive early in pregnancy, I think you might be referring to the "all or none" period.  If the embryo is exposed to fairly high levels of radiation during the first few weeks of pregnancy, one of two things happen: nothing at all and the embryo continues to develop normally or there will be a spontaneous abortion. The minimal dose level to cause this is reported as 6 rad (6,000 mrad). This is orders of magnitude higher than the dose you may have been exposed to.

Kelly Classic
Certified Medical Health Physicist