Answer to Question #4667 Submitted to "Ask the Experts"

Q

I am the sole PET/CT (positron emission tomography/computerized tomography) technologist in an outpatient facility and I have just found out that I am about five weeks pregnant. I follow radiation safety procedures very closely. My exposures are about average for a PET/CT technologist due to our patient volume (0 to 5 patients/day; 30 to 40 mrem per month). I want to know if I should continue working in this capacity. 511 keV gamma rays are very energetic and I don't want to harm my unborn child in any way. I have heard that you should not wear a lead apron due to bremstrahlung, but I have also been told that it wouldn't hurt to wear one. I receive unit doses and I still take call at our hospital for general nuclear medicine. (Is it okay to do ¹³³Xe ventilation studies?) Do you have any suggestions? As a first-time mom, I am very nervous and this has been weighing on my mind.

A

First, thank you so much for including information on how much radiation exposure you routinely receive each month. That is very helpful.

Let me start with regulatory stuff. The Nuclear Regulatory Commission's and most states' regulations indicate that the fetus of an occupationally exposed worker should not exceed 500 mrem during the gestation period. They also go on to state that exposure should be kept below 50 mrem/month as much as possible. Your exposure is below these levels.

Now, about radiation effects on the fetus. There is a lot of information stating that doses to the fetus below 10 rem will not produce effects, although some information now goes down to doses around 6 rem to produce effects. Your exposure is below these levels.

And how about the lead apron? It won't do you much good even if you do wear it. The lead apron is to protect someone from 80 kVp x rays (avg.). As you indicated, PET emitters emit gammas at 511 keV. The lead apron has 0.5 mm lead; 0.5 mm lead will stop over 90% of the low-energy x rays emitted by a machine in radiology. On the other hand, the half-value layer for a 511 keV photon is about a half centimeter or 5 mm lead. That means you need 5 mm of lead between you and the positron emitter to reduce your exposure by 50%.

For those in nuclear medicine to reduce their exposure, usually shielding of the syringe and distance from the patient are the best methods. That is because shielding yourself from higher-energy photon emitters is not practical.

Is it okay to do a ventilation study? Yes. It is unlikely you would inhale much of the ¹³³Xe and, if you did, you would exhale it back out. Very little, if any, would stay in your body.

In summary, the doses you currently are receiving are below regulatory limits and below levels of radiation shown in studies to cause biological effects. A lead apron really won't help reduce your exposure from PET emitters.

Should you go part-time or change jobs? I can't advise you on that. You're doing the right thing, though—getting all the information you can to make an informed decision.

Kelly Classic
Certified Medical Health Physicist