Answer to Question #1115 Submitted to "Ask the Experts"

Q

I am a nuclear medicine technician and am six weeks pregnant. I coordinate clinical trials, so my radiation exposure is sporadic (unlike a daily clinic schedule). However, some studies involve large amounts of ¹³¹I 60 mCi++. I am concerned about bremstrahlung x rays if I add more lead shielding. Do you have any comments or suggestions?

A

Bremsstrahlung radiation is produced when high-energy beta particles or electrons interact with materials and are slowed or stopped, giving up some of their energy as x rays. Beta particles emitted from radionuclides can usually be shielded pretty easily (depending on their energy) with a low-atomic-number material, for example, Plexiglas, to minimize the potential for bremsstrahlung production. ¹³¹I does emit a number of beta particles of only moderate energies. Since the ¹³¹I is inside the body, very few of the beta particles will actually be able to make it outside the patient. Thus, the principle external exposure to caregivers is from the photons emitted by ¹³¹I. If we don't take into account shielding by the patient, each 30 mCi or so gives an external exposure rate of 5 mR h^-1 at one meter. For 60 mCi, it is 10 mR h^-1 at one meter. For 200 mCi that some institutions are using, it is just over 30 mR h^-1 at one meter. If shielding is used, it takes just about 1 cm of lead to reduce these exposure rates by a factor of 10. Also, remember that minimizing time and increasing distance will reduce exposure as well. That being said, I will still go ahead and mention a few things about shielding beta particles with lead. In general, a leaded apron won't do much good for protection from ¹³¹I regardless of the bremsstrahlung radiation. The gamma-ray photon energies from ¹³¹I are too high to be stopped by 0.5 mm lead, which is typically used in the heavy-leaded aprons. And, if the beta particles were actually able to make it out of the patient, thinner lead could expose you to more radiation as bremsstrahlung radiation is produced as the beta particles strike the lead. Even though we don't want to shield with lead, if we did, we would want lead that is thick enough to stop the high-energy bremsstrahlung x rays also. The risks to the fetus will depend on the amount of exposure you're receiving. Presumably you're wearing a radiation badge to measure this exposure, but if we say that, for any one administration of 200 mCi, that takes 10 minutes with you 0.5 meters away from the source, your exposure would be about 20 mR with no shielding. With a centimeter of lead shielding, the exposure would be 2 mR, and with a quarter centimeter of lead it would be about 10 mR. The regulatory exposure limit for a declared pregnant worker or embryo/fetus is 500 mR. Exposures below 6,000 mR have not been shown to adversely affect the fetus. The greater risk with ¹³¹I as sodium iodide is actually ingestion or inhalation. It is extremely important that when handling ¹³¹I or touching anything in the room of an ¹³¹I sodium iodide patient that universal precautions for infectious material are used. Accidental ingestion or inhalation of ¹³¹I while pregnant will lead to fetal thyroid uptake, if the fetus is less than 12 weeks old. Since the fetal thyroid is very small, that is, composed of very few cells, the radiation dose will be quite large. The risk from other ¹³¹I-labeled compounds will depend upon their volatility and uptake into the pregnant worker.



[Editor's note: The exposures are expressed in mR, milliroetgen, rather than mrem or mSv, which are measures of dose to critical organs.]



Kelly Classic

Certified Medical Health Physicist