Answer to Question #215 Submitted to "Ask the Experts"

Q

I am looking for information to assist in determining "real" vs. "perceived" risk to pregnant female workers involved in nuclear pharmacy compounding. Specifically, I am interested in fetal dosimetry that might result from a needle-stick injury involving ¹³¹I. (This is a hypothetical situation.) In general, the amount of ¹³¹I contained in the syringe in this hypothetical situation would range from 3-300 mCi. The actual amount of ¹³¹I deposited into the finger would most likely be in the uCi range. Please, compare this risk to that of inhalational exposure. For purposes of this example, assume that DAC and ALI values are well below the action levels each year. All iodine handling is performed in a fume hood with proper air flow.

A

Thank you for your question. My answer uses a lot of data that I wouldn't begin to suggest you pass along to these workers "as-is." I think it is important, though, that you have this information when explaining to the workers what the "real" risk might be.

When discussing the risk of radiation exposure, I would suggest also including the natural incidence of abnormalities for a pregnancy. I find it quite astonishing for instance that growth retardation can be caused by a minimal amount of smoking (less than a pack a day) or drinking (two drinks a day) during pregnancy; the risk is as much as one abnormality in three births to drinking/smoking mothers. Or that developmental anomalies occur in two to four percent of live births. Compare this risk to a fetal dose of 1 rem where the risk of childhood cancer (before the age of 12) would be 1 in 3,333.

Anyway, here is the answer to your question: With iodine, route of administration (needle stick, inhalation, ingestion) appears to have little impact on how it is metabolized. If the route is a needle stick, we're essentially looking at injection since it will be absorbed rapidly even if it isn't injected directly in the blood stream. According to the literature, in an inhalation situation anywhere from 70 to 100 percent is removed and deposited in the GI tract where nearly 100 percent of that is absorbed while in the small intestine. Interestingly, iodine is considered to be removed from the lungs to body fluids almost instantaneously. So whether it is injection via needle stick or inhalation, we have to assume that the amount injected (or left from the needle stick) or inhaled gets into the blood stream of the mother. From here I'm going to use information from a reference I wouldn't want to go without and would recommend it to you if you have these questions frequently; I've cited it at the bottom of this answer. Note that there can be large uncertainties in the numbers listed in the table and paragraphs below. Once the amount of radioactivity administered to the mother has been determined, the table below can be used to calculated an estimated conceptus dose from ¹³¹I in an iodide form. The units are rad to the conceptus per administered activity to the mother.

But, your question is actually about risk. Since the conceptus thyroid isn't fully functioning until about week 13, we don't need to worry about decrease in the thyroid's ability to function until then. Possible other bioeffects during those first few weeks would include prenatal death (very early pregnancy) requiring about 10 rad (note though that prenatal death, e.g., spontaneous abortion, will occur naturally in 50 to 75 percent of all pregnancies), growth retardation requiring about 20-25 rad in humans, and small head size requiring about 75 rad in humans. From weeks 8 to 20, we've passed the point for prenatal death but have increased possibilities of small head size, severe mental retardation and seizures for conceptus doses in excess of 10 rad, and organ malformations requiring doses as low as 6 rad. From week 20 until birth, the potential for most consequences diminishes with the exception of thyroid effects. With iodine, thyroid malfunction becomes a possibility after week 13 and will typically be the bioeffect of concern with any maternal uptake of iodide simply because a significant amount of activity would need to be administered for the conceptus body dose to be of concern biologically. Between weeks 14 and 22 the percent uptake of iodine in the fetal thyroid is between 55 and 75 percent.

One study reported hypothyroidism following internal fetal radioiodine uptake after 12 to 225 mCi ¹³¹I sodium iodide was administered to the mothers. Hypothyroidism is the only well documented effect from exposure of the conceptus to radioiodine. Note that while literature hasn't suggested a threshold for in-utero hypothyroidism, studies in children and adults suggest a threshold of 20 rad.

During the entire gestation period, literature suggests an increased risk of childhood cancer due to exposure in-utero and with increased exposure there is increased risk. The likelihood of not developing childhood cancer changes from 99.93 percent with no exposure to 98.25 percent with 10 rad delivered to the conceptus during the first trimester. So, while there is increased risk, there is still a much better probability that a child will not develop cancer—exposure or not. Thyroid cancer risk is also increased but not likely to be a factor below 10 rad thyroid according to data collected from Chernobyl and the National Council on Radiation Protection and Measurements.

So, to answer your question, a few microcuries taken up by the mother at week 20 would give the conceptus an estimated body dose of about 10 mrad body and up to 1.7 rad to the thyroid. According to the literature, no effects would be expected. However, if the maternal uptake was 30 uCi (100 mrad body, 17 rad thyroid), regular thyroid function testing performed after the child is born would be encouraged.

Kelly Classic
Certified Medical Health Physicist

Reference:
Wagner LK, Lester RG, Saldana LR. Exposure of the pregnant patient to diagnostic radiations: a guide to medical management. Second edition. Madison, Wisconsin: Medical Physics Publishing; 1997.