Tuesday, January 10, 2012

Q & A: If Nuclear Accidents Can Lead to Thyroid Cancer, Why Doesn't the I-131 Treatment?

Thanks for your great post on radioprotective therapies (see Radiation Protection after a Nuclear Accident).

But I'm confused. If radioidine-131 released into the atmosphere during an accident can result in a dramatic increase of thyroid cancer in children, why don't people or cats treated with I-131 for hyperthyroidism develop thyroid cancers?

My Response:

Good question. After a nuclear accident, people and animals will get exposed to varying amounts of radioiodine as I-131, as well as many other radioisotopes including cesium-137, strontium-90, and plutonium-241 (1). Some will receive a small dose to their thyroid, whereas others will get more, but it's generally not enough to destroy the thyroid — it "tickles" the thyroid and causes DNA damage, which can lead to thyroid tumors (2).  The exposure also tends to happen over a period of a few days to weeks, not all at once.

When we treat a patient (human, cat, dog) with radioiodine, we are delivering a huge, single dose of radiodine to the thyroid. This destroys the hyperfunctional adenoma/carcinoma. It tends not to be taken up by the normal, suppressed tissue. It is unclear why the remaining normal thyroid tissue doesn't develop cancer, but it appears related to the large, single dose of radioiodine administered, as opposed to chronic exposure of lower levels of radiation. It has been reported that spreading the total I-131 dose over time (from a few days to a few weeks or longer) may lower risk of thyroid cancer, probably due to the opportunity for cellular repair mechanisms to operate (3).

Infants and children have the peak risk as the result of the increased radiation sensitivity of their thyroid glands. As a result, most human patients that develop thyroid cancer after exposure to radioiodine are babies and young children (4-6), whereas adult individuals are at a lesser risk. This fact probably also contributes to why we don't see thyroid cancer developing in adult human or feline patients treated with radioiodine.

The latest study in Hiroshima and Nagasaki Atomic Bomb survivors in Japan has indicated that a biological effect from a single brief external exposure to ionizing radiation nearly 60 years in the past still occurs and can be detected (7). In childhood, once exposed even to low doses of ionizing radiation, either externally or internally, the cancer-prone cell damage within the thyroid gland can be preserved for a long time.

What Can We Do in an Nuclear Emergency?

Since children are at the highest risk to exposure to radioactive iodine, potassium iodide should be available to all children (8). Also, because of the risk to the developing fetus, pregnant women should also take potassium iodide in the event of a nuclear accident. Compared to children and the fetus, adults are at a lower risk but still would benefit from thyroid blockage with potassium iodide.

Iosat Tablets, one FDA approved KI preparation
Potassium iodide pills (sometimes abbreviated as KI: the K stands for potassium, the I for iodine) don’t prevent radioactive iodine from entering the body. But these pills do keep the radioiodine from accumulating in the thyroid gland. By flooding the body with non-radioactive iodine, the pills keep the gland from absorbing the radioactive iodine. Here is how the CDC fact sheet summed it up (9):

Because KI contains so much stable iodine, the thyroid gland becomes “full” and cannot absorb any more iodine—either stable or radioactive—for the next 24 hours.
ThyroShield and ThyroSafe, the 2 other FDA approved KI preparations

What Potassium Iodide Preparations Are Available?

The potassium iodide (KI) products approved by the Food and Drug Administration (FDA) include the following (10):
Properly packaged, potassium iodide’s shelf life is at least 5 years and possibly as long as 11 years. If you take a very old pill, it may not work but won't hurt you or do any harm.

What are the Recommended Potassium Iodide Doses?

For humans, the FDA recommends the following doses based on the patient's age:
  • 0—1 months ........................................ 15 mg
  • 1 months—3 years ............................30-35 mg
  • 3—12 years ...........................................65 mg
  • >12 years ............................................130 mg
As I discussed in my last post, the recommended dose of potassium iodide for a dog 1.4 mg/kg (11). In other words, a 20-kg dog would receive 25-30 mg, whereas a small dog or cat would receive about 5-10 mg.  The easiest way to prepare a 15-mg dose for a newborn is to dissolve a 130-mg pill in 8 oz of a clear liquid and feed the newborn 1 oz of the liquid. Similar measures could easily be used in dogs and cats.

A single dose of KI protects the thyroid gland for up to 24 hours. A one-time dose at the levels recommended by the FDA is usually all that is needed to protect the thyroid gland. In the rare instance that radioiodine is present in the environment for longer than 24 hours, continuing the dose of KI every 24 hours for a few days would be needed.

  1. Report of the Chernobyl Forum Expert Group. Environmental Consequences of the Chernobyl Accident and their Remediation: Twenty Years of Experience. International Atomic Energy Agency, Vienna, 2006. 
  2. Awa A. Analysis of chromosome aberrations in atomic bomb survivors for dose assessment: studies at the Radiation Effects Research Foundation from 1968 to 1993. Stem Cells. 1997;15 Suppl 2:163-73.
  3. Ron E, Lubin JH, Shore RE et al. Thyroid cancer after exposure to external radiation; a pooled analysis of seven studies. Radiation Research 1995;141:256-277. 
  4. Jacob P, Kenigsberg Y, Zvonova I, et al. Childhood exposure due to the Chernobyl accident and thyroid cancer risk in contaminated areas of Belarus and Russia. British Journal of Cancer 1999;80:1461-1469. 
  5. Ivanov VK, Gorski AI, Maksiutov MA, et al. Thyroid cancer incidence among adolescents and adults in Bryansk region of Russia following the Chernobyl accident. Health Physics 2003;84: 46-60. 
  6. Cardis E, Kesminiene A, Ivanov V, et al. Risk of thyroid cancer after exposure to I-131 in childhood. Journal of the National Cancer Institute 2005;97:724-732. 
  7. Imaizumi M, Usa T, Tominaga T, et al. Radiation dose-response for thyroid nodules and autoimmune thyroid diseases in Hiroshima and Nagasaki Atomic Bomb Survivors 55-58 years after radiation exposure. Journal of the American Medical Association 2006;295:1011-1022. 
  8. National Research Council. Distribution and  administration of potassium iodide in the event of a nuclear incident, Washington, DC: National Academies Press, 2004.
  9. Center for Disease Control and Prevention website. Potassium iodide factsheet
  10. FDA website. Frequently asked questions on potassium iodide (KI). www.fda.gov
  11. Ribela MT, Marone MM, Bartolini P. Use of radioiodine urinalysis for effective thyroid blocking in the first few hours post exposure. Health Physics 1999;76:11-16.

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