Answer to Question #10056 Submitted to "Ask the Experts"

The short answer to your first question is that there is only one thallium-201. I am making a supposition here, but I believe I can explain the discrepancy. The dose from thallium-201 is about 600 mrem per mCi. An administration of 2.88 mCi would result in a dose of about 1,700 mrem. Please note that I have rounded the numbers, since the actual radiation dose you received would depend on a variety of factors such as metabolism, hydration, urination frequency, etc., so using more exact numbers implies more precision than actually exists. These doses represent the total exposure from initial injection to total decay. 
 
You should be aware that thallium-201 is not as commonly used for nuclear cardiology studies as it once was. Current imaging agents use technitium-99m and radiation doses are substantially less. During the shortage this year of technitium-99m, many hospitals switched to using thallium-201 for these studies. Since I am not a nuclear cardiologist nor privy to your medical condition, I cannot tell you whether thallium-201 or one of the technitium-99m imaging agents is more appropriate. 

Rest assured, you are not doomed to getting cancer as a result of your stress test. While there is clear and convincing evidence of health effects at high doses, the dose from the thallium-201 study doesn't qualify as high. Indeed, below 5,000-10,000 mrem, risks of health effects are either too small to be observed or are nonexistent. Since health effects below these levels have not been observed, theoretical models are used to predict effects. So, at worst, you can say that there is a small theoretical increase in your chance of getting cancer. (We are talking about a fraction of a percent change in your chance of cancer.) But you could also say that there is no evidence that a radiation dose at this level has any health effect. 
 
From naturally occurring radiation that we are all exposed to everyday, by age 32, you would have received a radiation dose over 9,000 mrem just from living on Earth (based on U.S. average background radiation levels).
 
It's like peanut butter. Seriously. Peanut butter contains small amounts of aflatoxin, a known carcinogen. I could avoid eating peanut butter altogether, but it also has nutritional value and, even more important, it makes jelly sandwiches taste better. In other words, there is a risk and a benefit. So, I could eliminate that small risk, but I would lose the benefit, too. You need to consider that the potential health benefit of the diagnostic information from the nuclear cardiology study may outweigh the small theoretical health risk from the radiation exposure. I encourage you to read "Radiation Risk in Perspective," HPS Position Statement PS010-2, on the subject.  

Kent Lambert, MS, CHP
Director, Radiation Safety