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

I recently underwent a nuclear medicine procedure using ²⁰¹Tl for a cardiac stress test. How many types are there?  Is there a strong one and a weak one?  My doctor told me I was exposed to 6 mSv from 107 MBq. On your website it indicates more (i.e., for 107 MBq about 17 mSv). Is that possible?

Another question I have is: I have since read a lot of articles and I am concerned over the radiation I was exposed to during this test. I refuse to take another one. I was 32 when I took the nuclear stress test. Do I still have a chance at living a long life without getting cancer or am I now doomed to get it in the future at a younger age than I should have because of this test? Are there a lot of people who get cancer within years of taking this test?

The short answer to your first question is that there is only one ²⁰¹Tl. I am making a supposition here, but I believe I can explain the discrepancy. An administration of 107 MBq would result in a dose of about 17 mSv. 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 ²⁰¹Tl is not as commonly used for nuclear cardiology studies as it once was. Current imaging agents use ^99mTc and radiation doses are substantially less. During the shortage this year of ^99mTc, many hospitals switched to using ²⁰¹Tl for these studies. Since I am not a nuclear cardiologist nor privy to your medical condition, I cannot tell you whether ²⁰¹Tl or one of the ^99mTc 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 ²⁰¹Tl study doesn't qualify as high. Indeed, below 50–100 mSv, 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 90 mSv just from living on Earth (based on US 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" on the subject.  

Kent Lambert, MS, CHP
Director, Radiation Safety