Answer to Question #9412 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

Q

I had a nuclear bone scan about two months ago using 925 megabecquerels (MBq) of 99mTc MDP (methylene diphosphonate). I was told that all of the 99mTc MDP would be out of my system within a few days. The technician said its completely safe and that all radiation is gone quickly and that the  99mTc has a short half life of six hours.

I was told that this product accumulates in/on the bones. I also recently read that the  99mTc decays to 99Tc which is also radioactive but is a "soft beta emitter" with a half life of over 200,000 years.

Do any of the 99Tc particles end up staying embedded in my bones to radiate them? I know that a lot of the 99mTc MDP is supposed to be excreted through urine—but what about the portions that end up in your bones? I don't like the idea of having a radioactive substance sitting in bones over the years.

A

Yes, 99mTc decays to 99Tc, which is radioactive. However its half-life is about 211,000 years. Every decay of 99mTc produces an atom of 99Tc, it's true, BUT, the decay rate of a nuclide is inversely proportional to its half-life.

Let me explain: the activity of the 99Tc produced is 462.5 MBq since half of the material is excreted in the urine and half (462.5 MBq) is incorporated in the skeleton. We can calculate this proportionality: (462.5 MBq × 6 hours)/(211,000 years × 365 days/year × 24 hours/day) = 0.00000148 MBq. This amount is below levels of natural background radioactivity in everyday materials, including in your body. Your body has perhaps 0.004 MBq of the naturally occurring nuclide 40K, which is constantly produced in the atmosphere and is incorporated into many materials, including our bodies! So there is really no hazard to worry about from this tiny amount of 99Tc.

Michael G. Stabin, PhD, CHP

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