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18 March 2010

Answer to Question #7645 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:

I recently had a heart scan with 29 mCi of sestamibi ^99mTc and 4 mCi of ²⁰¹Tl. Your effective dose chart states that 2 mCi of ²⁰¹Tl would be 1,700 mrem. This is much higher than the 29 mCi of sestamibi. Is this because of the extended length of time the thallium stays in the body? Would the 4 mCi I received translate to 3,400 mrem?

More importantly, how can I get a general idea of the exposure to my child lying against me for two hours three days after the test? I was told this was safe by this time, but it would greatly ease my mind if someone could explain the actual dose equivalent to me.

The effective dose for a radiopharmaceutical depends on a number of factors. First, we calculate individual organ doses—which depend on the amount of the compound taken up in each organ and its rate of removal—then we apply "weighting factors" to the doses to each organ and sum up the contributions to get a single number that we call effective dose. More information may be found on our Web site.

^99mTc has only a six-hour physical half-life, whereas ²⁰¹Tl has a 73-hour physical half-life. Biological clearance from the body is also faster for sestamibi (MIBI) than for ²⁰¹Tl chloride. These two factors combined lead to somewhat higher organ doses, and thus the higher effective dose, for ²⁰¹Tl chloride over sestamibi (^99mTc), even though less activity is used. Yes, you are correct about the estimated effective dose from your 4 mCi administration, although I think that this dose estimate for ²⁰¹Tl is a bit out of date. More recent values suggest a lower dose, perhaps more like 2,300 mrem for a 4 mCi administration.

It's difficult to do an accurate dose calculation for reclining next to someone for a short period, with some ²⁰¹Tl remaining in the body, in the situation you describe. I did a rough calculation, using a few different assumptions, and came up with values between 0.05-0.2 mSv (5-20 mrem). For comparison, we all receive about 3 mSv (300 mrem) every year just from natural radiation in the environment around us, and this can vary up or down, within the range of the most conservative value I estimated. So it was a very low radiation exposure, also equivalent to a few trips in an airplane.

I hope that information is helpful.

Michael G. Stabin, PhD, CHP
Associate Professor of Radiology and Radiological Sciences

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