Answer to Question #10393 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

When a dialysis patient has a myocardial perfusion imaging cardiac procedure, should thallium or technetium be used as the radionuclide imaging agent? I'm wondering about both aspects—harm to patients' kidneys and radiation exposure to dialysis workers.

A

There are a couple of different 99mTc imaging agents, now used in lieu of 201Tl, including sestimibi and tetrofosmin. The nuclear medicine group at our facility uses the tetrofosmin imaging agent—333 MBq during the resting phase and 925 MBq during the stress phase. Using the dose estimates from International Commission on Radiological Protection (ICRP) 80, Radiation Dose to Patients from Radiopharmaceuticals, Addendum 2 to ICRP Publication 53, the total effective dose from those two phases can be calculated to be 9.02 mSv and an equivalent dose to the kidney of 14.93 mSv.

When our nuclear medicine group performs the same study with 201TlCl, it administers 185 MBq and performs the stress portion of the study, allows redistribution of the 201TlCl, and then performs the resting portion of the study. Using the ICRP 80 information, the calculated effective dose for 201TlCl is 40.7 mSv and the equivalent dose to the kidney is calculated to be 88.8 mSv.

While the 99mTc tetrofosmin requires two administrations for a total administered activity of 1,332 MBq, the effective dose is 4.5 times lower and the kidney equivalent dose is about 6 times lower than a single 185 MBq administration of 201Tl. Thus, from the radiation dose perspective, the 99mTc tetrofosmin is a better choice. One would expect similar results for other 99mTc imaging agents. While the medical community has adopted the "as low as reasonably achievable" philosophy in terms of patient doses, it isn't clear whether or not kidney doses in this range would have a significant effect on patients' kidneys, but equivalent doses in this range would not be expected to result in observable (deterministic) effects.

In terms of doses to dialysis workers, a number of variables will affect potential equivalent doses to those individuals. One would expect a higher equivalent dose rate around the 99mTc patients following initial administration; however, due to the short half-life, the dose equivalent rate will drop to very low levels during the following couple of days. Of course, 201Tl has a longer half-life (73 hours), but considerably less is administered. Thus, the equivalent dose to dialysis workers would be somewhat dependent upon when dialysis was performed, compared to when either radiopharmaceutical was administered.

Regardless, National Council on Radiation Protection and Measurements Report No. 124, Sources and Magnitude of Occupational and Public Exposures from Nuclear Medicine Procedures estimates that "non-nuclear medicine hospital personnel" are likely to receive no more than 0.1 mGy (equivalent to 0.1 mSv) per year from nuclear medicine patients. Dialysis workers would most likely fall into that category. 0.1 mSv is a very small percentage of the average annual effective dose from natural background received by the average member of the U.S. population, estimated to be 3 mSv per year.

Mack L. Richard, MS, CHP

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