Answer to Question #13787 Submitted to "Ask the Experts"

I have worked in diagnostic nuclear medicine for almost 30 years, predominantly with ^99mTc. We never wear lead aprons, and I have always thought that this is because one apron would not prevent all 140 keV gamma radiation from getting through to the wearer, but would introduce Compton scattering which could theoretically reduce the transmitted radiation's energy to a more absorbable (and consequently harmful) level. When I explain this to patients, I joke that I'd need to wear about three lead aprons for the shielding to be completely effective—so I don't wear one at all. Is my reasoning correct, or should we be changing our current working practices?

This question is one that has been asked for a number of years, primarily by nuclear medicine technologists. Presumably you've searched the internet for some scientific papers that discuss this very issue and there are certainly quite a few of them. In general, you will find that a typical 0.25 mm Pb (lead) "equivalent" apron will reduce the equivalent dose to the wearer by about 50% for radiation in the 140 keV range (i.e., for ^99mTc). Obviously, the Pb apron attenuates the 140 keV photons which results in the primary reduction in intensity; however, you are correct that some lower energy radiation is actually created due to interactions in the Pb apron by the higher energy photons.

However, the main origin of these lower energy photons is not from Compton scattering, but from characteristic x rays associated with Pb. The K shell x-ray absorption edge in Pb is 88 keV; therefore, photon energies greater than that value can and do result in these characteristic x rays. A recent paper published in Radiation Protection Dosimetry shows the energy spectra from various radionuclides after the photons have passed through a Pb apron. Click on "read full text" to see the entire article. Cobalt-57 (⁵⁷Co) is frequently used as a comparison to ^99mTc due to the photon energies being similar. Take a look at Fig. 3.

The photopeaks at 72.81, 74.97, and 84.94 keV are from the K_a1, K_a2, and K_b1 energy levels respectively. From the graph, you can see these appear when the Pb apron is present. That reference provides energy spectra from other radionuclides and, as you might imagine, these photopeaks also appear in those spectra when the main photon energy exceeds 88 keV. With all that said, the actual energy spectra will likely vary because many of the newer Pb aprons don't really contain Pb, but a mixture of various metals, all of which will result in different energy spectra. Even with these additional photopeaks, the overall protection provided by the Pb apron for ^99mTc photons is still about 50%, so using a Pb apron when working with that radiopharmaceutical does provide some protection.

Mack L. Richard, MS, CHP