Answer to Question #14031 Submitted to "Ask the Experts"

I recently read an article online titled "An Accident of Internal Contamination with Plutonium and Americium at a Nuclear Facility in Japan: A Preliminary Report and the Possibility of DTPA Administration Adding to the Diagnosis."

Does this article mean that americium detection can be missed in a urine sample as it was not picked up in the original urine sample before diethylenetriaminepentaacetic acid (DTPA) administration? Would this mean that those who work in the nuclear field and have annual bioassays potentially have intakes which go undetected?

When actinide radionuclides such as americium-241 (²⁴¹Am) are inhaled or ingested, they initially reside in the respiratory or alimentary tracts, respectively, followed by metabolism into the bloodstream and retention in particular organs and tissues such as bone and liver. The radionuclide then recycles into the bloodstream where it can be reabsorbed by the same or another organ or tissue or excreted.

DTPA, when administered, increases the metabolism of heavy metals, including actinides, out of the bloodstream through the excretory process. This means that the amount, in this case, of ²⁴¹Am in a urine or fecal sample should actually increase following administration making it easier to detect.

As with all analyses for radionuclides in a matrix (e.g., urine, soil, feces), there is a limit to the detectability of ²⁴¹Am because of the limitations of the method itself. For alpha-emitting radionuclides with no or a low-energy accompanying gamma emission, alpha spectrometry is the common detection methodology. This requires that as much ²⁴¹Am as possible be chemically extracted from the sample followed by electroplating on a metal disc and counted in an evacuated chamber using a surface barrier or similar detector. Chemical recoveries (typically 50–90%) and detector efficiencies (30–40%) make up the detectability. What also must be considered is that the amounts being looked for are in the picocurie (pCi) or millibecquerel (mBq) range, tiny amounts of radioactivity.

It is possible for individuals working with actinides to have intakes that are undetected. This situation can be minimized in three ways. The first is by designing a routine radiobioassay program of frequencies of a few weeks, rather than a year, where the excreted fraction of the intake is greater. The second is to have a robust operational health physics program that detects exposures very soon after they occur so that those exposed workers can be sampled. The third would be to assign personal air monitoring where the worker wears a filter pump during work and the filters are collected and analyzed each day.

Urine and fecal sampling techniques can both be used for detection of ²⁴¹Am. Radioactivity found in the urine has metabolized systemically; that is, through the bloodstream to organs and tissues back to the bloodstream into the kidneys and out of the body. Therefore, the radioactivity in the sample is more representative of that which causes dose to those organs and tissues including the respiratory and alimentary tracts where the intake occurs.

Fecal sampling does have some advantages, but also some quirks. I am sure it is obvious that ingested material that is not absorbed into the bloodstream out of the alimentary tract will be excreted in feces. A sizeable fraction of inhaled material, over 30%, is deposited in the posterior nasal passage and pharynx where it is swallowed and digested. Once the material is swallowed, it forms a bolus in the stomach which travels through the small and large intestines to excretion. However, the models used for the alimentary tract treat each section of the alimentary tract exponentially, which means that some small amounts, in this case, of ²⁴¹Am would be expected in the feces immediately after intake. This is clearly an overestimation. However, within 24 hours there will be a detectable amount of ²⁴¹Am in the feces (dependent, of course, on the amount of intake), so fecal monitoring is useful to determine that an individual received an intake—but not necessarily its magnitude. The uncertainties in the model tend to smooth out after a week or two following the intake and fecal results can then be used to determine dose with more confidence.

The length of time that an intake is detectable is completely dependent on the size of the intake. It could be months, years, or even decades that ²⁴¹Am is still detectable, since the radiological half-life is 430 years and the body's rates of metabolism are very slow.

I hope my lengthy answer provides some clarity.

Charles "Gus" Potter, PhD, CHP