Answer to Question #11931 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations

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Q

This question pertains to 24-hour urine bioassay vs. whole-body counting (WBC). My understanding based on International Commission on Radiological Protection (ICRP) Publication 78 is that if we can detect gamma emitters though WBC, then we don't have to do urine bioassay over a 90-day monitoring period. Is the monitoring overkill if both methods are used to quantify intakes? Of course, we would consider both soluble and insoluble elements.

A

I assume that your operating/possession license does not contain any specific requirements for performing biological excreta measurements. If this is the case, then your understanding that WBC is satisfactory as a method to determine the intake of particular radionuclides, exclusive of any other measurements of biological excreta, is often legitimate. Certain limitations to using external monitoring do apply. I will refer to external monitoring rather than WBC in some of this discussion.

Naturally, the radionuclide(s) of concern must emit photons of sufficiently high energy and abundance that successful external measurements can be made with sufficient sensitivity to meet necessary reference/action levels; for many gamma-emitting radionuclides of common interest, this criterion is relatively easy to meet. For some radionuclides (such as several alpha-decay nuclides that emit no photons, low-yield photons, or low-energy photons), external monitoring is not well suited, and other bioassay techniques, typically urine and/or fecal analyses, are better suited to the estimation of intakes.

Also, when using external counting one must be careful to ensure that no significant contamination is present on the clothing or skin of the affected individual(s). When dealing with radionuclide intakes that result in highly preferential uptake by specific organs, such as radioiodine by the thyroid gland, it is preferable to use direct external monitoring of that organ or tissue, if possible. This is often done by selectively placing the detector close to the organ and using collimated detectors. Additionally, when very insoluble species are inhaled and the deposition is heavily confined to the lungs, often with slow removal therefrom, it is preferable to perform lung monitoring rather than WBC. For radionuclides that distribute throughout much of the body, such as radiocesium, WBC is desirable.

When conditions are met for doing acceptable external monitoring, either WBC or specific organ/tissue monitoring, this approach is often preferable to measurements of biological excreta, not only because it is faster, often more economical, and more easily accomplished than are excreta measurements, but also because it is often more accurate and reliable. This is true because the correlation of measured activity with intake via an external measurement is often subject to less uncertainty in the measurement(s) and in the mathematical model(s) necessary for correlation than when dealing with measurements of biological excreta. (A notable exception is urine monitoring of tritium, which provides an accurate and simple method for estimating intakes; of course, tritium, which emits no photons, cannot be assessed by external monitoring.)

While there is no specific prohibition against performing both external monitoring and biological excreta measurements, once both such methods are used you must then decide how you will use the results to obtain intake estimates. Will you estimate intakes from both procedures and average them together? Will you decide ahead of time that only one method will be used as the official method of record and the other will be used simply for confirmation (or some other purpose)? The reason I raise this issue is because, depending on the radionuclide(s) involved, there may arise significant differences between intakes estimated from the two results; this, in turn, introduces considerable uncertainty in your final intake estimate if both results are used to obtain an average result. This may not be a concern if your intakes are always a small fraction of allowed intakes or of other action levels, but large uncertainties can become problematic if calculated intakes are close to limiting values.

Clearly, my personal preference is to use external monitoring measurements as the preferred technique if the method meets all the necessary criteria to make acceptable accurate assessments.

George Chabot, PhD, CHP

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