Answer to Question #12910 Submitted to "Ask the Experts"

Category: Radiation Basics — Doses and Dose Calculations

The following question was answered by an expert in the appropriate field:

Q

Please discuss accurate thorium bioassay tests to determine amounts in the system as presented in International Commission on Radiation Protection (ICRP) Publication 137 and the best way to determine inhalation quantity or amount swallowed and amount retained by organs.

A

The parts of the question really encompass the whole of internal dosimetry, so I will not be able to provide all of the required detail; however, I do provide some references that I hope are helpful.

As of 2019, the ICRP has provided four complete sets of biokinetic models: ICRP 2, ICRP 26/30 and supplements, the models of the mid-1990s including ICRP publications 56, 66, 67, 69, and 71, and now the three-volume set including ICRP publications 130, 134, and 137. Each of these sets of models improves on the last by incorporating additional data into the development of the models and taking advantage of the computing power available at the time of development. That being said, the thorium biokinetic model in ICRP publication 137 is that published in ICRP publication 69 from which one can conclude that no changes were deemed necessary to that particular model.

The ICRP 130 set of models incorporates updated radiological data (ICRP 107), reference person parameters (ICRP 89), and the alimentary tract model from ICRP 100. The respiratory tract model from ICRP 66 is adopted with some changes, mostly the reduction in short-term clearance compartments, and liberally uses the "bound state" absorption compartment described in ICRP 66 but not implemented at the time of its publishing. While this would ideally add accuracy to calculations of intake and dose made using the model, it also implies an expectation for the dosimetrist to know the intake pathway (inhalation or ingestion) and chemical compound to achieve the most accurate result. There are five different options with associated dose coefficients for thorium: ingestion and four sets of compounds for inhalation (ICRP 137, Table 14.9).

The use of ICRP models for calculation of intake and dose typically requires determination of the fraction of intake in an organ, tissue, or excreta being measured. ICRP 67 provides lists of fractions in excreta for some times that correspond to typical routine bioassay period or accident scenario. However, these lists are inadequate for common scenarios where the indicative bioassay result occurs at a random (sometimes weeks or months) time post intake with follow-up measurements over the next several months. Therefore, a user would have to develop a more robust set of "intake retention fractions" useable for their particular situation (Potter 2002). A fitting technique such as that in Potter (2002) or the IDEA system (Castellani 2013) could then be used to determine the intake and dose coefficients from ICRP 137 to infer the dose. As is described in Potter (2002), functions can be developed that describe the retention in each organ or tissue in the model.

Determination of the intake pathway (inhalation or ingestion) is extremely difficult, perhaps impossible, using these techniques. While one could perhaps perform a calculation for each of the five scenarios and compare using a fitting statistic (r2, X2) or visually, there is always a likelihood that both inhalation and ingestion occurred or that the original compound changed chemically during process resulting in a mixed source term.

A "best way" in my experience is to use workplace data such as surface or airborne contamination measurements, nasal swabs, etc., to determine the intake pathway and then fit data to a default or pre-determined compound class. ICRP provides suggested default compound types for unspecified compounds, with Class S being recommended for thorium.

I am sure that as the internal dosimetry community adopts the models, there will be more information available about their use and inferred accuracy.

Charles Potter, PhD, CHP

References

Potter CA. Intake retention fractions developed from models used in the determination of dose coefficients developed for ICRP publication 68 – particulate inhalation. Health Phys 83(5):594–789;2002.

Castellani CM, Marsh JW, Hurtgen C, Blanchardon E, Berard P, Giussani A, Lopez MA. IDEAS guidelines (version 2) for the estimation of committed doses from incorporation monitoring data. EURADOS Report 2013-01, Braunschweig, DE:EURADOS; 2013.

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