Radiobioassay

DOE-HDBK-1184-2004

4.3 Radiobioassay

4.3.1

Urinalysis

Soluble materials ingested or inhaled into the body will be excreted via urine. Given

a known concentration of the contaminant in the urine and knowledge of the time of

exposure and rate of dissolution of the material in body fluids, one can derive a total

intake quantity. While determinations of the exposure time and urine concentration

may be relatively straightforward for STCs, determination of dissolution rates is

more difficult. Chapter 5 discusses the use of urinalysis in performing internal dose

assessments for intakes of STCs.

4.3.2

Fecal Analysis

Insoluble materials that are inhaled into the body will be excreted via feces.

Retention factors and total intake quantity can be calculated based on the models

described in Section 5.1, given a known concentration of the contaminant in the

feces and knowledge of the time of exposure, radioactive decay rate and solubility.

Determination of exposure time is straight forward, but no reliable protocol currently

exists for determining concentrations of STCs in feces2.

4.3.3

In-Vivo Analyses

In-vivo analyses depend on the detection of radiation emitted by radioactive

materials within the body by radiation detectors external to the body. In-vivo

analysis is not considered to be a viable means of assessing tritium intakes,

including STC intakes. The are two reasons for this: 1) Tritium decays by emission

of low energy beta particles that will not penetrate through the body to the external

detector, and 2) A significant number of bremastrahlung x-rays resulting from the

interaction of the tritium beta with the metal atoms of the host particle are not

expected to escape to an external detector unless the STC uptake is massive.

4.4 Organically Bound Tritium (OBT)

4.4.1

Soluble OBT

Soluble OBT migrates through the skin or lung into the bloodstream by the physical

processes of dissolution and diffusion. The two processes are inseparably linked

and often simply called "absorption." Soluble OBT is also readily absorbed through

the GI tract following ingestion. Rapid dispersion minimizes organ-specific (e.g.,

lung) differential doses. Following absorption into the body, soluble OBT is

excreted via urine. A biokinetic model is available which relates intakes of soluble

OBT to urine excretion rates, and a dose conversion factor is available for soluble

OBT intakes. Urine bioassay is therefore considered to be a viable approach to

estimating intake and dose from soluble OBT. More information on dose

assessment for soluble OBT is found in Section 5.2.5.1.

For intakes expected to result in doses greater than 2 rem CEDE, one may want to collect and store fecal samples

pending development of a fecal biokinetic model and assay technique.