Methods of Estimating Intake - doe-std-1128-98

DOE-STD-1128-98

Biokinetic models are used in conjunction with bioassay data to evaluate the intake, uptake,

and retention of plutonium in the organs and tissues of the body. Intake estimates can then

be used to calculate committed effective and organ dose equivalents. It is essential that

good professional judgment be used in evaluating potential intakes and assessing internal

doses. Carbaugh (1994) has identified a number of considerations for dose assessments.

Computer codes are commonly used for assessment of intakes, dose calculation, and

bioassay or body content projections. La Bone (1994a) has provided an overview of what

should be considered in selecting a computer code, as well as descriptions of a number of

internal dosimetry codes available in 1994. Internal dosimetry code users should

understand how the code works and be aware of its limitations. Computer codes merely

provide the logical result of the input they are given. Use of a particular computer code

does not necessarily mean a dose estimate is correct.

As used in this section, the definition of "intake" is the total quantity of radioactive material

taken into the body. Not all material taken into the body is retained. For example, in an

inhalation intake, the ICRP Publication 30 respiratory tract model predicts that, for 1-m

particles, 63% of the intake will be deposited in the respiratory tract; the other 37% is

immediately exhaled (ICRP, 1979). For a wound intake, material may be initially deposited

at the wound site. Once the material has been deposited, it can be taken up into systemic

circulation either as an instantaneous process (e.g., direct intravenous injection of a

dissolved compound) or gradually (e.g., slow absorption from a wound site or the

pulmonary region of the lung). Both the instantaneous and slow absorption processes are

often referred to as uptake to the systemic transfer compartment (i.e., blood). Once material

has been absorbed by the blood, it can be translocated to the various systemic organs and

tissues.

An understanding of this terminology is important to review of historical cases. In the past

sites reported internal doses not as dose equivalent estimates but as an uptake (or projected

uptake) expressed as a percentage of a maximum permissible body burden (MPBB). The

standard tabulated values for MPBBs were those in ICRP Publication 2 (ICRP, 1959).

Many archived historical records may have used this approach. DOE Order 5480.11

(superseded), required calculation of dose equivalent. Now, 10 CFR 835 (DOE, 1998a), has

codified the calculation of intakes and committed doses.

5.8.1 Methods of Estimating Intake

There are several published methods for estimating intake from bioassay data

(Skrable et al., 1994a; Strenge et al., 1992; ICRP, 1988b; King, 1987; Johnson and

Carver, 1981). These methods each employ an idealized mathematical model of the

human body showing how materials are retained in and excreted from the body over

time following the intake. An intake retention function (IRF) is a simplified

mathematical description of the complex biokinetics of a radioactive material in the

human body. These functions are used to predict the fraction of an intake that will be

present in any compartment of the body, including excreta, at any time post-intake.

Intake retention functions incorporate an uptake retention model that relates uptake to

bioassay data and a feed model that relates intake to uptake and bioassay data. ICRP

Publication 54 (1988a) and Lessard et al. (1987) have published compilations of

IRFs. Selected IRFs calculated using the GENMOD Computer Code (incorporating

the Jones excretion function) for the lung, urine, and fecal excretion are show in

5-30