Internal Dose Evaluation

DOE-STD-1121-98

7.0 INTERNAL DOSE EVALUATION

Radiation protection guides are expressed in terms of limiting values of dose to workers. As

summed with deep dose equivalent, 10 CFR 835.202 limits committed effective dose equivalent for

individuals and committed dose equivalent for their organs and tissues. Committed effective dose

equivalent and committed organ dose equivalent are calculated for intakes in specific calendar years to

evaluate conformance with limiting values for occupational exposure and for reporting doses to workers.

A committed effective dose equivalent is calculated (1) to evaluate conformance with limiting values for

control of the workplace, (2) to measure the effectiveness of the facility's radiation protection programs,

and (3) to provide a summary to the worker of the dose equivalent that may be received in subsequent

years as a result of any intake during the calendar year. The need may also arise to calculate doses over

other time periods such as from the date of intake to the first year following the intake, to the date when

the person would turn age 75 (i.e., "the expected lifetime dose"), and to the date of death.

There are three conceptually distinct methods to assess internal dose:

assessment of intake directly from air samples or other workplace data, followed by the assessment

of dose from intake

assessment of intake from bioassay data and biokinetic models, followed by the assessment of dose

from intake

direct assessment of dose time-integrated retention from bioassay data, with assignment of a putative

intake that is consistent with the dose.

Assessments of internal dose using mathematical biokinetic models should be based, as appropriate,

direct, in vivo measurements of a radionuclide(s) in various source organs of the body; or

indirect, in vitro measurements on excreta.

If bioassay data are not available or are of questionable value, assessments of inhaled radionuclides

should be based on workplace data, preferably on air sample measurements. The initial assessment of a

radionuclide intake or retained quantity may be based on air monitoring or other workplace measurement

data as well as available bioassay measurement data. However, assessments based only on workplace

monitoring data should be regarded as provisional and should be updated if and when bioassay

measurement data of sufficient quality become available. Evaluations of dose equivalent resulting from

an intake of a radionuclide proceed from an assessment of the amount of the radionuclide in organs and

tissues of the body as a function of time. The radionuclide distribution and retention depends on the

physical and chemical forms of the radionuclide, its radiological properties, the physiological

characteristics of the individual, the route(s) of intake, and the magnitude of intake(s).

Except for radon, thoron, and their short-lived progeny, internal dose equivalent is defined in terms

of the energy imparted to target tissues from the radiations emitted by radionuclides in source organs and

tissues of the body. The purpose for analyzing radionuclide intake and retained quantity as a function of

time is to identify the organs and tissues in which the radionuclide is deposited and to evaluate the

cumulated activity (e.g., transitions in BqAs or :Ci-days; 1 :Ci-day = 3.1968E9 transitions) in source

organs. Since it is often difficult to precisely determine the cumulated activity in all source organs

directly from bioassay measurements, biokinetic models have been developed to describe empirical

relationships between intake, number of transitions, and bioassay measurement values.