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
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.