Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
means, generally by scientific judgment using all relevant information available. In
the case of a measurement of activity in the total body or in a biological sample, Type
A uncertainties are generally taken as those that arise only from counting statistics and
can be described by the Poisson distribution, while Type B components of uncertainty
are taken as those associated with all other sources of uncertainty.
(360) Examples of Type B components for in vitro measurements include the
quantification of the sample volume or weight; errors in dilution and pipetting;
evaporation of solution in storage; stability and activity of standards used for
calibration; similarity of chemical yield between tracer and radioelement of interest;
blank corrections; background radionuclide excretion contributions and fluctuations;
electronic stability; spectroscopy resolution and peak overlap; contamination of
sample and impurities; source positioning for counting; density and shape variation
from calibration model and assumptions about homogeneity in calibration (Skrable et
al, 1994). These uncertainties apply to the measurement of activity in the sample.
With excretion measurements, the activity in the sample is used to provide an
estimate of the subject’s average excretion rate over 24 hours for comparison with the
model predictions. If the samples are collected over periods less than 24 hours then
they should be normalised to an equivalent 24 hour value. This introduces additional
sources of Type B uncertainty relating to biological (inter-and intra-subject)
variability and sampling procedures, which may well be greater than the uncertainty
in the measured sample activity. Sampling protocols can be designed to minimize the
sampling uncertainty, as shown by Sun et al (1993) for plutonium urinalysis and
Moeller and Sun (2006) for indoor radon exposure.
(361) In vivo measurements can be performed in different geometries (whole body
measurements, and organ or site-specific measurement such as measurement over the
lung, thyroid, skull, or liver, or over a wound. Each type of geometry needs
specialized detector systems and calibration methods. The IAEA (1996a) and the
ICRU (2003) have published reviews of direct bioassay methods that include
discussions of sensitivity and accuracy of the measurements.
(362) Examples of Type B components for in vivo monitoring include counting
geometry errors; positioning of the individual in relation to the detector and
movement of the person during counting; chest wall thickness determination;
differences between the phantom and the individual or organ being measured,
including geometric characteristics, density, distribution of the radionuclide within the
body and organ and linear attenuation coefficient; interference from radioactive
material deposits in adjacent body regions; spectroscopy resolution and peak overlap;
electronic stability; interference from other radionuclides; variation in background
radiation; activity of the standard radionuclide used for calibration; surface external
contamination of the person; interference from natural radioactive elements present in
the body; and calibration source uncertainties (IAEA, 1996a; Skrable et al, 1994).
(363) For partial body measurements it is generally difficult to interpret the result in
terms of activity in a specific organ because radiation from other regions of the body
may be detected. Interpretation of such measurements requires assumptions
concerning the biokinetics of the radionuclide and any radioactive progeny produced
in vivo. An illustration using 241 Am is given in the IAEA Safety Series Report on
Direct Methods for Measuring Radionuclides in the Human Body (IAEA, 1996a). A
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