Occupational Intakes of Radionuclides Part 1 - ICRP

4896
4897
4898
4899
4900
4901
4902
4903
4904
4905
4906
4907
4908
4909
4910
4911
4912
4913
4914
4915
4916
4917
4918
4919
4920
4921
4922
4923
4924
4925
4926
4927
4928
4929
4930
4931
4932
4933
4934
4935
4936
4937
4938
4939
4940
4941
DRAFT REPORT FOR CONSULTATION
dose to an organ, for example, are considered to be those associated with the
underlying mean absorbed dose.
(394) The physical and anatomical parameters contributing to uncertainties in the
mean absorbed dose for internal emitters are:
Energy and intensity of the nuclear and atomic radiations emitted by the
radionuclide and by any radioactive progeny;
Interaction coefficients of the emitted radiations in tissues;
Elemental composition of the tissues of the body;
Volume, shape, density of the organs of the body; and
Parameters describing the spatial relationship of the source regions (regions
containing the radionuclide) and the target regions (radiosensitive organs and
tissues for which dose values are desired).
(395) Limitations are present in the computational model representing the anatomy
and in the numerical procedures used to calculate the energy absorbed in the target
tissues. The magnitudes of these uncertainties vary with radiation type, the energy of
the radiation, and the specific source-target pair. The adoption of computational
phantoms based upon medical imaging data (often referred to as voxel phantoms) has
reduced the uncertainties associated with cross-irradiation of tissues by photon and
neutron radiations to some extent by providing more realistic spatial relationships of
some source and target regions (ICRP, 2009). However the absorbed dose is
frequently dominated by the contributions from non-penetrating radiations. For source
and target regions that cannot be resolved in the medical image data, e.g., source and
target regions in the respiratory and alimentary tracts and in the skeleton, uncertainties
are associated with the computational models used to represent these regions.
(396) The anatomical models are static and thus do not address uncertainties in the
spatial position of the organs due to breathing and posture other than reclining.
Reference values for the masses and elemental composition of the organs of the body
have been defined in Publication 89 (ICRP, 2002a) and used in the reference
computational models of the anatomy (computational phantoms) noted above.
(397) The parameters of the dosimetric model contributing to uncertainties in the
absorbed dose are those physical parameters associated with the nuclear
transformation processes that determine the energy and intensity of the emitted
radiation and parameters which govern the transport radiations in the body. An
uncertainty less than 10% has been assigned to attenuation and absorption coefficients
for photons with somewhat higher uncertainties ascribed to soft tissue stopping power
values for alpha and electron particles. Improvements in the basic nuclear data have
reduced the uncertainties in the physical half-lives of radionuclides and the branching
fractions of decay modes. The simplified procedures used in the dosimetric
calculations to address the delayed beta and gamma radiations of spontaneous fission
can contribute substantial uncertainties in the mean absorbed dose in some tissues.
(398) The dosimetric calculations must associate an anatomical region (source
region) with each biokinetic compartment. Many biokinetic models partition the
systemic activity among a few identified organs/tissues and include a compartment
referred to as ‘Other tissue’ which represents the residual. The dosimetric procedure
distributes the activity in the ‘Other tissue’ compartment uniformly among all tissues
not explicitly noted in the model. Substantial uncertainty may be associated with the
134