Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
calculated. The target cells identified in ET1, ET2, BB and bb, and the masses of
tissue containing target cells in each region for dose calculations, are given in Table 8.
(171) In each of these regions there are also several possible source regions. In the
bronchiolar region (bb), particles retained in the airway wall (bbseq) are taken to be in
a macrophage layer at a depth of 20-25 μm (i.e. below the target cells); activity
‘bound’ to the epithelium is uniformly distributed in it; and account is also taken of
irradiation from activity present in the AI region. In the original HRTM, activity in
the fast phase of clearance (compartment bb1, Figure 5) was taken to be in the mucus
layer above the cilia; activity in the slow phase of clearance (bb2) was taken to be in
the mucus between the cilia. In the revised HRTM, there is only one phase of
clearance.
(172) For each source/target combination, Publication 66 provides absorbed
fractions for non-penetrating radiations: α, β and electrons; in each case as a function
of energy. Since these absorbed fractions are not represented in the voxel phantoms
because of inadequate spatial resolution, the values were derived in Publication 66
using a single cylindrical geometry to represent each region of the conducting airways
(ET1, ET2, BB, bb): the representative bronchus for BB being 5 mm diameter and the
representative bronchiole for bb being 1 mm diameter. The absorbed fractions for the
BB and bb source regions were derived as the thickness-weighted sum of the slow
and fast clearing source regions, as tabulated in Publication 66.
(173) To take account of differences in sensitivity between tissues, the equivalent
dose, Hi, to each region, i, is multiplied by an apportionment factor, Ai, representing
the region's estimated sensitivity relative to that of the whole organ. The
recommended values of Ai are also given in Table 8. In Publication 103 (ICRP, 2007)
the extrathoracic and thoracic lymph nodes were included in the tissue ‘lymphatic
nodes’, which is itself included in the list of remainder tissues and organs (Table 2),
and so are no longer included in the extrathoracic and thoracic airways respectively as
they were in the original HRTM. The fractions, Ai, of wT that they were assigned in
Publication 66 are reassigned to other regions in Table 8. The weighted sum of the
equivalent dose, Hi, to each region, is the equivalent dose to the extrathoracic or
thoracic airways respectively:
H = H
= H BB A
(174) The tissue weighting factor, wT of 0.12 specified for lung in Publication 103
(ICRP, 2007) is applied to the equivalent dose to the thoracic region, HTH. The
extrathoracic airways are included in the list of remainder tissues and organs (Table
2).
Table 8. Target tissues of the respiratory tract
Tissue Region Target cells Depth of
target cell
nuclei a , μm
H
TH
BB
77
AET
+ 1 H ET 2 A
+ H bb Abb+
H
ET ET 1
ET 2
AI
A
AI
Mass of target tissue a,b ,
kg
Assigned
fraction a,c
Ai of wT