Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
effective doses, which are dominated by contributions from other tissues (ICRP,
2006). Information on retention in alimentary tract tissues is given, where available,
in individual element sections of this report series.
3.3.5 Alimentary Tract Dosimetry
(189) The HATM allows explicit calculations of dose to target regions for cancer
induction within each alimentary tract region, considering doses from radionuclides in
the contents of the regions, and considering mucosal retention of radionuclides when
appropriate.
(190) The oesophagus and oral cavity will receive very low doses from ingested
radionuclides because of short transit times in these regions (ICRP, 2006). However,
they were included because a specific wT is assigned to the oesophagus (ICRP, 2007),
and because retention in the mouth, on teeth for example, can result in a substantial
increase in dose to the oral mucosa (which was added to the organs and tissues
constituting the remainder in Publication 103).
(191) In general, the alimentary tract regions of greater importance in terms of doses
and cancer risk are the stomach and particularly the colon. While the small intestine
may receive greater doses than the stomach, it is not sensitive to radiation-induced
cancer and is not assigned a specific wT value (ICRP, 2007), but is included in the
remainder.
(192) An important refinement in the HATM is the methodology used to calculate
doses in the various regions from non-penetrating alpha and electron radiations. Thus,
while the Publication 30 approach was to assume that the dose to the wall was one
half of that to contents of the region, with an additional factor of 0.01 included for
alpha particles to allow for their short range (ICRP, 1979), the HATM takes explicit
account of the location of the target tissue in the mucosal layer of the wall of each
region. The targets relating to cancer induction are taken in each case to be the
epithelial stem cells, located in the basal layers of the stratified epithelia of the oral
cavity and oesophagus and within the crypts that replenish the single cell layer
epithelium of the stomach and small and large intestines.
(193) This new methodology generally results in substantially lower estimates of
doses to the colon from alpha and beta-emitting radionuclides than obtained using the
Publication 30 model. This is because of the loss of the alpha particles and electrons
energies in the colon contents and in the mucosal tissue overlying the target stem cells
(at a depth of 280 – 300 m). This reduces energy deposition in the target tissue for
electrons and results in zero contribution to dose in the target tissue from alpha
particles emitted within the contents. In the absence of retention of radionuclides in
the alimentary tract wall, doses from ingested alpha emitters to all regions of the
alimentary tract will be solely due to their absorption to blood and subsequent
irradiation from systemic activity in soft tissues.
(194) The consequences of this decrease in local colon dose on the total committed
effective dose will vary according to the radionuclide. Examples given in Publication
100 (ICRP, 2006) for 55 Fe, 90 Sr and 239 Pu show that this decrease of local dose to the
colon has little or no impact on the effective dose since the dominating contributions
are from equivalent doses to organs and tissues from activity absorbed to blood. In
general, the effect on effective dose is small for radionuclides with large fA values or
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