Human Alimentary Tract Model

Draft document: Human Alimentary Tract Model
Submitted by Dr. Gunnar Brix, Federal Office for Radiation Protection, Division of Medical Radiation Hygiene and Dosimetry
Commenting as an individual

In my opinion the draft "Human Alimentary Tract Model for Radiological Protection" is very good. With its Annexes, also not yet finished, it will become even more detailed and it will be a good basis for dose calculations. There are, however, 3 main concerns: 1) I regret that in general the secretion into the alimentary tract is not covered very detailed and that especially the gall bladder is not at all considered. When calculating doses to workers and members of the public a gall bladder model may not be very important, but for the calculation of doses due to administration of radiopharmaceuticals to patients it may be very important (in some cases the gall bladder is expected to be the most exposed organ). I would prefer to have a unique ICRP model for the alimentary tract which can be used for all purposes of radiological protection rather than to have the need of additional models for the use of radiopharmaceuticals. 2) In the last paragraph of Chapter 1.3 it is written "Doses to the oral cavity are considered as doses to skin". In my opinion this sentence may cause severe problems in radiation protection which cannot yet overlooked totally: Due to ICRP Publication 60 and several national and international regulations the local skin dose (averaged over 1 cm²) is limited to 500 mSv. Due to the sentence cited above this had to be valid for the oral cavity, too. However, such a local dose cannot be calculated with the models given in this report (the dose to the oral cavitiy is not homogeneously distributed, for example, if retention of a beta-emitter on teeth is assumed). Moreover, the oral cavity is also part of the extrathoracic region (ET) of the respiratory tract model and especially for the anterior nasal passage (ET1) it can be stated in the same way that its dose can be considered as a skin dose. But then, due to the limitation of the local skin dose, almost all annual limits on intake had to be lowered considerably by up to a factor of 20 or even more. These effects should be considered. 3) In Chapter 5.4 three "Radionuclide-specific examples of the use of the model" are given. The first example describes the absorption pathway due to the old gastro-intestinal model (which is also included in the new alimentary tract model and which will be used in many cases if no special knowledge on the absorption processes is available). The other two examples, however, use a model structure different to the model structure given in Figure 5.1. Therefore these examples may be disturbing because they don't show how to use the alimentary tract model and they give the impression that the alimentary tract model given in this document is not suitable. I would prefer to describe the assumed biokinetics by the model proposed in Figure 5.1.