A worker inhaled I-131. Thyroid measurements and urine sampling started one day after inhalation. The activity values in the urine samples are normalized to daily excretion. Calculate the inhaled activity, the committed effective dose and the equivalent dose to the thyroid.









This is in principle a very simple case because most of the intake parameters are known. After having entered the measured data (blue table in the screenshot), you may evaluate the data assuming a single intake by inhalation one day before the first measurement. I-131 is allocated typically to absorption type F. The particle size may be assumed to be 5 µm because the subject is a worker.


























The results, however, are looking not that good. There is a significant discrepancy between the measured data and the fitted retention- and excretion function. The measured activity in the thyroid is about one order of magnitude lower than the model retention function and the measured activity in the urine is – after four days – almost one order of magnitude higher than the model excretion function. This is a typical finding when the assumed solubility is too high.
































So you should try to adjust the absorption type. The software will tell you that the best fit can be achieved when assuming absorption type S.




However, if you apply absorption type S, the software will tell you that the respective dose coefficients are not available. So there is no way to calculate the dose for this case scenario. This might look like a bug but actually it is very reasonable. As mentioned already under the second exercise, IDEA doesn’t allow the user to do something which is not in agreement with the regulations or recommendations. These regulations and recommendations say that I-131 should be allocated always to absorption type F. In fact you cannot find in the ICRP recommendations any compound of I-131 which could be allocated to absorption type S and so IDEA puts on the emergency brake if the user would try to apply type M or type S for I-131. The emergency brake, however, is put on only for workers, because the recommendations refer only for occupationally exposed persons.





















So you can avoid the brake when allocating the person to a member of the public. When doing so you will get a perfect fit of measured data (see screenshot on the left).








































But in spite of the good fit you will get very strange results in terms of dose.  So in this case the critical organ is the lung with the organ dose being 9,9 mSv. The dose to the thyroid is only 2 mSv. Here, finally, the alarm bell is ringing…


















This exercise is an excellent example illustrating how IDEA is shortening the free pathway for making mistakes. If you evaluate such case with IMBA you will get perfect fitting of data but there will be no warning that something might be wrong with the data. There might be several reasons such as wrong positioning of detector for thyroid measurement or contamination of urine sample. Moreover, if thyroid measurements would have been performed with NaI(Tl) detectors, Ba-133 could have been identified by mistake as I-131 (contrary to Iodine, Barium has type S compounds). All these reasons are not very likely but they are much more likely as the existence of a type S compound of Iodine.










Prof. Dr.-Ing. Hans Richard Doerfel

IDEA System GmbH, Am Burgweg 4, D-76227 Karlsruhe, Germany.

E-Mail: info@idea-system.com