Thyroid Dose Reconstruction for Children with Thyroid Cancer in Belarus

Zoltán Kis1, 3 Katalin Eged2 and Béla Kanyár2

1 GSF-Institut für Strahlenschutz, Neuherberg, Germany
2 Department of Radiochemistry, University of Veszprém, Veszprém, Hungary
2 Nuclear Medicine, Csolnoky Ferenc County Hospital, Veszprém, Hungary
   
Corresponding author: Zoltán Kis, Ph. D.
    Nuclear Medicine, Csolnoky Ferenc County Hospital
    Kórház út 1., Veszprém, Hungary
    Telephone: (+36) 88 420 211
    Fax number: (+36) 88 421 457
    E-mail: kissz@fiz5.mars.vein.hu

CEJOEM 1999, Vol.5. No.3-4.:261-273

Key words:
Thyroid cancer, radiation dose, dose reconstruction, 131I and 137Cs ground deposition, Chernobyl, Belarus
 
Abbreviations:
D = thyroid dose
DF = age dependent committed dose conversion coefficient for thyroid by ingestion of 131I (Gy.Bq-1)
GDCs-137 = ground deposition of 137Cs (Bq.m-2)
GDI-131 = ground deposition of 131I (Bq.m-2)
RI-131/Cs-137 = ratio between the 131I and 137Cs deposition values on soil surface
TIAm = activity concentration of 131I in milk (Bq.L-1)
Vm = milk consumption (L.d-1)

Abstract:
Years after the Chernobyl accident the incidence of thyroid cancer statistically significantly increased in the Belarussian population, especially in children. Retrospective dose assessments mainly based on direct thyroid measurements were carried out for the population. Unfortunately, only relatively few direct measurements of population exposure for children were provided during the first weeks after the accident. Therefore direct data on the thyroid uptake of the very short-lived isotopes of iodine are not available widely. Due to the minor data there are some uncertainties in the calculations of the dose for children in general or especially to children with thyroid cancer calculation even using these data.
    Dose reconstruction can, however, help assuring thyroid doses for wider population. Until now the results of dose reconstruction models have not been precise enough due to the uncertainties both related to deposition data available and dietary habits in Belarus. The aim of this paper is to present more reliable dose assessments based on radiocaesium deposition to investigate the relationship between the release of 131I during the reactor accident and the increased incidence of thyroid cancer observed in children of Belarus.
    Thyroid doses were assessed considering “the worst and the best” cases i. e., the calculations were only carried out for dry (“worst case” scenario) and 100% wet depositions (“best case” scenario), respectively. The database of children treated for thyroid cancer in Belarus contained 426 entries until 1995. Among those there were some deficient data therefore the number of cases having investigated decreased to 381.
    The most estimated thyroid doses in the “worst case” scenario are less than 40 Gy, and more than 80% of the cases are even less than 5 Gy, more than 40% of cases are less than 1 Gy. Due to the lack of reliable meteorological data the “best case” scenario was divided into three subscenarios according to the amount of rain assumed (1 mm, 3 mm and 10 mm rain). The highest doses are 31 Gy, 8.8 Gy and 2.8 Gy, respectively. The number of cases for doses below 1 Gy is 321, 352 and 380 (84%, 92% and 99.7%), respectively.
    Having compared the reconstructed doses to doses from thyroid measurements as references it can be stated that the dose range derived here covers a very similar range with a similar distribution. The highest coincidence occurs in the case of wet deposition with amounts of rain of 1 and 3 mm. In the case all doses are below 2 Gy as given for non-evacuated settlements. The reconstructed mean doses, however, overestimate the measured ones as it was expected and intended in the investigations.
    Dose reconstruction by the means of 137Cs atmospheric deposition values can be a reliable method for estimating thyroid doses due to 131I contamination if parameters of the radioecological model adjusted to the local conditions are reliable. Moreover, it seems to be more realistic if some rainfall is supposed in the cases when meteorological data are deficient. In this way, it turns generally out that the greater the average dose in an “oblast” (region) the greater the number of the new incidence of thyroid cancer cases.

Received:  17 January 2000
Accepted:  08 March 2000
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