Dose Characteristics of Radiation-Induced (I-131) Thyroid Cancer

Analysis of absorbed doses by the Thyroid Gland (TG) of residents of the regions of Belarus affected by the Chernobyl accident showed that the highest doses absorbed by this organ (600-700 mGy) were received by infants and children, who in April 1986 were in the age from 0 to 4 years old. These doses did not differ much between men and women. The curve of absorbed dose values versus age is described well by the exponential dependence. The incidence of Thyroid Cancer (TC) in age groups of both sexes (from 0 to 19 years), depending on the absorbed dose by the TG, was characterized by a linear relationship with a coefficient of determination equal to 0.89. Persons from the younger age group (0-4 years), who received the highest absorbed doses, had a higher morbidity. Persons from the age group of 15-18 years old at the time of an accident received lower doses by their TG and demonstrated lower incidence of TC. Extrapolation of the direct dependence of the latent period of cancer on the received absorbed dose in those age groups suggests that the shortened latent period of this disease recorded in Belarus may be associated with the formation of very high doses absorbed by TG (more than 1.2 Gy) in some children from affected areas.


Introduction
The first scientifically confirmed stochastic effect after the Chernobyl accident was an increase in the incidence of One of the ways of the intake of those radionuclides was inhalation.
Moreover, as a result of sedimentation from a radioactive cloud, iodine radionuclides contaminated the soil and, due to the functioning of trophic chains, they reached the human body, accumulating in the tropic organ -the Thyroid Gland (TG). In the parenteral route of the intake, whole milk played an important role.
In addition to iodine, Cs-137, Cs-134, and Sr-90 radionuclides that contaminated the soil contributed to the formation of both external and internal irradiation. The fact of such a short latent period of TC, that was characteristic to the children, is alarming. The growth of incidence of this disease had begun to be recorded 4-5 years after the accident, i.e. since 1990 [1]. The reasons for this are not clear yet. In this regard, additional studies of this phenomenon are needed for the better understanding of medical consequences of the Chernobyl accident, which is what this work is devoted to.

Materials and Methods
As the material for this study we took the data on the incidence of TC from the Belarusian cancer registry for 1986-2016 for rural residents of two districts of the Brest region (Luninets and Stolin districts) affected by the Chernobyl accident [4]. Residents of this area fell under the "iodine blow", i.e. a radioactive cloud passed over the territory of their residence in the night of April 26, 1986.
As a result, the territory of the two districts was contaminated by the I-131 in the range from 15 to 100 Ci / km2. The cohort from the cancer register of two districts included 235 persons, of whom 183 were women and 52 were men. The study included only rural residents of the mentioned areas, who at the time of the accident were in the age between 0 and 19 years old. Since in the previous analysis of TC in residents affected by the Chernobyl accident, which was carried out by UNSCEAR experts in 2016, age groups were identified, in this study we used the same approach and the following age groups were identified in the studied cohort at the time of the accident at the Chernobyl nuclear power plant: 0-4 years, 5-9 years, 10-14 years and 15-18 years [5]. The composition of age groups is presented in Table 1 SigmaPlot 13 (Systat Software Inc, Germany) application programs.
The reliability of the results was assessed using the SigmaPlot program, and the value of p <0.05 considered as the evidence of the existence of significant difference. This study was approved by the Ethics Commission of the Belarusian Association of Physicians (10.12.2020).

Results
As  These results are well illustrated by the curve shown in Figure   2. The dependence of the dose on the age is well described by the exponential curve and for persons of the age from 0 to 10 years it is characterized by a steeper segment. Considering the effect of doses absorbed by TG received during an accident, it is of a great interest to clarify the dose patterns in relation to the age at which TC was diagnosed ( Figure 3). It can be seen that in age groups a linear relationship with a coefficient of determination of 0.98 is revealed between those parameters. Moreover, if the victim was exposed    According to these data, a short latency period (5-6 years) discovered immediately after the Chernobyl accident in Belarus could be associated with very large values of absorbed doses by the TG. It follows from Figure 6 that this phenomenon could be caused by absorbed doses of more than 1.2 Gy. In addition, initial mutation at the time of exposure can play the role in tumour latency [8].

Discussion
As follows from the presented data, it makes some sense to process the data from the cancer register after the division of all the persons into the age groups.  Gy, at least in some cases, which could be the reason for the early onset of TC.