Chernobyl Radiation-induced Thyroid Cancers in Belarus

Assessment of incidence and mortality for thyroid cancers carried out for the Belarusian population is described in the present report. It is found that in the period of 1987-2000 about 4,400 radiation-induced thyroid cancers appeared in Belarus: 692 cancers among children and 3,709 cancers among adolescents and adults. The number of lethal thyroid cancers in this period of time in Belarus was assessed as about 350 cases. The excessive absolute risk, EAR, of thyroid cancer incidence assessed for the period of 1987-2000 on the basis of given data on the morbidity and the assessed collective thyroid dose of irradiation is (2.5 – 5.0) per 104 PYGy. The EAR value of thyroid cancer mortality is assessed as (0.20 0.40) per 104 PYGy. The excessive relative risk, ERR, of thyroid cancer incidence is assessed as (11.2 – 22.4)/Gy. The radiation risks of thyroid cancers found in the present report are higher than the risk coefficients established for atomic bomb survivors that were irradiated with dose rates some thousand times higher than populations of Belarus affected by the Chernobyl accident. The absence of marked latency period is another feature of radiation-induced thyroid cancers caused in Belarus as a result of this accident. INTRODUCTION As a result of the Chernobyl accident a large amount of radioactive substances escaped from the destroyed reactor. Especially high release was in case of volatile radionuclides. According to the existing assessments about 50% of the I isotope inventory came into the environment [1]. This caused the radioactive contamination of many countries of the Northern Hemisphere [2]. However, the main fraction of radioactive materials released by the accident deposited in Belarus. The total amount of the I deposited in Belarus is assessed as approximately 3.3⋅10 Bq or about 18.5% of the total amount of I that escaped from the Chernobyl reactor [3]. Practically the whole territory of Belarus was contaminated with this and other iodine isotopes. This resulted in very high doses delivered to thyroid gland of a large amount of people in Belarus. It is well known that the highest thyroid doses in Belarus were received by the inhabitants of the south districts of the Gomel region and of the southeast districts of the Brest region [4]. According to the data of Yu.Gavrilin et al.[4], the arithmetic mean thyroid dose in the age group 0-6 years of the children living in settlements of the Khoiniki district settled in the 30-km zone was about 4.7 Gy and maximal doses were about 50 Gy. Very high thyroid doses were received also by the inhabitants of Brest and Mogilev regions. This is the reason for a very high increase in the thyroid cancer incidence in Belarus after the Chernobyl accident, especially among the children of Gomel and Brest regions. V.Kazakov, E.Demidchik and L.N. Astakhova [5] published the first reliable data on the significant increase in the thyroid cancer morbidity among the children of Belarus after the Chernobyl accident. However, specialists met these data with a big skepticism. There were serious reasons for this skepticism. At first, the data of [5] have shown an unusual short latent period of about 2-3 years. At second, no similar data about the increase in the incidence of thyroid cancer among the children of the affected areas of the Ukraine and Russia were known at the time of publication [5]. This was very strange because all specialists believed that the Ukraine was affected by the Chernobyl accident more significantly than Belarus. It is now clear that such assumption was fully incorrect.