SYSTEM FOR RECONSTRUCTION OF INTERNAL THYROID DOSES OF UKRAINIAN RESIDENTS EXPOSED TO IODINE RADIONUCLIDES AS A RESULT OF THE CHERNOBYL ACCIDENT

Abstract

The increased incidence of thyroid cancer among individuals
exposed to iodine radionuclides during childhood and
adolescence is the main long-term consequence of the Chornobyl
nuclear power plant (ChNPP) accident. Already 5–6 years after the
accident, a sharp increase in the number of cases of thyroid cancer
was recorded in children and adolescents living in radioactively
contaminated areas of Ukraine. This phenomenon has led to a
number of international radiation-epidemiological studies aimed
at quantitatively assessing the risk of developing radiation-induced
cancer and other thyroid diseases in individuals with the irradiated
thyroid, which, in turn, requires determining the thyroid radiation
doses of individuals involved in the study. This study aimed to
review the current TD20 thyroid dosimetry system and the results
of its application to reconstruct thyroid radiation doses received
by the Ukrainian population in April–July 1986 as a result of iodine
radionuclides intake into the body. Material and methods. Residents
of Ukraine exposed to iodine radionuclides as a result of the
Chornobyl accident were divided into four dosimetric groups based
on the availability of thyroid radioactivity measurements and other
information necessary for thyroid doses calculation. The methodology
for reconstructing thyroid doses depends on the dosimetric
group to which an individual belongs. Thyroid doses were calculated
for the following exposure pathways: intake of 131I, as well as

132Te+132I and 133I through inhalation of radioactively contaminated
air; and consumption of food products such as milk, dairy products,
and leafy vegetables in the period from April 26 to July 14, 1986.
Results. The mean arithmetic value of thyroid radiation dose from
131I radionuclide intake among 13.204 members of the Ukrainian-
American cohort study (dosimetric group 1) was 527 mGy, and the
median was 198 mGy. The mean arithmetic dose of thyroid radiation
due to 131I intake among 146.425 individuals whose thyroid activity
was measured in May-June 1986 (dosimetry group 2) was 230 mGy,
and the median dose was 94 mGy. The highest averaged and population-
weighted thyroid doses (dosimetric group 3) were received
by residents of Chernihiv (arithmetic mean is 150 mGy, median is
60 mGy), Kyiv (130 mGy and 51 mGy), and Zhytomyr (120 mGy and
49 mGy) Oblasts. Total prenatal and postnatal thyroid doses were
calculated for 2.582 members of the Ukrainian cohort of individuals
exposed to 131I in utero (dosimetric group 4) and ranged from 0
to 2.7 Gy. The average total thyroid dose from 131I was 87 mGy and
the median was 17 mGy. The contribution of 132Te+132I and 133I to
the total thyroid dose was the largest for residents of Pripyat (22–
40%, depending on age at the time of exposure). For residents of
areas near the Chornobyl Nuclear Power Plant, it was 8–11%, and
for residents of other areas it does not exceed 3%. Conclusions.
The paper presents an updated system for reconstructing internal
thyroid doses (TD20) of Ukrainian residents exposed to iodine radionuclides
following the Chornobyl accident, as well as the results
of applying this system to reconstruct thyroid doses: 13.204 members
of the Ukrainian-American cohort study, 2.582 members of the
Ukrainian cohort of individuals irradiated in utero, 146.425 individuals
who had thyroid radioactivity measurement in May-June 1986,
and residents of 30.353 settlements in 24 Oblasts of Ukraine, the
Autonomous Republic Crimea, and the cities of Kyiv and Sevastopol.