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The radioiodine thyroid committed dose depends on the scale of radiation
contamination of the environment, the time during which radioiodines are
present in the environment, and on the quantity of radioiodines taken up
and retained within the thyroid gland. In general the radioiodine uptake
in the gland is inversely related to the daily intake of stable iodine
(14). Radioiodines can reach the thyroid gland through inhalation of contaminated
air, through ingestion of contaminated food and through the skin if there
is prolonged contact with contaminated material. With the exception of
the area close to the Chernobyl power station, where contamination through
the skin was important, in all other places only inhalation and alimentary
components were significant for the thyroid adsorbed dose. Although short-lived
radioiodines with their high dose rate of radiation are likely to be more
carcinogenic per unit absorbed than 131I (15), only thyroid committed dose
of this latter radioiodine was evaluated. The contribution of contaminated
air to total thyroidal radiation amounted in the Ukraine and Belarus to
80%, in Poland to 30% and in other European countries contribution varied
from 2% in Switzerland to 80% in Belgium (16). Evaluation of the thyroid
absorbed dose was done either by direct methods or by indirect one, based
either on a modified MIRD scheme (17,18) or on the 5-compartment model
of iodine metabolism developed by Johnson (19). This second indirect method
is using 131I in air, milk and other foods, standard values for inhalation
rate, milk and food consumption and parameters such as age, sex, thyroid
size, and thyroid iodine uptake rates. According to former USSR data (3,20)
about 200000 direct measurements of 131I in the thyroid were performed
in Ukraine, Belarus and Russia, but because of obsolete equipment used,
their accuracy may be questioned. In regions of Ukraine close to Chernobyl
the thyroid dose equivalent was estimated by using the relationship between
the thyroid dose and the 137-Cs deposition (21). With this approach the
highest mean thyroid dose for infants was calculated to be about 3.3 Gy
and for adults 0.5 Gy. In Kiew thyroid committed doses for 1-3 years, 4-6
years, 7-9 years and 12-15 years old children were 104, 62, 19 and 41 mGy
respectively (22). There is a difference in the collective thyroid dose
calculated for Ukraine with a value of about 29000 persons/Gy (22) and
64000 persons/Gy (23). In highly contaminated regions of Belarus, thyroid
recorded doses in children varied from 0.3 Gy to 50 Gy and in adults from
0.1 Gy to 50 Gy(24,25). In the Russian Federation the highest radioactive
contamination was recorded in Bryansk and Kaluga regions. The average thyroidal
absorbed dose in children varied between 10 mGy and 2.2 Gy and in adults
the highest recorded dose was 2 Gy (24,25,26). In Poland about 1500 measurements
were performed by a direct, standardized method (27), while the population
study was based on the Johnson model (28) assuming, however, appropriate
iodine intake . The highest thyroid doses were recorded in children 1 year
old or under being in the range of 60-80 mGy regardless of prophylactic
measures taken. The average country-wide thyroid committed dose in children
up to 16 years old was estimated to about 20 mSv giving a collective thyroid
dose of about 210000 persons/Sv. The average thyroid committed dose in
adults was about 6 mSv giving a collective thyroid dose of about 162000
person/Sv. Taking into account the results of a population study (29),
which clearly showed, that the entire population of Poland is mildly or
moderately iodine deficient, the Polish data may be underestimated and
both average individual and collective thyroid dose may be higher. In Western
Europe approximately 3000 direct measurements were performed and data on
thyroid absorbed dose are therefore very general. Average thyroid absorbed
dose estimates taken from different sources (16,20-28) are presented in
Table 1.
Table 1 Average 131I thyroid committed dose in some Western European countries and range of 131I thyroid committed doses in children and adults in Belarus, Poland, Russian Federation and Ukraine in first year after the Chernobyl accident. Data from Belarus, Russian Federation and Ukraine are from heavily contaminated
regions.
It should be recognized, that both in former USSR and in Europe as a whole accurate and detailed estimates of thyroid accumulated dose of radiation are not available and that dose reconstruction will be extremely difficult if at all possible. |
