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The possibility, that the Chernobyl catastrophe can lead to an
increase in the incidence of thyroid cancer was first time reported in
1991 (52). The study from Belarus presented thyroid cancer in 18 children
diagnosed between 1986 and 1989. As thyroid cancer in children is very
rare with an incidence of about 1/million/year, the findings seemed significant
for a population of children amounting to about 2.5 million. At the same
time, however, the results of the International Chernobyl Project (53)
led to conclusion that "There were significant non-radiation related health
disorders in the population of both surveyed contaminated and surveyed
control settlements studied under the Project, but no health disorders
that could be attributed directly to radiation exposure". In addition to
this general conclusion the Report of the International Advisory Committee
stated (53) that abnormalities in either TSH, free T4, mean thyroid size
and the size distribution in the population, as well as the number of thyroid
nodules were the same in the population of surveyed contaminated and control
settlements and similar to those reported for other countries. The significance
of these conclusions published also elsewhere (54) was such, that the response
to the Belarus and Ukraine data on the rise of incidence of thyroid cancer
after Chernobyl were received with scepticism. It was even suggested that
the pathological findings of removed thyroid tissue could be grossly over-interpreted.
The first report about the rise in the incidence of thyroid cancer in children
was published in international journals in September 1992 (55) and the
Belarus findings were supported by a group of international experts (56).
Soon thereafter several other reports were published (24,25,57-59) all
presenting significant increases in the incidence of thyroid cancers in
children. The results of the IPHECA pilot project presented in 1995 showed
(24), that until the end of 1994 there were 333 cases of childhood thyroid
cancer in Belarus, 208 in the Ukraine and 24 cases in the Russian Federation.
The incidence of thyroid cancer in 1994 (rounded) per million of children
in Belarus, the Russian Federation and the Ukraine was 36, 22 and 31 respectively
(vs 0.9, 0. 0 and 0.7 in 1986) thus showing a significant rise. More than
98% of thyroid cancers diagnosed until the end of 1994 were of the papillary
type, and more than half of them the solid variant. Very few were microcarcinomas
and the majority showed aggressive histologic features including local
and extrathyroidal invasion (24,25,57,59-61). In general, the histological
findings were similar to those described in childhood thyroid cancer (62,63).
From the age of children at diagnosis it can be concluded that very young
children were much more sensitive to the radiation than teenagers. The
incidence of thyroid cancers in girls (59,7%) only slightly exceeded the
incidence in boys (40,3%). It is of interest that of the children who developed
thyroid cancer 66% had a thyroid radiation dose of less than 0,3 Gy, 22%
a committed dose from 0.3 to 1.0 Gy and only 12% a dose of 1.0 Gy or more
(59). The Belarus Cancer Registry also shows that 241 thyroid adenomas
were diagnosed in children between 1987 and August 1995, while only 1 case
was seen in 1986 (64). Finally, both the Belarus and Ukraine studies on
the geographical and time distribution of thyroid cancers and adenomas
suggest, that they are related to radiation contamination after the Chernobyl
accident (24,25). During the International Conference jointly sponsored
by the European Commission, the International Atomic Energy Agency and
the World Health Organization (65), drs. Williams, Becker, Dimidchik, Nagataki,
Pinchera and Tronko presented data showing a further increase of thyroid
cancer incidence in children both in Belarus (424 cases diagnosed till
end of 1995) and the Ukraine (211 cases diagnosed till end of 1994). The
authors stated that although absolute proof of the relationship between
exposure to radioiodine and the increase in the incidence of childhood
cancer is not available, the circumstantial evidence is very strong.
Regardless of all recent data there is still scepticism whether the rise of thyroid tumors incidence can be related to 131I radiation (63). Sceptics suggest that diagnostic and therapeutic doses of 131I have never showed to lead to a significant increase of thyroid cancer incidences. As the significant rise in the number of thyroid cancer in childhood can not be questioned, the most probable explanation then for the increase would be that increased awareness leading to intensive screening, or that thyroid cancer is a consequence of radiation contamination by compounds other than 131I like short-lived isotopes of iodine and isotopes of tellurium and caesium. The aggressiveness of tumors and type of cancer is against the first of these arguments. The majority of thyroid cancers diagnosed in Belarus or Ukraine were in more than 98% of cancers of the papillary type. In regions of iodine deficiency at least 25-30% of cancers are of the follicular type. Regarding the second argument, it is true that 131I in humans has never shown to cause thyroid cancer, but such data were never derived from very young children. On the other hand, as already mentioned, the Chernobyl catastrophe led to a significant release of short-lived radioiodines which are as tumorigenic as X-ray radiation. From the core inventory of Reactor No 4 it can be assumed (10), that the amount of short-lived radioiodines could well exceed that of 131I. The Marshall Island experience has shown that a 8:1 mixture of short-lived radioiodines and 131I was highly carcinogenic (41). This factor and the relative iodine deficiency which is present in many regions of Belarus and Ukraine can explain the high incidence of thyroid papillary cancer in childhood. Although a higher incidence of thyroid cancer in adults is also suggested by the Belarus Cancer Register, further data are needed to come to a definite conclusion. Further studies should be also performed to assess the number of benign thyroid tumors in Belarus, Ukraine and Russia, as well as the possible impact of the Chernobyl accident in other European countries. |
