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Radioiodine and thyroid cancer
Ionising
radiation is the only definitely established cause of thyroid
cancer in humans, although only a small proportion of thyroid
cancers can be accounted for by radiation. The thyroid gland
is highly susceptible to ionising radiation presumably because
of its superficial location, high level of oxygenation, and
high cell turnover rate. A pooled analysis of seven studies
revealed that thyroid cancer was induced even by low doses
of brief external gamma radiation in childhood, but rarely
developed after exposure in adulthood. Data from the atomic
bomb survivors underline the strong modifying effect of age
at exposure, with no excess risk seen in individuals older
than 20 years. During the first 14 years after the Chernobyl
accident, approximately 1800 thyroid cancers were diagnosed
in the three most contaminated countries among children younger
than 15 years, whereas only three or four childhood thyroid
cancers were registered annually in the same area before the
accident. No increased thyroid cancer as a consequence of
the Chernobyl accident has been identified in adults.
The
major concern regarding medical use of ionising radiation
has been the possibility that thyroid examinations or treatments
using radioiodine cause thyroid cancer. The annual number
of thyroid examinations using radioiodine is currently 5 per
1000 individuals in the western world. Patients treated with
131I for hyperthyroidism are almost entirely adults
and no increased risk of thyroid cancer is seen among these
patients. It is also likely that the doses, ranging from 100
to 300 Gy, received by the thyroid gland induce cell killing
instead of carcinogenic transformation.
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