:: 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.