Irradiation and second cancers. The thyroid as a case in point

The thyroid gland is highly sensitive to radiation during childhood: the ri sk of thyroid tumours is increased for mean doses as low as 100 mGy and for higher doses, the risk increases linearly with the dose. Excess relative r isk is important, being 7.7 for 1 Gy delivered to the thyroid gland during childhood. The risk of thyroid tumours is modified by several factors: a) a ge at exposure: in childhood, the risk decreases with increasing age at exp osure and is not significant after 20 years; b) gender: females are two tim es more likely than males to develop thyroid tumours; c) genetic predisposi tion due to a defect in DNA repair mechanisms, and dietary and hormonal fac tors may modify the risk; d) the influence of fractionation and dose rate i s not well established. Radioiodine 131 (1311) used for medical purposes ha s almost no tumourigenic effect on the adult thyroid gland. The consequence s of the Chernobyl accident have clearly shown that the risk of thyroid can cer after exposure to 1311 in childhood is important, and that such exposur e should be prevented by potassium iodine prophylaxis. RET/PTC rearrangemen ts are found in 60-80 % of papillary carcinomas and in 45 % of adenomas occ urring after radiation exposure. They are found in 5-15 % of papillary carc inoma and in no follicular adenomas that occurred in the absence of radiati on exposure. ((C) Academie des sciences / Elsevier, Paris.)