Therapeutic radiation at a young age is linked to secondary thyroid cancer

We estimated the risk of thyroid cancer among 9170 patients who had survived 2 or more years after the diagnosis of a cancer in childhood. As compared with the general population, patients had a 53-fold increased risk (95% confidence interval, 34-80). Risk increased significantly with time since treatment for the initial cancer. Detailed treatment data were obtained for 23 cases and 89 matched controls from the childhood cancer cohort. 68% of the thyroid cancers arose within the field of radiation. Radiation doses to the thyroid of > 200 cGy were associated with a 13-fold increased risk (95% confidence interval, 1.7-104). The risk of thyroid cancer rose with increasing dose, but this was derived almost entirely from the increase from < 200 to > 200 cGy. The risk of thyroid cancer did not decrease, however, at radiation doses as high as 6000 cGy.