Neoplasms in youthful populations following X-ray treatment in infancy

Abstract

The results of current mail surveys are summarized for two youthful populations in Rochester, New York and Ann Arbor, Michigan, each with a history of X-ray treatments in infancy for thymic enlargement. Studies of one subgroup in the Rochester series, examined because of a high incidence of tumors, particularly thyroid tumors, are also presented. The high incidence of neoplasms, previously described for the irradiated Rochester population is even more striking than before. The new tumors developed mainly in the thyroid and in the irradiated bones of the rib cage. In the examined subgroup, the projected incidence of thyroid nodularity was 28%. Although the incidence of neoplasms in the Ann Arbor study was not as high as that in the Rochester group, it seems to be higher than that in the general population, particularly in the case of thyroid neoplasms. The incidence of thyroid and extrathyroid tumors in the Rochester series is dose-dependent and the frequency of thyroid neoplasms is age-dependent until age 20. Some evidence is presented suggesting that (1) the dose response of thyroid tumors is linear in the lower dose range, and (2) there is no threshold or, at least, the threshold is below 20 rads. The reason for the effectiveness of opposing AP ports in inducing thyroid neoplasms in the Rochester study is given. Although the dose dependency of the thyroid neoplasms indicates that radiation exposure played an etiologic role, evidence is presented showing that other factors, particularly thyroid stress during adolescence, play a powerful secondary or promoting role in tumor induction. In this respect, the induction of thyroid neoplasms in man follows the multistage theory of carcinogenesis so well established for the development of endocrine tumors in animals. The relatively benign clinical course of all persons with thyroid carcinoma suggests that the disease was still in the hormone (TSH) dependent stage. The clinical characteristics of the radiation-induced neoplasms are described. Estimates of the risk of developing thyroid cancer, nodular thyroid disease and leukemia after radiation exposure are also given.