This paper provides an overview of what is known concerning the oncogenic effects of radiation exposure during childhood and adolescence. The study populations include atomic bomb survivors, patients given radiotherapy to treat cancer or benign diseases, patients who received rather massive exposures during diagnostic radiographic procedures, persons exposed to environmental radiation, and children who received occupational exposures when working in underground mines (Table I).
ATOMIC BOMB SURVIVORS
Studies of survivors of the atomic bombs dropped on Hiroshima and Nagasaki in 1945 have provided valuable information on cancer risks following radiation exposure [ 11. The increase in recorded deaths due to either leukemia or solid tumors following exposures between 0.5 and 1 Gy are both statistically significant. The dose-response relationship for leukemia appears linear quadratic with a suggestion of a leveling off of risk at the highest doses. For radiation-induced solid tumors, the dose-response curve appears consistent with a straight line. There also is a suggestion at the highest doses that there might be a plateauing of effect, possibly related to the dominance of cell-killing over transformation at very high doses. A recent major publication on cancer incidence included articles clearly defining the oncogenic risks of radiation for many sites [2,3]. Cancer types with the highest relative risk coefficients include leukemia, breast, thyroid, and lung cancer. The particular mix of tumors following relatively low-dose whole-body exposures to the atomic bombs is different from that following therapeutic exposures in childhood, when sarcomas, brain tumors, and several other sites appear more often. It is also clear that children are at higher relative risk of radiation-induced cancers than adults (Table 11). For practically all sites the relative risk at 1 Gy is greater for those under the age of 20 at exposure than those over the age of 20. This age difference is particularly notable for leukemia, thyroid, breast, and skin cancer. Lung cancer is the only site for which the relative risk is elevated more among adults than children. On the other hand, the absolute excess risk