THYROID-CANCER AFTER EXPOSURE TO EXTERNAL RADIATION - A POOLED ANALYSIS OF 7 STUDIES

The thyroid gland of children is especially vulnerable to the carcinog enic action of ionizing radiation. To provide insights into various mo difying influences on risk, seven major studies with organ doses to in dividual subjects were evaluated. Five cohort studies (atomic bomb sur vivors, children treated for tinea capitis, two studies of children ir radiated for enlarged tonsils, and infants irradiated for an enlarged thymus gland) and two case-control studies (patients with cervical can cer and childhood cancer) were studied. The combined studies include a lmost 120,000 people (approximately 58,000 exposed to a wide range of doses and 61,000 nonexposed subjects), nearly 700 thyroid cancers and 3,000,000 person years of follow-up. For persons exposed to radiation before age 15 years, linearity best described the dose response, even down to 0.10 Gy. At the highest doses (greater than or equal to 10 Gy) , associated with cancer therapy, there appeared to be a decrease or l eveling of risk. For childhood exposures, the pooled excess relative r isk per Gy (ERR/Gy) was 7.7 (95% CI = 2.1, 28.7) and the excess absolu te risk per 10(4) PY Gy (EAR/10(4) PY Gy) was 4.4 (95% CI = 1.9, 10.1) . The attributable risk percent (AR%) at 1 Gy was 88%. However, these summary estimates were affected strongly by age at exposure even withi n this limited age range. The ERR was greater (P = 0.07) for females t han males, but the findings from the individual studies were not consi stent. The EAR was higher among women, reflecting their higher rate of naturally occurring thyroid cancer. The distribution of ERR over time followed neither a simple multiplicative nor an additive pattern in r elation to background occurrence. Only two cases were seen within 5 ye ars of exposure. The ERR began to decline about 30 years after exposur e but was still elevated at 40 years. Risk also decreased significantl y with increasing age at exposure, with little risk apparent after age 20 years. Based on limited data, there was a suggestion that spreadin g dose over time (from a few days to > 1 year) may lower risk, possibl y due to the opportunity for cellular repair mechanisms to operate. Th e thyroid gland in children has one of the highest risk coefficients o f any organ and is the only tissue with convincing evidence for risk a t about 0.10 Gy. (C) 1995 by Radiation Research Society