THYROID-CANCER AMONG PERSONS GIVEN X-RAY TREATMENT IN INFANCY FOR AN ENLARGED THYMUS GLAND

A cohort of 2,657 infants in Rochester, New York, who were given x-ray treatment for a purported enlarged thymus gland, along with 4,833 sib lings, have been followed by mail surveys through about 1986, which re presents an average of 37 years of follow-up, to determine their incid ence of thyroid cancer. Estimated thyroid doses ranged from 0.03 to >1 0 Gy, with 62% receiving <0.5 Gy. There were 37 pathologically diagno sed thyroid cancers in the irradiated group and five in the sibling co ntrols. The dose-response relation was essentially linear, with no evi dence of an additional dose-squared component. The estimated relative risk at 1 Gy was 10 (90% confidence interval 5-23). Thyroid cancer rat es were elevated even at low doses; i.e., a dose-response analysis ove r the range of 0-0.3 Gy showed a significant positive slope. The risk ratio was declining over time but was still highly elevated to at leas t 45 years after irradiation. An examination of potential risk factors showed that older age at first childbirth was significantly associate d with thyroid cancer risk. An evaluation of interactions between poss ible risk factors and radiation suggested that Jewish subjects and wom en with older ages at menarche or at first childbirth were at greater risk for radiogenic thyroid cancer.