Radiation and Host Factors in Human Thyroid Tumors Following Thymus Irradiation

Abstract
Thyroid tumor data from the 1971 survey of the Rochester, New York, USA, thymus-irradiated population are further analyzed to study radiobiological and host factors. The analyses were based on about 2650 irradiated subjects and 4800 sibling controls who had 5 yr or more of follow-up. There were 24 thyroid cancers and 52 thyroid adenomas in the irradiated group and no thyroid cancers and 6 adenomas among the controls. The overall risk estimates were 3.8 thyroid cancers/106 PY(person-years)-rad and 4.5 thyroid adenomas/106 PY-rad. The dose-response data (thyroid dose range of 5 to > 1000 rad) for thyroid cancer indicate a linear and a dose-squared component, but no dose-squared component is evident for thyroid adenomas. At lower total doses (< 400 rad) there was a suggestion that dose fractionation diminished the thyroid cancer response, but a similar fractionation effect was not found for thyroid adenomas. The temporal pattern of tumors suggested an extended plateau of excess tumor production, rather than a wavelike temporal pattern. There was no evidence for an inverse relationship between thyroid radiation dose and thyroid cancer latency. Female and Jewish subjects had a higher risk of radiation-induced thyroid cancer than did their respective counterparts. The additive and multiplicative models of radiation effects were compared with respect to sex differences; neither model provided a superior fit to the data. The tentative nature of the conclusions is stressed because of the relatively small number of thyroid cancers.