Of 2 872 individuals who had received radiation treatment in childhood because of enlarged thymus, 24 developed cancer of the thyroid and 52 developed benign thyroid tumors. A comparison group consisted of 5 055 children who had received no such treatment (brothers and sisters of those who had received radiation treatment). During the follow-up period none of the comparison group developed thyroid cancer, but six developed benign thyroid tumors. What is the relative risk for thyroid cancer?