Of 2,872 persons who had received radiation treatment in childhood because of an enlarged thymus, cancer of the thyroid developed in 24 and a benign thyroid tumor developed in 52. a comparison group consisted of 5,055 children who had received no such treatment (brothers and sisters of the children who had received radiation treatment). during the follow-up period, none of the comparison group developed thyroid cancer, but benign thyroid tumors developed in six. calculate the relative risk for benign thyroid tumors: