Statistics question about relative risk

1. Of 2,872 persons who had received radiation treatment in childhood because of enlarged thymus, cancer of the thyroid developed in 24 and benign thyroid tumor developed in 52.A comparison group consisted of 4500 children who had received no such treatment (brothers and sisters of those 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 6.

1. Of 2,872 persons who had received radiation treatment in childhood because of enlarged thymus, cancer of the thyroid developed in 24 and benign thyroid tumor developed in 52.A comparison group consisted of 4500 children who had received no such treatment (brothers and sisters of those 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 6.

Calculate the relative risk for benign tumor