Multiple primary malignancies in childhood cancer.

A total of 2,609 cases of childhood cancer (9,012 person-years), among which there were nine cases of second primary malignant neoplasms, were collected from six institutions. The expected number of second primary cancers was 0.88, and the observed/expected (O/E) ratio was 10.22. The estimated incidence of multiple primary cancers in children with primary cancers was 99.9 per 100,000. The tumor registry of Kanagawa Prefecture had 1,317 cases of childhood cancer with 3,326.4 person-years, among which six multiple primary cancer cases were reported. The O/E ratio was 18.4 with 180.3/100,000 person-years at risk in this series. A total of 51 cases of multiple primary cancer, including 20 synchronous cases, were reported by four major registries of childhood cancers. Frequent association with nervous tissue tumors was noted. In 17 cases the second tumor followed retinoblastoma, and most second tumors were related to radiation therapy. In the remaining 14 cases the second tumors varied; hematopoietic neoplasms in seven, thyroid carcinoma in three, and bone and connective tissue tumors in four.