Ploidy of malignant mediastinal germ-cell tumors.

The ploidy of 19 primary mediastinal malignant germ-cell tumors (GCT) was measured. Six tumors were (near) diploid. The median DNA index of the 13 remaining tumors was 1.91, indicating that most aneuploid tumors were near tetraploid. Seminomas and nonseminomatous GCTs were similar with respect to proportion of diploid tumors and median DNA index of the aneuploid tumors. The ploidy of mediastinal malignant GCTs is remarkably different from that of testicular GCTs of adults, but more similar to that of testicular GCTs of infancy. The pathogenesis of malignant mediastinal GCTs is probably different from that of testicular GCTs of adults, and more similar to the pathogenesis of infantile testicular GCTs. The difference in ploidy between mediastinal malignant GCTs and GCTs of the adult testis may help to differentiate between a primary extragonadal malignant GCT and a mediastinal metastasis from a primary testicular GCT.