The terminology used for mediastinal germ cell tumors is similar to that of germ cell tumors of the gonads.¹ They have been categorized for therapeutic purposes (in ascending order of aggressive behavior) into teratomas, pure seminomas, and malignant nonseminomatous germ cell tumors (embryonal carcinoma, yolk sac tumor, choriocarcinoma, and mixed germ cell tumors)^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18} (Tables 119.1 and 119.2).

Pure teratomas composed of mature elements are often classified as “benign” mediastinal germ cell tumors because of lack of metastatic potential. Overall teratomas are the most common single histologic subtype.¹⁴

Germ cell tumor with somatic-type malignancy contains one or more components of non-germ cell malignant tumor, which may be a sarcoma or a carcinoma.^1,8,9,10 Germ cell tumors with somatic malignancy have been classified into two clinical and pathologic types: those induced by chemotherapy or irradiation and naturally occurring somatic malignancy.¹ They have also been classified as somatic malignancies with solid-type carcinoma, and somatic-type hematologic malignancies.¹

Teratomas of the mediastinum are frequently subclassified as immature or mature, based on the presence of fetal-type tissue, especially in children.^2,15 In contrast to ovarian teratomas, mediastinal teratomas with immature elements that do not contain mixed components (most often embryonal carcinoma) are rare. Therefore, although sometimes considered malignant, pure immature teratomas of the mediastinum are not usually graded, as is the case with ovarian teratomas (see Chapter 121).

The occurrence of extragonadal germ cell tumors in the mediastinum is explained on the basis of migration of the primitive germ cells during embryonic life from the wall of the yolk sac to the primitive gonad.²⁰ However, since thymic epithelial stem cells and their plasticity have only partially been characterized, a somatic stem cell derivation of at least some mediastinal germ cell tumors has not been excluded to date.

The only established risk factor for mediastinal nonseminomatous germ cell tumor development is Klinefelter syndrome (reported risk 50 to several hundredfold).²¹ Apart from the well-established association between hematologic malignancies and mediastinal nonseminomatous germ cell tumors, the frequency of other neoplasms is not increased in patients with mediastinal germ cell tumors, arguing against a role of common cancer susceptibility genes in the development of mediastinal germ cell tumors.¹