Mediastinal embryonal carcinoma is a germ cell tumor composed of large primitive cells of epithelial appearance with abundant clear or granular cytoplasm, resembling cells of the embryonic germ disk and growing in solid, papillary, and glandular patterns.¹

Embryonal carcinoma of the mediastinum is a tumor of young males (M/F ratio, >10:1).^2,3 It occurs in pure form or as a component in mixed germ cell tumors at about equal frequencies.⁴ Embryonal carcinomas (pure or mixed) account for up to 12% of all mediastinal germ cell tumors.^2,4 Between 0% and 26% of nonseminomatous malignant germ cell tumors of the mediastinum are pure embryonal carcinoma, depending on the series.^4,5,6,7 The mean age of adult patients is 27 years (range: 18 to 67 years).^2,8

Pure embryonal carcinoma does not occur in the mediastinum of children.⁹ Embryonal carcinoma as part of a mixed germ cell tumor occurs after age 5 in boys.^9,10,11 Embryonal carcinoma is commonly associated with teratoma (56%), choriocarcinoma (22%), or seminoma (22%).^2,3 The association with yolk sac tumor (YST) is the most common combined form of mixed germ cell tumor in adults⁴ and is also not uncommon in children, sometimes with seminoma or choriocarcinoma.⁹

A minority of patients with mediastinal embryonal carcinoma show features of Klinefelter syndrome similar to other germ cell tumors.¹²

Patients present with thoracic or shoulder pain, respiratory distress, hoarseness, cough, fever, and superior vena cava syndrome.³ Gynecomastia is uncommon. A quarter of patients have pulmonary metastasis at presentation, and virtually all patients exhibit increased serum α-fetoprotein (AFP) levels, while β-hCG levels are elevated in cases with a choriocarcinoma component.¹

Embryonal carcinomas present as large tumors with invasion of the surrounding organs and structures. Grossly, the cut surface often reveals large areas of necrosis and hemorrhage. Viable tumor tissue is soft, fleshy, and gray or white to pink or tan. In mixed germ cell tumors, cystic spaces may be conspicuous.

Pure embryonal carcinomas show a more solid growth pattern than do other nonseminomatous germ cell tumors. Embryonal carcinomas form sheets, tubules, or vague papillary structures composed of large polygonal or columnar cells (Fig. 123.1). The nuclei are large, round or oval, and often vesicular and can be hyperchromatic or have a light chromatin. They can be crowded and overlapping.