Mediastinal seminoma may be pure, or associated with mixed germ cell tumors. The reported frequency of pure seminomas among primary mediastinal germ cell tumors ranges from 3% to 39%, ranking seminoma second in frequency following teratoma.^{1,2,3,4,5,6,7} Seminoma also forms a component in 5% to 20% of mediastinal mixed germ cell tumors.^3,8

With the exception of single cases,⁹ all reported mediastinal seminomas have occurred in adolescent boys or men.^1,2,3,4,5,6 The age ranges from 13 to 79 years, with approximately two-thirds of the cases occurring in the third and fourth decade.^1,2,3,4,5,6

In contrast to patients with retroperitoneal seminomas, intratubular germ cell neoplasia of the testis rarely if ever occurs in association with mediastinal germ cell tumors. There are rare reports of intratubular germ cell testicular neoplasia occurring in patients with mediastinal mixed germ cell tumors.^10,11,12

Presenting symptoms of mediastinal seminomas are related to the local mass effect and comprise chest pain, respiratory distress, cough, hoarseness, and the superior vena cava syndrome.^1,2,3,4,5,6 Fever and formation of multilocular thymic cysts may result from local inflammatory reactions.¹³

Mediastinal seminomas occur in patients with Down syndrome¹⁴ and may present with anti-Ma2 encephalitis, a paraneoplastic disorder characterized by limbic, diencephalic, and/or brainstem dysfunction.¹⁵

Macroscopically, mediastinal seminomas are well-circumscribed, fleshy tumors with a homogeneous, slightly lobulated to multinodular, tangray or pale cut surface.¹⁶ Hemorrhage and yellowish foci of necrosis may be observed. Thymic seminomas may present as a multilocular cystic lesion showing small focal areas of induration within the cyst walls.¹³

Mediastinal seminoma is a primitive germ cell tumor composed of fairly uniform round or polygonal cells with distinct cell borders, and a round nucleus with one or more prominent nucleoli, resembling primordial germ cells.¹⁷ The histologic and immunohistochemical features are presented in Table 120.1. The cytoplasm of the tumor cells is usually clear reflecting the glycogen and lipid content. Less commonly, the tumor cells have an eosinophilic cytoplasm. The tumors typically grow in confluent multinodular clusters, sheets, cords, strands, or irregular lobules (Fig. 120.1). Between the tumor cell aggregates, delicate fibrous septa associated with a prominent inflammatory infiltrate of small mature lymphocytes, plasma cells, and eosinophils are observed. Occasionally, lymphoid aggregates with germinal centers are present. Granulomatous reaction and fibrosis are common and occasionally so extensive that the neoplasm is obscured. The seminoma cells may be arranged in a nested pseudoglandular/alveolar, cribriform, or tubular pattern with sparse lymphocytes. Syncytiotrophoblastic cells have only rarely been reported in mediastinal seminoma, in one series seen in one of 23 cases.¹⁸ Necrosis occurs in 35% of tumors, and mitotic figures are not uncommon (mean 4 per 10 HPF).¹⁸