Mediastinal seminomas may contain elements of thymic tissue, suggesting the thymus as a primary site of origin. These tumors rarely secrete identifying hormones. A biopsy (CT-guided core or minimal surgical incision) is required to establish their identity. Immunohistochemistry will help to further identify and differentiate these tumors if nonseminomatous elements are present.16 Occasionally, they may be associated with elevated beta-human chorionic gonadotropin but not alpha-fetoprotein. They often metastasize (60–70%), often to lymph nodes. However, despite their metastatic potential, they have a very favorable prognosis (88% 5-year survival), akin to gonadal seminomas, when treated with chemotherapy or radiotherapy for local disease.1,3
Mediastinal seminomas and germ cell tumors may demonstrate histologic and morphologic features as well as chemosensitivity similar to testicular germ cell tumors. However, the nonseminomatous germ cell tumors (NSGCTs) demonstrate a clinical course and biologic behavior pattern associated with markedly inferior prognosis compared with gonadal germ cell tumors.16,18–20 The salient divergent points between malignant mediastinal and testicular germ cell tumors are the inferior sensitivity of mediastinal NSGCTs to chemotherapy and the treatment of tumors with elevated tumor markers after chemotherapy. Attempts to deal with the inferior chemosensitivity of mediastinal NSGCTs have included dose escalation and intensive chemotherapy regimens with peripheral stem cell transplantation.19–21
Surgical resection of testicular NSGCT is indicated for residual masses with normalized serum tumor markers, patients with a late relapse, or for desperation salvage resections. Mediastinal NSGTC tumors, however, that relapse following chemotherapy have poor response to salvage chemotherapy, unlike testicular tumors. Furthermore, the presence of elevated serum tumor markers after chemotherapy does not reflect the status of residual viable tumor. However, elevated serum tumor markers after chemotherapy and, in particular, viable tumor after resection are adverse prognostic markers.19,20,22 A large series of mediastinal NSGCTs has shown that in postchemotherapy treated tumors, elevated serum tumor markers occur in 28% of patients with benign tumors, 44% of patients with persistent germ cell tumors, and 66% of patients with malignant non-germ cell cancers.19 Expectations of postchemotherapy pathology for mediastinal NSGCTs include necrosis (25.5%), teratoma (34%), and persistent germ cell tumor (31.4%).