17: Mediastinal Disease

Epidemiology

• Thymic tumors are the most common primary neoplasm of the anterior mediastinum, accounting for about 20 percent of anterior mediastinal masses. Their peak incidence is in middle-aged adults between the ages of 30 and 50 years. Although lymphomas are the most common neoplasm of the mediastinum, primary mediastinal lymphomas are rare and account for less than 10 percent of lymphomas. Mediastinal germ cell tumors, including both seminomas and nonseminomas, account for about 20 percent of anterior mediastinal masses and, with the exception of teratomas, occur almost exclusively in males. All of these tumors have a peak incidence in young adults in their third decade of life. In general, extragonadal germ cell tumors are rarely primary tumors, with the mediastinum frequently being a site of metastasis.

Pathophysiology

• Pathophysiologic processes range from the antibody-mediated effects of myasthenia gravis (MG) seen with thymoma to the mass effects of tumors as noted in superior vena cava (SVC) syndrome. The primary mediastinal lymphomas include Hodgkin, non-Hodgkin, and lymphoblastic lymphomas. Seminomas, which occur exclusively in men, are generally malignant but highly responsive to treatment, whereas nonseminomas can range from benign teratomas that have a very low percentage of malignant transformation (<1 percent) to primary mediastinal nonseminomatous germ cell tumors that have a very poor prognosis.

Clinical features

• The majority of patients with thymoma are asymptomatic at presentation with an incidentally discovered mass on imaging. Those who do present symptomatically have symptoms associated with local compression or invasion, such as chest pain, cough, or dyspnea. About one-half of patients with thymoma have MG and present with muscle weakness, diplopia, ptosis, or blurry vision. Lymphomas commonly present with “B-type” symptoms of fever, night sweats, weight loss, and malaise, in addition to mediastinal compression symptoms. Most patients with seminomas and nonteratomal nonseminomas present with systemic endocrine changes such as gynecomastia. In contrast, most patients with teratomas will have compression of nearby structures causing cough and dyspnea, or direct connection with the bronchial tree causing expectoration of teratoma contents.

Diagnostics

• The primary diagnostic modalities include plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), radionuclide scans, serum markers, and biopsy.

Treatment (medical/surgical)

• Treatment of thymoma includes complete surgical resection. Patients with larger malignant tumors (>8 cm) often receive neoadjuvant chemotherapy, and patients with Masaoka Stage III or IV often receive comprehensive treatment with chemotherapy, radiotherapy, and surgery. Treatment of MG includes immunosuppression with prednisolone, cyclosporine, and azathioprine; rituximab and cyclophosphamide for severely drug-resistant patients; cholinesterase inhibitors such as pyridostigmine; plasma exchange; and thymectomy. Treatment of lymphomas includes chemotherapy, radiotherapy, and immune modulators such as rituximab. Treatment of teratomas is surgical resection. Treatment of seminomas is cisplatin-based chemotherapy. For patients suffering from relapse, high-dose chemotherapy with autologous stem cell transplantation is suggested. In general, surgical resection is not performed for relapse due to the excessive morbidity from the desmoplastic reaction induced by chemotherapy. Treatment of nonseminomas includes chemotherapy and surgical excision for residual disease, given the poor results noted with ...