Imaging of Mediastinal Disease

1

Describe the anatomy of the mediastinum.

The mediastinum is located centrally within the thorax between the pleural cavities laterally, the sternum anteriorly, the spine posteriorly, the thoracic inlet superiorly, and the diaphragm inferiorly. It is usually divided into anterior, middle, and posterior compartments to help categorize tumors and diseases by their site of origin and location. Processes that arise in these different compartments are generally related to the anatomic structures located within the compartments. Figure 20-1, A , shows the normal appearance of the mediastinum on frontal chest radiography.

2

What are the three compartments of the mediastinum?

• The anterior mediastinal compartment is a space posterior to the sternum and anterior to the heart and trachea extending from the thoracic inlet to the diaphragm; it contains the thymus, thyroid gland, fat, and lymph nodes.

• The middle mediastinal compartment is a space that contains the heart and pericardium, the ascending aorta and aortic arch, brachiocephalic vessels, venae cavae, main pulmonary arteries and veins, trachea and bronchi, fat, and lymph nodes.

• The posterior mediastinal compartment is a space posterior to the middle mediastinal compartment that contains the descending thoracic aorta, esophagus, azygos and hemiazygos veins, autonomic ganglia and nerves, thoracic duct, fat, and lymph nodes.

Figure 20-1 B , delineates the three compartments of the mediastinum on lateral chest radiography.

3

List the differential diagnosis of major anterior mediastinal lesions.

In the following list, an asterisk (*) indicates the most common causes:

Thymic Masses

• Lymphoma*

• Thymoma*

• Thymic carcinoma

• Thymic carcinoid

• Thymolipoma

• Thymic cyst

• Thymic hyperplasia

Thyroid Masses

• Thyroid goiter*

• Thyroid cyst

• Thyroid adenoma

• Thyroid carcinoma

Germ Cell Tumors

• Teratoma and teratocarcinoma*

• Seminoma

• Mixed germ cell tumors

4

List the differential diagnosis of major middle mediastinal lesions.

In the following list, an asterisk (*) indicates the most common causes:

• Goiter

• Lymphadenopathy

• Metastatic disease* (lung cancer is the most common etiologic factor)

• Lymphoma (non-Hodgkin lymphoma and Hodgkin lymphoma) or leukemia*

• Granulomatous infection (fungus, tuberculosis, nontuberculous mycobacterium)

• Sarcoidosis

• Inhalational lung disease (silicosis, coal workers' pneumoconiosis, or berylliosis)

• Castleman disease

• Aortic abnormalities: aneurysm,* dissection,* traumatic aortic rupture

• Bronchopulmonary foregut cysts

• Tracheal tumor

• Esophageal abnormalities: neoplasms (carcinoma, leiomyoma, leiomyosarcoma), achalasia

• Hiatal hernia* (often contains air-fluid level) ( Figure 20-2 )

  

• Cardiac tumor

• Left ventricular or thoracic aortic aneurysm or pseudoaneurysm

• Pulmonary artery aneurysm

• Neurogenic tumor of the vagus nerve

5

List the differential diagnosis of major posterior mediastinal lesions.

In the following list, an asterisk (*) indicates the most common causes:

• Neurogenic tumors* (peripheral nerve, sympathetic ganglion, or parasympathetic involvement)

• Primary or metastatic bone tumor of the thoracic spine

• Osteomyelitis or paraspinal abscess of the thoracic spine

• Extramedullary hematopoiesis

6

List the differential diagnosis of fat-containing mediastinal lesions.

• Lipoma

• Mature teratoma

• Thymolipoma

• Well-differentiated liposarcoma

• Mediastinal lipomatosis

• Fat-containing hernia (hiatal, Bochdalek, or Morgagni hernia)

• Posterior mediastinal angiomyolipoma

7

List the differential diagnosis of cystic mediastinal lesions.

• Cystic lesions with thin, smooth wall:

  • Pericardial cyst

  • Bronchogenic cyst

  • Esophageal duplication cyst

  • Thymic cyst

  • Neurenteric cyst

  • Mediastinal pancreatic pseudocyst

  • Intrathoracic meningocele

• Cyst lesions with thick wall, mural nodularity, or internal septations:

  • Thymic teratoma

  • Any mediastinal tumor with necrosis or cystic change

  • Mediastinal abscess

8

Name different collections that may occur within the mediastinum.

• Fluid: mediastinal edema or pericardial effusion

• Blood: mediastinal hematoma or hemopericardium

• Pus: mediastinal abscess, pericardial abscess, or acute mediastinitis

• Air: pneumomediastinum and pneumopericardium

• Fat: mediastinal lipomatosis

• Fibrosis: fibrosing mediastinitis

• Cells: mediastinal tumor

9

What clinical symptoms and signs can be associated with mediastinal lesions?

Compression or invasion of the trachea or bronchi, recurrent laryngeal nerve, or esophagus may produce cough, dyspnea, chest pain, respiratory infection, hoarseness, or dysphagia. Compression or invasion of the adjacent cardiovascular structures may produce superior vena cava syndrome; cardiac dysrhythmias; constrictive pathophysiology; cardiac tamponade; or, rarely, sudden death. About 75% of mediastinal tumors in asymptomatic patients are benign, whereas about 66% of those in symptomatic patients are malignant.

10

What is a thymoma?

A thymoma is an uncommon thymic epithelial tumor that is the most common primary tumor of the thymus and of the anterior mediastinum. It occurs in men and women equally and most often occurs after age 40. It is an epithelial neoplasm composed of a mixture of epithelial cells and mature lymphocytes. About 33% are invasive, and the remainder are encapsulated. Complete surgical resection is the major treatment for a thymoma. Radiation therapy or chemotherapy may be used for an invasive thymoma, an incompletely resected thymoma, or a disseminated thymoma. Most patients with an encapsulated thymoma are cured with surgical resection, and many with microscopically invasive thymoma are cured with surgery and adjunctive radiation therapy. Patients with macroscopic invasion often have a prolonged course with slowly growing metastatic disease. The Masaoka-Koga staging system is used in patients with thymoma ( Box 20-1 ).

11

Describe the clinical presentation of a thymoma.

Most patients are asymptomatic, although 33% may be symptomatic because of compression or invasion of adjacent structures. Of patients, 50% may have a paraneoplastic syndrome, such as myasthenia gravis, hypogammaglobulinemia, or pure red blood cell aplasia. About 30% to 50% of patients with a thymoma may develop myasthenia gravis, whereas 15% of patients with myasthenia gravis have a thymoma. Ten percent of patients with a thymoma may develop hypogammaglobulinemia, whereas 5% of patients with hypogammaglobulinemia have a thymoma. Five percent of patients with a thymoma may develop red blood cell aplasia, whereas 50% of patients with red blood cell aplasia have a thymoma.

12

Describe the imaging findings of a thymoma.

One typically sees a well-defined, rounded, or lobulated anterosuperior mediastinal soft tissue mass arising from one of the thymic lobes with asymmetric growth toward one side of the midline, occasionally with necrotic, cystic, hemorrhagic, or calcific changes ( Figure 20-3 ). About 33% of thymomas invade through the capsule and involve adjacent tissues or structures such as the mediastinal fat, pleura, pericardium, great vessels, heart, or lung and may extend through the diaphragm into the peritoneal cavity or retroperitoneum. Metastatic disease is most commonly to the pleura, often mimicking malignant pleural mesothelioma with unilateral pleural thickening, masses, or diffuse nodular circumferential pleural thickening encasing the ipsilateral lung. Pleural effusions, lymphadenopathy, or distant hematogenous metastases are present less commonly.