The mediastinum is divided anatomically into the following compartments (according to Felson): superior, anterior, middle, and posterior. In Radiology, we use a modified division based on radiographic landmarks. The anterior mediastinum is bounded anteriorly by the sternum. The posterior boundary is made up of a line from the intersection of the trachea with the sternum drawn inferiorly to intersect the diaphragm. The line follows the posterior border of the heart and IVC (inferior vena cava). This second line makes up the anterior boundary of the middle mediastinum. The middle and posterior mediastinum are divided by a line 1 cm posterior to the anterior border of the vertebral bodies. In all cases, the differential diagnosis is limited by the location of the mass. Some have altered the division so that the esophagus, and all tissue posterior to it, is included in the posterior mediastinum.
Anterior mediastinal masses
The anterior mediastinum can be further divided. Thoracic inlet lesions are those that extend above the clavicles. In the adult these are almost always caused by thyroid masses, usually goiters. Other less common causes include, lymphoma (especially when associated with palpable neck nodes) and cystic hygroma (lymphangioma). The latter usually manifests in childhood.
Cardiophrenic angle masses also have a limited differential diagnosis. The most common cause of a mass in this location is prominent pericardial fat. Comparison with prior films will document the stability of this lesion. The right cardiophrenic angle is the most common location of pericardial cysts. They may also occur on the left. Foramen of Morgagni hernia also occur in this location.
Anterior mediastinal masses limited to the retrosternal space also have a limited differential diagnosis. These include Lymphoma, Thymoma, Germ Cell tumors and vascular lesions such as ascending aortic aneurysm.
Middle mediastinal masses
The structures in this location include the trachea, esophagus, aortic arch and great vessels, pulmonary arteries, and lymph nodes. It is helpful to consider four categories when assessing a middle mediastinal mass: 1) lymphadenopathy, 2) primary tumors of the above structures 3) vascular lesions, and 4) duplication cysts.
Lymphadenopathy can occur from lymphoma or metastatic disease. Infectious diseases such as primary tuberculosis, Histoplasmosis, Coccidioidomycosis and Blastomycosis can cause mediastinal adenopathy. Inflammatory processes such as Sarcoid, Silicosis and Coal-worker's pneumoconiosis also can present with mediastinal adenopathy.
Primary tumors which can arise in the middle mediastinum include tumors of the trachea and esophagus. However, they usually cause symptoms before a large mass is evident on the chest radiograph.
Aneurysms of the aortic arch or great vessels may present as a mediastinal mass. Aortic transection following trauma can also cause widening of the mediastinum.
Duplication cysts of the foregut (tracheobronchial tree and esophagus) may present as a middle mediastinal mass. The most common location of bronchogenic cysts is around the carina. However, they can occur in a paratracheal or retrocardiac location. Esophageal duplication cysts may occur in the middle mediastinum but are more commonly seen posteriorly.
Posterior mediastinal masses
Neurogenic masses make up the majority of posterior mediastinal masses. These include nerve root tumors such as schwannomas or neurofibroma and sympathetic ganglion tumors such as neuroblastomas, ganglioneuroblastoma and ganglioneuroma. Paragangliomas (chemodectoma and pheochromocytoma) are rare causes of posterior mediastinal masses.
Metastatic lymphadenopathy and lymphoma may cause a posterior mediastinal mass. Aortic aneurysm, paraspinous abscess, extramedullary hematopoiesis, lateral meningocele and Bochdalek hernia are other causes of a posterior mediastinal mass.
 C22,
 C23
|
CPAL, mass in thoracic inlet extending into mediastinum |
 C24
|
CT, mass due to enlarged thyroid |
 C25
|
Anterior mediastinal mass (lymphoma) |
 C26, C27
|
CPAL, anterior mediastinal mass (Germ cell tumor) |
 C28
|
Lateral (same patient as 3E, 3F) after treatment |
 C29
|
"Normal" |
 C30
|
Enlarged aorta |
 C31
|
CT, aortic dissection |
 C32,
 C33
|
CPAL, middle mediastinal mass |
 C34
|
CT, mass due to esophageal cancer |
 C35
|
KUB, paraspinal mass at TID |
 C36,
 C37
|
MR, abnormal signal TID, mass extends into spinal cord |