Mediastinal lesions & hilar enlargement

Mediastinal lesions

A lateral view can be used to localize to the anterior, superior or middle mediastinum a mass observed on the frontal view. The anterior mediastinum consists of the retrosternal space and the heart, the middle consists of the structures along the trachea, esophagus, and between the hilar shadows, while the posterior includes the areas on both sides of the thoracic column.

Anterior mediastinal masses may be caused by retrosternal goitre (Fig. 35), tumor/cyst in the thymus (Fig. 36 a - c), dermoid cyst, and other

Figure 36. Thymoma (T) in the anterior mediastinum. a) PA view b) Lateral view c) The CT section shows the thymoma with peripheral calcification. It is close to the ascending aorta (A). Pulmonary artery (P). Vertebral column (C). a

c b

Figure 37. Malignant thyroid tumor with pulmonary metastases. Compression of the trachea from the left (arrows). Bilateral pulmonary metastases.

Figure 38. Mass (T) in the middle, lower part of the mediastinum, behind the heart shadow. Picture after oral contrast medium showed oesophageal cancer.

germinal tumors. Goitre usually causes deviation and compression of the trachea (Fig. 37). The thymus is examined in patients with myasthenia gravis. Other possible masses are lymphoma, aneurysm of the ascending aorta, and tumors arising from the sternum and underlying soft tissues. Near the diaphragm, anterior mediastinal masses are usually pericardial fat pad, pericardial cyst or anterior diaphragmatic (Morgagni) hernia (Fig. 25). Masses in the right cardiophrenic angle are common, but are seldom of clinical significance.

Masses localized to the middle mediastinum (Fig. 38) are most frequently aneurysms of the aortic arch, bronchogenic cysts, lesions of the oesophagus and enlarged lymph nodes. Contrast medium in the oesophagus may contribute to the correct diagnosis.

Posterior mediastinal masses are frequently neurogenic tumors, which may extend into or originate from the spinal canal. Spindle-shaped enlargement of the paravertebral soft tissue shadow may be due to spondylitis and hematomas subsequent to fractures of the vertebral column. Metastases to the vertebral column with bone destruction may also lead to swelling of the soft tissue shadow, which may also be caused by enlargement of paravertebral lymph nodes. Expansive processes in the lungs and pleura may be adjacent to the paravertebral soft tissues and resemble mediastinal masses.

Figure 39. Pneumomediastinum. Stripes of air (arrows) along the outlines of the heart and mediastinum and up on the right side of the neck. Tracheal tube and feeding tube. Thorax drain on right side.

CT scans can precisely de fine the location of mediastinal masses and their effect on normal structures; consequently, CT scans are usually done for the evaluation of mediastinal masses. Likewise, MR! can provide the same information and in some cases, better characterize the nature of the mass. MRI is favoured in the evaluation of mediastinal masses in some circumstances, especially when a vascular mass is suspected (Fig. 6). MRI is also favoured for the evaluation of paracardiac masses (Fig. 11) and posterior mediastinal masses.

Damage of aerated organs in the neck or in the medastinum may give rise to mediastinal emphysema, with striped air loculi in the mediastinum (Fig. 39), which often spreads into the soft tissues of the neck and the chest wall.

Mediastinitis may arise after perforation of the esophagus or thoracotomy and may also cause an increase in width of the mediastinum, or be localized as a mediastinal abscess. It is generally accompanied by bilateral pleural fluid. After radiotherapy of the mediastinum, fibrosis caused by the radiation in the lung adjacent to the mediastinum may lead to a straight lateral border, corresponding to the limit of the field (Fig. 40). Mediastinal fibrosis may give rise to shrinking and narrowing of the mediastinal vessels and the development of venous collaterals. MRI or magnetic resonance angiography is now usually employed for the definitive diagnosis of obstruction, occlusion or thrombosis of the brachiocephalic veins and superior vena cava (Fig. 7).

Figure 40. Linear border of the right mediastinal outline and fan-shaped opacity in the right apex caused by radiation fibrosis after radiotherapy for breast cancer.

Hilar enlargement

The hilar shadows are made up of the pulmonary vessels. When these are enlarged, it is necessary to decide whether the enlargement is due to dilated vessels, or enlarged lymph nodes or other masses. Bilateral enlarged hilar shadows, which seem to ramify, suggest dilated pulmonary vessels. Conditions that give rise to this include pulmonary hypertension, chronic embolism, excessive pulmonary blood flow such as with left-to-right shunts, anemia, and pregnancy, or post-stenotic dilatation associated with pulmonary arterial stenosis.

Enlargement of the hilar shadows without branching suggests a nonvascular nature. A polycyclic border (Fig. 41 a, b) is characteristic of enlarged hilar lymph nodes. With bilateral enlargement of hilar nodes, the most important differential diagnoses are sarcoidosis and lymphoma. With unilateral enlargement, the most important differential diagnoses are metastases from lung cancer, malignant lymphoma, and infections such as tuberculosis or histoplasmosis. The radiological finding in lung carcinoma may be the same as that in unilateral hilar expansion.

Figure 41. Hilar gland enlargement in sarcoidosis. a) PA view. Lobulated border of left-sided hilar enlargement (arrows). b) Lateral view shows the changes more clearly (arrows)    a

b

Alf Kolbenstvedt, Arnulf Skjennald and Charles B. Higgins