The mediastinum

Primary tumours of the mediastinum (Fig. 47.24)

Secondary, involvement from direct infiltration of an intrathoracic primary or metastatic disease from elsewhere occurs more frequently than primary mediastinal tumours. Thymoma, neurogenic tumours and lymphoma account for most primary tumours of the mediastinum. Rarer conditions such as germ cell tumours, mesenchymal and endocrine tumours account for the remainder.

Thymoma

This is the most common mediastinal tumour accounting for 25 per cent of the total. These are tumours of the thymic epithelial cells of Hassall’s corpuscles and are sited in the anterior and superior compartments. They generally occur after childhood and present as lobulated, occasionally calcified, masses in the anterior mediastinum (Fig. 47.25). They may appear encapsulated and are often associated with the autoimmune disease myasthenia gravis. The tumours vary in their behaviour from completely benign to aggressively invasive. The only reliable indicator of malignancy is capsular invasion. Diagnosis and treatment are best achieved by complete thymectomy, but radiotherapy may be the only treatment option if the lesion is advanced.

Eighty per cent of thymomas are benign and the prognosis is good. There is, however, a small risk of recurrence. A rarer type of thymoma has similar histological appearance to the benign thymoma, but it has capsular invasion and is occasionally aggressive with widespread metastases. Rarer still is the thymic carcinoma. This is not associated with myasthenia and resembles an SCC or an undifferentiated large cell tumour. The prognosis is generally poor in spite of combination treatment of excision, radiotherapy and cytotoxic therapy.

Germ cell tumour

Germ cell tumours account for 13 per cent of all mediastinal masses and cysts, and are usually found in the anterior mediastinum (Fig. 47.26). They tend to occur in young adults and 75 per cent are benign and cystic, although they may cause compression of neighbouring structures. They contain elements from all three cell types (mesoderm, endoderm and ectoderm) and are best treated by surgical excision in case malignant transformation should occur. Malignancy is suspected if elevated levels of serum alpha-fetoprotein, human chorionic gonadotrophin and carcinoembryonic antigen are detected.

Lymphoma

Lymphoma is a common cause of a mediastinal mass lesion, particularly the anterior mediastinum, leading to obstruction of the superior vena cava. Lymphomas arise from the thymic lymph tissue or the lymph nodes of the mediastinum. They can be classified into Hodgkin’s and non-Hodgkin’s types but details of the pathological differences and their implications are outside the scope of this chapter. Hodgkin’s lymphomas presenting in the thymus tend to be localised and usually have favourable histology and a favourable response to treatment. Non-Hodgkin’s lymphomas are usually high grade and are more common in young to middle-aged females. A tissue diagnosis is essential so that the appropriate treatment can be planned. The overall prognosis in non-Hodgkin’s lymphomas is poor.

Mesenchymal tumours

A small number of mesenchymal tumours occurs within the mediastinum and approximately half are malignant. Lipomas are common in the anterior mediastinum, whereas the malignant form, liposarcoma, rends to occur posteriorly. Fibrosarcoma and mesothelioma may also occur.

Thyroid

Ectopic thyroid tissue (and parathyroid) may be found in the anterior mediastinum. Neoplasia and hyperplasia may occur but these are uncommon. More often the tumour is merely a mediastinal extension of a thyroid lesion.

Neural tumours

These may derive from the sympathetic nervous system or the peripheral nerves and are more prevalent in the posterior mediastinum. They may be painful but are more often discovered accidentally on routine chest radiography (Fig. 47.27).

Tumours of the sympathetic nervous system are more common in young patients less than 10 years of age.

Neuroblastoma. This rare childhood tumour consists of poorly differentiated primitive neural cells. It metastasises widely and has a poor prognosis.

Ganglioneuroma. This is a mixture of mature ganglion cells and spindle cells which does not progress after puberty and generally has a good prognosis.

Ganglioneuroblastoma. This tumour has a mixed pattern of mature and immature cell types and has an intermediate prognosis.

Adults are more prone to develop Schwannomas and neurofibromas. The histological appearances are similar to those found in conventional sires and they present as a mass lesion with pressure effects or as asymptomatic radiographic findings.

Tumours of the nerve sheath and fibres (neurofibromas and Schwannomas) have a wide range of presentations and behaviours. Multiple neurofibromas may be part of a familial syndrome (von Recklinghausen’s disease, neurofibromatosis). Tumours in the paravertebral gutter may have a component within the intervertebral canal (a so-called ‘dumb-bell’ tumour).

Phaeochromocytomas. These arise from the sympathetic chain and produce the characteristic endocrine syndrome.

The mediastinum may be involved by metastatic tumour mimicking a primary mediastinal lesion. In elderly people, this is the most common cause of mediastinal lymphadenopathy.

Symptoms of mediastinal masses

Most asymptomatic mediastinal masses discovered by routine chest radiography will be benign. In contrast, masses presenting with symptoms, in particular pain, are much more likely to be malignant.

Symptoms are generally secondary to compression or invasion of a structure within the mediastinum.

Superior vena caval obstruction

Tumours located behind the sternum have little space in which to grow and consequently the low-pressure superior vena cava is the first to be compressed. Venous engorgement of the upper extremities and face occurs and persists even when the patient sits upright. The most common cause of this syndrome is carcinoma of the bronchus and radiotherapy usually provides good symptomatic relief.

Tracheal and oesophageal compression

This may be extrinsic or from mural invasion; symptoms include dysphagia, dyspnoea and occasionally stridor. Radiotherapy or intraluminal stenting may provide some relief.

Neural invasion

A left-sided hilar lesion may infiltrate the recurrent laryngeal nerve and paralyse the vocal cords leading to hoarseness and a bovine cough. Paralysis of the phrenic nerve causes a raised hemidiaphragm on the affected side and indicates irresectabiliry if bronchial carcinoma is present. Homer’s syndrome is a result of invasion of the sympathetic chain superiorly.

Pericardial invasion

Direct invasion may cause changes similar to pericarditis with arrhythmias and ECG changes. Chronic tamponade may occur from the slow accumulation of fluid in the pericardium.

Neural tumours

Invasion of the spinal cord may lead to progressive paraplegia.

Investigation and treatment

Investigation of a mediastinal lesion follows the same pattern as investigation for pulmonary lesions with more emphasis on radiology, and mediastinotomy and mediastinoscopy. If a resection is planned, the best approach to the anterior and superior mediastinum is through a median sternotomy. The posterior mediastinum can be reached through a postero­lateral thoracotomy at the appropriate level. Any operation must be carefully planned because the pitfalls are numerous and may be difficult or impossible to counter.