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
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.