The mediastinum is the central space of the chest between the two pleural cavities. It is bound by the sternum to the front, the spine to the rear, and by the mediastinal pleura at the sides; above it is linked to the neck through the upper opening of the chest and below with the diaphragm.
Mediastinal cancers are divided according to the topography (anterior, medial or posterior mediastinum) and whether benign or malignant.
- Anterior mediastinal cancers are thymoma, germ cell tumours (teratoma, teratocarcinoma, embryonal carcinoma, seminoma, choriocarcinoma), vascular and lymphatic cancers, lipoma, carcinoid tumours, fibroma and fibrosarcoma.
- Medial mediastinal cancers are cysts, paraganglioma, pheochromocytoma, carcinoid cancers, heart and trachea cancers.
- Posterior mediastinal cancers are neurogenic cancers (schwannoma, neurofibroma, ganglioneuroma, ganglioneuroblastoma), enterogenic cysts and oesophageal cancers.
The most frequent cancers are of the lymph nodes (lymphoma or lymph node metastases). The mediastinum is the site of onset of various types of cancers both of benign and malignant nature, among which thymus tumours and tumours originating from neural elements (neuromas) are the most frequent and treated by the thoracic surgeon.
Thymic cancers
The thymus is a lymphoepithelial organ located for the greater part in the anterior mediastinum and a smaller part located in the neck. It is a transient organ; highly developed in the foetus and in the first years of life, it undergoes involution after puberty. Thymic tumours are the most frequent mediastinal neoplasia in the adult population, representing about 50% of anterior mediastinal masses; their incidence is 0.05 per 100,000 people per year. Thymic tumours are a very heterogeneous group of neoplasms, originating from the epithelial cells of the thymus, although in most cases are made from varying percentages of epithelial cells and lymphocytes; the most frequent histological subtype is the thymoma, while thymic carcinoma is very rare and sometimes thymolipomas, carcinoid tumours, lipomas and germ cell tumours can be found. The thymoma is derived from the epithelial cells of the thymus, a rare disease, still famous for its associations with the mysterious neuromuscular disorder called Myasthenia Gravis. Quantitatively it represents 90% of tumours of the thymus. The average age of thymoma occurrence is 53 years, although they can occur at any age with an equal distribution in both genders. Thymomas have an unpredictable evolution, with cases that remain indolent for a long time up to very aggressive neoplasms with aspects of local infiltration and stories of multiple recurrences and distant metastases.
Because of its painless course, the diagnosis of thymoma can often be random. At least 30% of patients are asymptomatic at the time of diagnosis. When the often very vague and hazy symptoms are present, they are due to local compression or infiltration of surrounding structures. The most common are chest pain, cough, dyspnoea, paralysis of a hemidiaphragm through phrenic nerve involvement, but dysphonia may also be present through infiltration of the recurrent nerve and signs relating to superior vena cava syndrome.
Thymomas may be associated with a number of paraneoplastic syndromes: the most frequent is without doubt Myasthenia Gravis, which is present in 30-45% of patients, while pure red cell aplasia and hypogammaglobulinaemia occur in 2-5% of cases.
Hypogammaglobulinaemia is present in 2-5% of patients with thymoma A. Thymoma A rarely develops distant metastases, however in some cases it may be locally-invasive with infiltration of surrounding organs and spread to the parietal pleura and/or pericardium. The degree of local invasiveness is decisive in the choice of treatment.
A CT scan of the chest is needed to determine the extent of the mediastinal tumor, the relationship with adjacent structures and possible pleural and/or pericardial effusions. It can also be instrumental in diagnosing small tumours not appreciable with a standard chest X-ray. Surgical biopsy of the lesion is not recommended in cases of encapsulated thymic tumours because of the risk of dissemination; it is necessary in the case of unresectable tumours or for differential diagnosis with other malignant neoplasms affecting the anterior mediastinum for a correct therapeutic choice.
Neuromas
These are perhaps the most common form (25%) of mediastinal cancer, affecting mainly the posterior mediastinum (costovertebral gutter where nervous structures are more concentrated such as the origin of the intercostal nerves, ganglia and sympathetic chain). In children, the malignant and symptomatic forms outweigh the benign and asymptomatic forms found in adults. In adults, the most frequent forms are the Schwannoma and Ganglioneuroma, both benign in nature. In both cases, the location site is the posterior mediastinum, more precisely the paravertebral region. In a small percentage of cases, these cancers may expand into the spinal canal and compress the spinal cord into the "hourglass" radiology appearance. In paediatric age, the percentage of neurogenic tumours of malignant nature, including neuroblastoma, is relevant. Symptoms, when present, are mainly chest pain and/or various neurological symptoms such as paralysis or medullary nervous irritation.
In the majority of cases, these are detected occasionally with chest radiography. The test used to diagnose this disease with certainty is a chest CT scan, to which a spinal MRI might be added, in cases of suspected extension to the spinal canal.