Definitions

1. •Mediastinum - that part of the thorax contained between the two pleural cavities.
   

2. •Superior compartment - above the aortic arch
   

1. ◦Thyroid
   

2. ◦Aneurysms
   

3. ◦Oesophageal
   

4. ◦Neurogenic
   

3. •Anterior compartment - is bordered by the sternum anteriorly, the pericardium posteriorly and the mediastinal pleura laterally. (the four T's)
   

1. ◦Thymus
   

2. ◦Thyroid
   

3. ◦Teratoma (and other germ cell tumours)
   

4. ◦Terrible Lymphoma and other lymphoid diseases (sarcoid, Castlemann's NSCLC, SCLC)
   

5. ◦(also pericardial cyst and Morgagni hernia inferiorly)
   

4. •Middle (visceral) compartment - from the anterior pericardium back to the pre-vertebral fascia and bounded by both pleura, includes the heart, trachea, main bronchi and oesophagus. (BLAB)
   

1. ◦Bronchogenic carcinoma
   

2. ◦Lymphoma
   

3. ◦Aneurysms (and other vascular, including cardiac tumours)
   

4. ◦Bronchogenic cyst
   

5. •Posterior compartment - better referred to as the paravertebral sulci, includes those structures medial to the pleura but excluding the vertebral column. (NOBA)
   

1. ◦Neurogenic
   

2. ◦Oesophageal (duplication and para-oesophageal hernia)
   

3. ◦Bone
   

4. ◦Aneurysms
   

6. Mediastinal tumours fall into three general categories, the commonest being secondary bronchogenic carcinoma. Often the primary tumour is small and assymptomatic, even being undetectable in the case of small cell carcinoma. The second group entails systemic malignancy manifesting itself in the mediastinum. Lymphomas are characteristic of this group common though metastases from breast, renal and gastro-intestinal tract cancers are also common. Primary tumours of the organs or tissues of the mediastinum make up the third group and include thymomas, neurogenic tumours, germ cell tumours, ectopic thyroid and parathyroid tumours, and the rare lipomatous, vascular and peripheral ectodermal (Askin) tumours.
   

Mediastinal tumours have similarities in presentation, investigation and surgical approach depending on the anatomical compartment in which they arise. The anterior compartment is bordered by the sternum anteriorly, the pericardium posteriorly and the mediastinal pleura laterally. The main structure in the anterior compartment is the thymus though retrosternal thyroid or parathyroid tumours as well as germ cell, lymphoma and rare vascular tumours also occur there. The usual surgical approach is via median sternotomy though unilateral disease or the presence of associated pleural disease may be better approached by a lateral thoracotomy.

The visceral (middle) compartment, from the anterior pericardium back to the pre-vertebral fascia and bounded by both pleura, includes the heart, trachea, main bronchi and oesophagus. Though the trachea, bronchi and oesophagus are mediastinal structures they are dealt with elsewhere because of common features with tumours of the respiratory and gastro-intestinal tract respectively.

The posterior compartment, better referred to as the paravertebral sulci includes those structures medial to the pleura but excluding the vertebral column. The common tumours in this area are the neurogenic tumours arising from intercostal nerves and the sympathetic chain. Though such tumours arise in the posterior mediastinum they can encircle vital structures of the visceral compartment preventing complete excision. Cartilage and bone tumours of the necks of the ribs can mimic posterior mediastinal tumours. The surgical approach to the posterior mediastinum is via posterior thoracotomy with paravertebral extension where a tumour permeates through an intervertebral foramen.

Presentation

Mediastinal tumours in children are usually symptomatic with respiratory symptoms such as cough, stridor and dyspnoes. Malignant lesions are often accompanied by lethargy, fever, malaise and chest pain. In adults many lesions are assymptomatic, found incidentally on routine chest radiographs. However, obstructive symptoms do occur when the tumour impresses on the superior vena cava, oesophagus or tracheo-bronchial tree and cardiac tamponade can be caused by large anterior compartment tumours. Invasion of phrenic, recurrent laryngeal or sympathetic chain nerves may also cause symptoms of breathlessness, hoarseness or Horner’s syndrome respectively.

Diagnosis

Imaging of the mediastinum is crucial in distinguishing tumours from other benign cystic lesions (thymic, bronchogenic, enteric duplication, neuroenteric, mesothelial and cystic hygroma) and granulomatous lesions (sarcoidosis, histoplasmosis and tuberculosis).

A CT scan or MRI scan will outline the exact site of the lesion and will give clues to the diagnosis, a variegated appearance suggesting teratoma. These scans will also give an indication of malignant invasion of adjacent structures and pleural metastases which in the case of thymoma produce a "droplet" pattern.

Fine needle aspiration cytology is frequently inadequate to differentiate thymoma from lymphoma and almost never provides enough tissue to differentiate between types of lymphoma, an important consideration as Hodgkin’s and non-Hodgkin’s lymphoma are treated by different modalities in the first instance. A core biopsy is sometimes safe and productive though the proximity of the aorta and other great vessels dissuades some radiologists. Mediastinoscopy, mediastinotomy via anterior mini-thoracotomy or thoracoscopy may be required to provide enough issue for the pathologist to make a full diagnosis. In patinets who are unstable due to compression or obstruction of a vital organ treatment with steroids, radiotherapy or chemotherapy may need to be commenced before a full diagnosis is obtained surgically.

Thymic tumours are unique in that they are associated with a number of paraneoplastic or "parathymic" syndromes. The rare paraganglionic neurogenic tumours may be also be functional in that they produce biogenic amines. In this regard they resemble phaeochromocytoma. Vanillylmandelic acid or homovanillic acid may be detectable in the urine. Haematological markers of germ cell tumours (beta-HCG and alpha feto-protein) should be sought. Both markers are negative in benign teratoma but both tend to be elevated in malignant non seminomatous tumours. The beta hCG may be elevated in seminoma but the presence of an elevated alpha feto-protein suggests that there are non seminomatous elements which need to be treated as such.