21st February 2025

Differentials for mediastinal masses

The mediastinum is the space between the lungs within the thorax and is separated into three compartments: anterior, middle and posterior. This separation is helpful in identifying the potential causes of a mediastinal mass.

Table 6 Differentials for mediastinal mass by compartment

Anterior mediastinal masses

Around 50% of mediastinal masses are in the anterior mediastinum and the most common causes can be remembered by the 4 Ts: Thymus, Thyroid, Teratoma (and other germ cell tumours) and ‘Terrible’ lymphoma. 

Thymic mass:

• Around a third of anterior mediastinal masses are thymic neoplasms (thymoma or more rarely, carcinoma)
• Thymomas are associated with paraneoplastic syndromes, particularly myasthenia gravis

Germ cell tumour:

• The mediastinum is the most common place outside the testes and ovaries to find germ cell tumours
• Benign germ cell tumours include teratomas and dermoid cysts
• Malignant tumours can be split into seminomatous (more common) and non-seminomatous germ cell tumours (GCT)

Lymphoma:

• Lymphomas are normally found in the anterior mediastinum and tend to grow quickly compared to other causes of anterior mediastinal masses