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Mediastinal masses: compartments and differential diagnosis
The ITMIG model divides the mediastinum into three compartments — prevascular, visceral and paravertebral — each with its own differential diagnosis; density (fat, fluid, calcium) and enhancement complete the diagnosis.
Why it matters in the exam
Prevascular (anterior) compartment: thymic tumours (thymoma — sometimes with myasthenia gravis), lymphoma, germ cell tumours (teratoma with fat, calcium and fluid; seminoma in a young man) and a thyroid gland extending into the chest and connected to the thyroid in the neck. In children the normal thymus is large and may look like a "sail" on the radiograph. To distinguish thymic hyperplasia from tumour, chemical shift MRI is used: in hyperplasia there is signal drop on the opposed-phase image because of microscopic fat; in tumour there is none.
Visceral (middle) compartment: enlarged lymph nodes, foregut cysts — bronchogenic cyst, usually subcarinal, of fluid density or higher because of protein, bright on T2 and non-enhancing — oesophageal duplication cyst that may cause dysphagia in a child, oesophageal disease and aortic aneurysm. A pericardial cyst lies in the cardiophrenic angle, mostly on the right, of water density and without enhancement. Paravertebral (posterior) compartment: neurogenic tumours — schwannoma and neurofibroma in an adult, neuroblastoma (with calcifications) and ganglioneuroma in a child — and also extramedullary haematopoiesis.
Lymph node mapping in lung cancer is also asked: stations 1–9 are mediastinal (N2 or N3), and station 10 (hilar) and beyond is N1; the boundary between N1 and N2 is the reflection line of the mediastinal pleura. Station 7 is subcarinal, station 5 is in the aortopulmonary window and station 6 is para-aortic. On the radiograph: a finding above the clavicles with a sharp border is posterior (cervicothoracic sign), and a hilum seen "through" a mass means the mass is not in the hilum (hilum overlay sign). Contrast CT is the basic examination; MRI helps with complex cysts, the thymus and spinal canal involvement.
Key points
- Three compartments according to ITMIG.
- Anterior: thymoma, lymphoma, teratoma, thyroid.
- Middle: nodes, foregut cysts.
- Posterior: neurogenic tumours.
- Thymic hyperplasia versus tumour — chemical shift.
Summary table
| Compartment | Boundaries | Main differential | Distinguishing clue |
|---|---|---|---|
| Prevascular | Behind the sternum, in front of the heart and great vessels | Thymoma, lymphoma, teratoma, thyroid | Fat and calcium — teratoma; myasthenia — thymoma |
| Visceral | Trachea, oesophagus, heart and great vessels | Lymph nodes, bronchogenic cyst, oesophagus | Non-enhancing subcarinal cyst |
| Paravertebral | Beside the vertebral bodies | Schwannoma, neurofibroma, neuroblastoma | Foraminal widening; calcifications in a child |
| Cardiophrenic angle | Beside the heart and diaphragm | Pericardial cyst, fat | Water density, mostly on the right |
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Sources
- ITMIG Classification of Mediastinal Compartments and Multidisciplinary Approach to Mediastinal Masses, 2017 — CT-based compartment model
- IASLC Lung Cancer Staging Project: Lymph Node Map, 2009 — Nodal stations and N1–N2 boundary
- ACR Appropriateness Criteria: Imaging of Mediastinal Masses, 2021 — Initial imaging and MRI for characterisation
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.