# Mediastinal Masses: A Compartment-Based Approach

## Mediastinal Compartment Models

### ITMIG Classification (International Thymic Malignancy Interest Group, 2014)

The ITMIG classification has replaced the traditional Felson three-compartment model (anterior, middle, posterior) with a more anatomically precise system based on cross-sectional imaging. The prevascular compartment occupies the space anterior to the pericardium, ascending aorta, and brachiocephalic vessels, extending from the thoracic inlet to the diaphragm. The visceral compartment is bounded anteriorly by the prevascular compartment and posteriorly by a vertical line 1 cm behind the anterior margin of each vertebral body, and contains the trachea, esophagus, heart, and great vessels. The paravertebral compartment lies posterior to the visceral compartment and includes the vertebral bodies, paraspinal soft tissues, and neural foramina.

### ITMIG Compartments and Differential Diagnosis

| Compartment | Boundaries | Common Masses |
|-------------|-----------|---------------|
| Prevascular | Anterior to pericardium, ascending aorta, brachiocephalic vessels | Thymoma, lymphoma, germ cell tumor, thyroid goiter |
| Visceral | Between prevascular and paravertebral; contains trachea, esophagus, heart, great vessels | Lymphadenopathy, bronchogenic cyst, esophageal tumor |
| Paravertebral | Posterior to visceral compartment; includes vertebral bodies and neural foramina | Schwannoma, neurofibroma, neuroblastoma, ganglioneuroma |

### Why Compartment Matters

Determining which compartment a mass originates from narrows the differential diagnosis significantly. When a mass spans compartments, the epicenter determines the compartment of origin. CT with IV contrast is the primary imaging modality, with MRI providing superior tissue characterization in selected cases.

## Prevascular Compartment Masses

### The 4 T's Mnemonic (Plus Additions)

The classic mnemonic for anterior mediastinal masses includes thymoma and thymic pathology, terrible lymphoma, teratoma and germ cell tumors, and thyroid (intrathoracic goiter). Additional entities include parathyroid adenoma, thymic cyst, thymic carcinoma, and thymolipoma.

### Thymoma

Thymoma is the most common primary neoplasm of the prevascular mediastinum in adults, with peak incidence at ages 40 to 60 and equal sex distribution. On imaging, it appears as a well-circumscribed, homogeneous or heterogeneous soft tissue mass that may contain calcification, cystic change, or hemorrhage. It arises from the thymus and has a round or lobulated contour. The Masaoka staging system, which is surgical, progresses from Stage I (encapsulated, no invasion) through Stage II (microscopic or macroscopic capsular or fat invasion) and Stage III (invasion of adjacent structures such as the pericardium, great vessels, or lung) to Stage IV (pleural or pericardial dissemination in IVA, or hematogenous/lymphogenous metastases in IVB).

| Masaoka Stage | Description |
|---------------|-------------|
| I | Encapsulated, no microscopic capsular invasion |
| II | Microscopic or macroscopic invasion into capsule or surrounding fat |
| III | Invasion of adjacent structures (pericardium, great vessels, lung) |
| IVA | Pleural or pericardial dissemination |
| IVB | Hematogenous or lymphogenous metastases  |  A characteristic spread pattern of invasive thymoma is pleural drop metastases, seen as nodular pleural thickening ipsilateral to the mass. Thymoma is associated with myasthenia gravis in 30 to 50% of cases, and also with pure red cell aplasia and hypogammaglobulinemia. |

### Thymic Carcinoma

Thymic carcinoma is an aggressive malignancy that is more heterogeneous and invasive than thymoma at presentation. Necrosis, calcification, and lymphadenopathy are more common, and the prognosis is substantially worse.

### Lymphoma

Hodgkin lymphoma commonly presents with prevascular lymphadenopathy and is the most common mediastinal lymphoma in young adults. It produces an anterior mediastinal mass with a lobulated contour that may encase or displace the great vessels and may contain areas of necrosis or cystic degeneration. After treatment, residual masses may represent thymic rebound or fibrosis, and PET is critical for distinguishing residual disease from inactive fibrosis. Primary mediastinal large B-cell lymphoma is an aggressive non-Hodgkin lymphoma variant arising in the thymus, predominantly affecting young women. It presents as a large anterior mediastinal mass with SVC compression and may invade the chest wall or pericardium.

### Germ Cell Tumors

Mature teratoma is the most common mediastinal germ cell tumor. It is well-circumscribed and contains a characteristic mixture of fat, soft tissue, calcification, and/or fluid. A fat-fluid level is virtually diagnostic. Teeth or bone may be present. Mature teratomas are benign and surgical excision is curative. Seminoma presents as a homogeneous soft tissue mass without calcification or fat and rarely elevates AFP, though beta-HCG may be mildly elevated. Non-seminomatous germ cell tumors are heterogeneous masses with necrosis, hemorrhage, and invasion, with markedly elevated AFP and/or beta-HCG. They include embryonal carcinoma, yolk sac tumor, and choriocarcinoma, occur almost exclusively in young males, and are associated with Klinefelter syndrome.

### Thyroid (Intrathoracic/Substernal Goiter)

The key diagnostic feature is continuity with the cervical thyroid gland. On non-contrast CT, substernal goiter has characteristically high attenuation (greater than 100 HU) due to intrinsic iodine content, and it shows intense, prolonged enhancement after IV contrast. Punctate or coarse calcification and tracheal compression with deviation are common findings.

## Visceral Compartment Masses

### Lymphadenopathy

Lymphadenopathy is the most common cause of a visceral compartment mass, occurring in subcarinal, paratracheal, aortopulmonary window, and peribronchial locations. Etiologies include metastatic disease, lymphoma, sarcoidosis, infection (tuberculosis, fungal), and Castleman disease. Sarcoidosis produces symmetric bilateral hilar and right paratracheal lymphadenopathy (the 1-2-3 sign), which may show eggshell calcification. Metastatic nodes are often heterogeneous and may be necrotic, with lung and esophageal cancers being the most common primaries.

### Foregut Duplication Cysts

Bronchogenic cyst is the most common mediastinal cyst, usually found in a subcarinal or right paratracheal location. It is a well-circumscribed, thin-walled, fluid-filled mass that shows no enhancement on post-contrast images. CT attenuation is water density (0 to 20 HU) when the contents are simple, but it can be much higher (greater than 40 HU) when proteinaceous or hemorrhagic material is present, potentially mimicking a solid mass. MRI clarifies the cystic nature by showing T2 hyperintensity with variable T1 signal depending on contents. Esophageal duplication cysts lie adjacent to the esophagus and may have a muscular wall, while neurenteric cysts are associated with vertebral anomalies.

### Esophageal and Tracheal Masses

Esophageal carcinoma presents as eccentric wall thickening that may appear as a mediastinal mass with lymphadenopathy. Leiomyoma, the most common benign esophageal tumor, presents as a smooth, well-circumscribed submucosal mass. Tracheal masses, including squamous cell carcinoma and adenoid cystic carcinoma, cause luminal narrowing visible on CT.

## Paravertebral Compartment Masses

### Neurogenic Tumors (Most Common Posterior Mediastinal Mass)

Schwannoma is a well-circumscribed, round mass arising from a nerve root that may extend through the neural foramen in a dumbbell configuration (with both intraspinal and extraspinal components). On MRI, it is T2 hyperintense and may show a target sign with central hypointensity and peripheral hyperintensity. Neurofibroma is similar to schwannoma, and plexiform neurofibromas are pathognomonic for neurofibromatosis type 1. The neuroblastic tumor spectrum includes neuroblastoma (aggressive, in children under 5, with calcification in 80 to 90%, crossing the midline, and metastasizing to bone), ganglioneuroma (benign, in adolescents and young adults, well-circumscribed and elongated along the paravertebral space), and ganglioneuroblastoma (intermediate behavior).

### Other Paravertebral Masses

Lateral meningocele is a CSF-density outpouching through the neural foramen associated with NF1. Extramedullary hematopoiesis produces bilateral paravertebral masses in patients with chronic anemias (thalassemia, sickle cell disease) and appears lobulated and homogeneous. Vertebral body tumors or metastases with soft tissue extension into the paravertebral space, and paraspinal abscesses from tuberculosis (Pott disease) or pyogenic infection presenting as rim-enhancing collections, are additional considerations.

<image>A coronal CT diagram of the mediastinum with the three ITMIG compartments color-coded and labeled. The prevascular compartment (blue) occupies the space anterior to the pericardium and great vessels. The visceral compartment (green) contains the trachea, esophagus, heart, and major vessels. The paravertebral compartment (orange) occupies the space on either side of the thoracic spine posteriorly. Representative masses are shown as overlaid silhouettes in each compartment: a thymoma in the prevascular space, subcarinal lymphadenopathy in the visceral space, and a schwannoma in the paravertebral space. Each mass is labeled with its name and compartment.</image>

<image>An axial CT image diagram of a mature mediastinal teratoma. The mass is shown in the prevascular compartment with its characteristic heterogeneous contents: a region of fat attenuation (dark), soft tissue density (gray), calcification or tooth-like structure (bright white), and a fat-fluid level with the fatty component layering anteriorly. The mass is well-circumscribed with a thin wall. Labels point to each tissue component. A caption states that the presence of fat, soft tissue, and calcification within a prevascular compartment mass is virtually diagnostic of a mature teratoma.</image>

<image>A diagram showing three different neurogenic tumors of the paravertebral compartment. Panel 1 shows a schwannoma: a well-circumscribed, round mass arising from an intercostal nerve with a dumbbell extension through the widened neural foramen into the spinal canal. Panel 2 shows a ganglioneuroma: an elongated, well-defined mass draped along several vertebral body levels. Panel 3 shows a neuroblastoma: an irregular, heterogeneous mass with stippled calcifications crossing the midline and encasing vessels, with an adjacent rib erosion. Each panel is labeled with the tumor type, typical patient age, and key imaging features.</image>

## Clinical Pearls

The compartment of origin is the most important first step in generating a differential diagnosis for a mediastinal mass. A prevascular mass with fat, calcification, and fluid in a young patient is virtually diagnostic of a mature teratoma, and the fat-fluid level is pathognomonic. High intrinsic CT attenuation (greater than 100 HU on non-contrast CT) in a prevascular mass contiguous with the cervical thyroid is diagnostic of substernal goiter. Thymoma must be considered in any prevascular mass in a middle-aged adult, and the clinical team should always be asked about myasthenia gravis symptoms. A paravertebral mass extending through a widened neural foramen (dumbbell morphology) is a neurogenic tumor until proven otherwise, and MRI with contrast is essential to evaluate the intraspinal component before surgery. Bronchogenic cysts may have high CT attenuation from proteinaceous or hemorrhagic contents and mimic a solid mass; the absence of enhancement and MRI characteristics confirm the cystic nature.

## References

- Carter BW, et al. "ITMIG Classification of Mediastinal Compartments and Multidisciplinary Approach to Mediastinal Masses." *RadioGraphics*, 2017
- Rosado-de-Christenson ML, et al. "Thymoma: Radiologic-Pathologic Correlation." *RadioGraphics*, 1992
- Whitten CR, et al. "A Diagnostic Approach to Mediastinal Abnormalities." *RadioGraphics*, 2007