Cystic Mediastinal Mass

Christopher M. Walker, MD

DIFFERENTIAL DIAGNOSIS

Common

Bronchogenic Cyst
Thyroid Goiter
Pericardial Cyst

Less Common

Necrotic Lymph Nodes
Necrotic or Cystic Neoplasms

Rare but Important

Other Foregut Duplication Cysts
Lymphangioma
Pseudocyst from Pancreatitis
Mediastinal Abscess
Lateral Meningocele
Thymic Cyst

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Key diagnostic feature is location and clinical presentation
10% of mediastinal masses in adults and children are cysts
Most mediastinal cysts are congenital in origin

Helpful Clues for Common Diagnoses

Bronchogenic Cyst
- Most common foregut duplication cyst
- Occur in middle or posterior mediastinum
- Paratracheal or subcarinal in location
- Round and smooth in contour
- Wall typically thin or imperceptible
- Water to soft tissue density
- Radiograph shows
  - Smooth and sharply marginated round mass
  - May displace bronchi or trachea
  - Rarely cause collapse of a lobe secondary to mass effect on bronchi
- Distinguish from soft tissue neoplasm by
  - Lack of enhancement
  - Characterization by MR
- May abruptly increase in size secondary to hemorrhage or infection
- MR shows
  - High T1 signal secondary to proteinaceous content
  - High T2 signal in nearly all cases
Thyroid Goiter
- 10% of mediastinal masses
- Radiograph shows
  - Leftward tracheal deviation
  - Mass in superior mediastinum
- Noncontrast CT shows
  - High-attenuation cystic or heterogeneous lesion
  - Connection to thyroid on sequential images
  - Coronal images are key to demonstrate connection
- Factors that suggest thyroid malignancy
  - Lymphadenopathy or metastases
  - Invasion of adjacent structures
Pericardial Cyst
- Smooth and well marginated
- Most contact diaphragm
- Majority right-sided and asymptomatic
- Low Hounsfield units by CT
- Single layer of mesothelial cells

Helpful Clues for Less Common Diagnoses

Necrotic Lymph Nodes
- Rim-enhancing lymph node with central low density indicating necrosis
- Infectious causes include tuberculosis and histoplasmosis
- Malignant causes include lymphoma or lung cancer
- Extrathoracic malignancies, such as head and neck carcinoma, seminoma, or gastric carcinoma
Necrotic or Cystic Neoplasms
- Germ cell tumors (teratoma, seminomas, and nonseminomatous tumors)
  - Teratomas are anterior mediastinal, well defined, cystic in appearance, ± fat and calcification
  - Seminomas are anterior mediastinal, homogeneous in density, ± low-attenuation areas, in younger men
  - Nonseminomas are anterior mediastinal, heterogeneous with areas of necrosis or cystic areas
- Large thymomas or thymic carcinomas
  - Anterior mediastinal mass
  - May have cystic or necrotic centers
  - So-called “drop metastases” to pleura may be seen
  - Thymic carcinoma invades great vessels and mediastinal structures in 40%
  - Thymic carcinoma may demonstrate hematogenous metastases

Helpful Clues for Rare Diagnoses

Other Foregut Duplication Cysts
- Esophageal duplication cyst
  - Similar appearance to bronchogenic cyst
  - Occur within esophageal wall or contact esophagus
  - Lined by gastrointestinal mucosa
- Neurenteric cyst
  - Posterior mediastinal mass
  - Connection to meninges through a vertebral defect
  - Vertebral anomalies include scoliosis or hemivertebrae
  - Identical appearance to other duplication cysts
  - Composed of neural and gastrointestinal components
Lymphangioma
- Most common in childhood; extend down from neck
- Commonly localized to mediastinum in adults
- Unilocular or multilocular ± thin septations
- May drape over structures and can grow to large size
- MR demonstrates heterogeneously increased T2 signal
Pseudocyst from Pancreatitis
- Located in lower mediastinum with access via esophageal or aortic hiatus
- Clinical history of pancreatitis ± lesion tracking from abdomen
Mediastinal Abscess
- Recent history of median sternotomy, esophageal perforation, or head and neck infection
- Typical rim-enhancing lesion with central low density
- May demonstrate air bubbles with communication to adjacent infection
- Difficult to differentiate from postoperative hematoma/seroma
  - May require needle aspiration
  - Postoperative hematoma/seroma should resolve after 2-3 weeks
Lateral Meningocele
- Strong association with neurofibromatosis type 1 or connective tissue disorders
- Posterior mediastinal cystic mass with extension into spinal canal
- Associated scoliosis and interpediculate widening
- MR or myelogram are diagnostic by showing connection to spinal canal
Thymic Cyst
- Usually incidental; may be unilocular or multilocular
- Thin walls
- Congenital or acquired secondary to radiotherapy after Hodgkin disease
- Fluid density
- Occasionally may contain fat or hemorrhage

Image Gallery

Frontal radiograph shows a round retrocardiac opacity obscuring a portion of the descending aorta

. Differential considerations would include lymphadenopathy.

Coronal CECT shows a round lesion of fluid attenuation abutting the descending aorta

. The most common location for this lesion is in the subcarinal space.

(Left) Esophagram shows external mass effect on the anterior esophagus

. The underlying mucosa is intact indicating this is an external process. Cross-sectional imaging would need to be performed for further characterization. (Right) Coronal CECT shows a well-circumscribed, low-attenuation mass