Thymic cyst

Thymic cysts are cysts that occur within, or arise from, the thymus.

Epidemiology

Thymic cysts are uncommon lesions and are estimated to account for approximately 1-3% of all anterior mediastinal masses ⁴. Approximately 50% of congenital thymic cysts are incidentally discovered during the first 2 decades of life. Acquired cysts may present much later.

Pathology

The presence of Hassall's corpuscles in the cyst wall is diagnostic especially in congenital lesions. The cysts often contain turbid fluid or gelatinous material.

They can be seen in a variety of settings:

• congenital: 
  • contains thymic tissue in their wall
  • often unilocular
  • rare and derive from a patent thymopharyngeal duct.
• acquired: often multi locular
  • secondary to thoracotomy
  • following chemotherapy or radiotherapy for mediastinal malignancy
  • inflammatory
  • in association with thymic tumours
  • may be unilocular or multilocular
    • multilocular thymic cysts are the sequelae of a variety of inflammatory processes and usually occur in asymptomatic men

Location

Can be broadly divided into:

• cervical: cervical thymic cyst (CTC)
• mediastinal/thoracic: mediastinal thymic cyst (MTC)

Larger lesions can sometimes occur in combination.

Associations

Large multilocular thymic cysts are seen in approximately 1% of children with human immunodeficiency virus infection ⁵. 

Radiographic features

Radiograph

May not even be visible 9espaically small lesions). If visible thymic cysts often indistinguishable from other non-lobulated thymic -anterior mediastinal lesions on radiographs.

CT

Typically manifest as unilocular or multilocular cystic masses with well-defined walls. Lesions can be lobulated and may have soft-tissue attenuation components. Some thymic cysts may have increased CT attenuation if haemorrhage or infection occurs as a complication. Curvilinear calcification of the cyst wall occurs in a small proportion of cases.

MRI

Uncomplicated

In uncomplicated cases, signal characteristics are similar to any simple cyst and are:

• T1: low signal
• T2: high signal 
• T1 C+ (Gd): no intrinsic enhancement

Complicated

If hemorrhage or infection occurs, the cysts can demonstrate high signal intensity on both T1 and T2-weighted images.

Differential diagnosis

If cervical consider

• branchial cleft cyst: any type dependant on location

If mediastinal consider

• cystic thymoma
• cystic teratoma
• lymphangioma
• cystic degeneration of a seminoma
• pericardial cyst

See also

• paediatric mediastinal masses