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. They are however reported to be the second most common type of primary mediastinal cyst ⁷. 

Clinical presentation

A large proportion of patients with thymic cysts (i.e. around 60%) are considered to be asymptomatic. Approximately 50% of congenital thymic cysts are incidentally discovered during the first 2 decades of life. Acquired cysts may present much later. If symptoms are present, the most common have to reported to be cough, dyspnoea, and chest pain ⁷. 

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 multilocular
  • secondary to thoracotomy
  • following chemotherapy or radiotherapy for mediastinal/intrathoracic 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

Thymic cysts can fluctuate both in size and attenuation over time on CT and MRI ⁸.

RadiographPlain radiograph

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

CT

Typically manifestmanifests 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. According to one study, cysts had a mean attenuation value of around 23 HU and a maximal attenuation value of 58 HU ^8,9.

MRI

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

If hemorrhage or infection occurs, the cysts can demonstrate high signal intensity on both T1 and T2-weighted images and are also referred to as complicated cysts.

Differential diagnosis

If

• if the lower cervical region, consider

• branchial cleft cyst: any type dependant on location

If

if

mediastinal, consider

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

See also

• paediatric mediastinal masses