Thymic cyst

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

Epidemiology

Thymic cysts are uncommon lesions, estimated to account for approximately 1-3% of all anterior mediastinal masses ⁴. However, they are reported to be the second most common type of primary mediastinal cyst ⁷. 

Associations

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

Clinical presentation

A large proportion of patients with thymic cysts (around 60%) are considered to be asymptomatic. Approximately 50% of congenital thymic cysts are incidentally discovered during the first two decades of life, but acquired cysts may present much later. If symptoms are present, the most common are cough, dyspnea, 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 tumors

  • 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.

Radiographic features 

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

Plain radiograph

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

CT

Thymic cysts 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 hemorrhage 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.

Another study suggested a diameter of ≤3 cm and an unenhanced CT value >20 HU are independent factors for a pre-operative diagnosis, suggesting a lesion diagnosis is more than a simple thymic cyst and warrants further evaluation ⁷.  

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. 

Treatment and prognosis

Pure thymic cysts are considered benign. Some authors recommend surgical resection ^seven, while others suggest more conservative approaches for lesions under 3 cm in asymptomatic individuals ¹².

Follow up protocol for incidentally detected lesions on unenhanced CT

Some publications recommend that if a small (< 3 cm) thymic cyst is found on low dose unenhanced CT follow up imaging (e.g. with contrast enhanced CT) recommended at around 3-6 months ⁷.

Differential diagnosis

• if the lower cervical region, consider

  • branchial cleft cyst: any type dependent on location

• if mediastinal, consider

  • cystic thymoma

  • cystic teratoma

  • lymphangioma

  • cystic degeneration of a seminoma

  • pericardial cyst

See also

• pediatric mediastinal masses