Thymic cyst

Last revised by Tom Foster on 28 Jul 2023

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

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

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

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 are cough, dyspnea, and chest pain 7

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

Can be broadly divided into:

Larger lesions can sometimes occur in combination.

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

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

Typically manifests 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 ≤3 cm and an unenhanced CT value >20 HU being independent factors for a pre-operative diagnosis suggesting a diagnosis of lesion being more than simple thymic cyst and warranting further evaluation 7.  

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 occur, the cysts can demonstrate high signal intensity on both T1 and T2-weighted images and are also referred to as complicated cysts. 

Pure thymic cysts are considered benign. Some authors recommend surgical resection 7 while other suggest more conservative approaches for lesions under 3 cm in asymptomatic individuals 12.

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.

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Cases and figures

  • Case 1: mediastinal thymic cyst
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  • Case 2: probable
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  • Case 3: mediastinal thymic cyst
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  • Case 4: cervical thymic cyst
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