Thymic cysts are relatively rare benign lesions. They comprise approximately two-thirds of anterior mediastinal cysts, but only 5% of all mediastinal cysts.^1,2 Other cystic lesions in the mediastinum include bronchogenic cysts, cystic teratomas, hydatid cysts, enteric cysts, and lymphangiomas.

Thymic cysts are commonly classified as either congenital or acquired. Congenital thymic cyst is considered to arise as a congenital defect due to persistence of embryonal remnants.³ A relative high proportion of congenital ectopic thymic cysts are diagnosed in the neck, where they are easily detected.^4,5

Congenital thymic cysts are usually unilocular and lack inflammatory processes.^6,7,8 In contrast, acquired cysts are always multilocular and accompanied by inflammation.^{3,9,10,11,12,13,14} Multilocular cysts most likely result from the cystic transformation of medullary duct epithelium-derived structures (including Hassall corpuscles) induced by an acquired inflammatory process.³

Thymic cysts comprise 3% of published anterior mediastinal masses and 15% of mediastinal cysts.^3,12 Congenital cysts are more common in children.^6,15 Acquired thymic cysts occur in middle-aged and older adults, with a male predominance.^3,10,14 Thymic cysts are most often asymptomatic and discovered incidentally on routine chest x-ray.^3,12 Patients may also present with acute symptoms of chest pain or discomfort, sometimes associated with dyspnea.³