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Mediastinum

Cystic lesions

Thymic cyst

Reviewer: Hanni Gulwani, M.D. (see Reviewers page)
Revised: 24 February 2013, last major update December 2012
Copyright: (c) 2003-2013, PathologyOutlines.com, Inc.

See also proliferating multilocular thymic cyst below

General
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● Thymus derived from third and fourth branchial pouch, as is parathyroid gland
● Usually presents as incidental mass in anterosuperior mediastinum
● Congenital (unilocular) or acquired (multilocular)
● Rarely occur post-operatively
● Mixed multilocular thymic cyst: has parathyroid or salivary gland tissue

Epidemiology
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● Usually ages 20-50 years

Clinical features
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● May be associated with thymic carcinoma (Am J Surg Pathol 2011;35:1074), mediastinal Hodgkin lymphoma, but not non-Hodgkin lymphoma

Case reports
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● 6 year old boy with huge cervico-thoracic thymic cyst (Interact Cardiovasc Thorac Surg 2003;2:339)
● 23 year old man with epithelioid granulomas within cyst (Ann Diagn Pathol 2012;16:38)

Xray description
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● Rounded, circumscribed masses in anterior mediastinum, may have peripheral rim of calcification

Gross description
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● Up to 18 cm
● Unilocular with thin wall and serous fluid or multilocular with turbid, cheesy or hemorrhagic material, thick wall and fibrous adhesions
● Either centered in thymus or connected to it by a small pedicle

Gross images
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Huge thymic cyst

Thin walled cyst with thyroid

Micro description
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Unilocular cysts:
● Have thin wall with a few layers of bland squamoid cells and thymic tissue in wall, no inflammation, no cholesterol granulomas, no hemorrhage

Multilocular cysts:
● May have more layers of squamoid, cuboidal, columnar, micropapillary or mixed glandular epithelium
● May have pseudoepitheliomatous hyperplasia
● Usually cholesterol granulomas
● Commonly lymphocytes, granulation tissue, hemorrhage
● Cysts separated by thick fibrous septae
● 50% have Hassall’s corpuscles or other thymic tissue, but not in cyst wall
● No cartilage or smooth muscle is present

Micro images
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Thymic cyst	Flat epithelial lining #1	#2	Proliferative epithelium lining cyst wall
Bland squamoid epithelium	Papillary outpouchings	Cholesterol clefts	Thymic tissue in cyst wall	Squamous epithelial lining

Differential diagnosis
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● Cystic degeneration in Hodgkin lymphoma
● Seminoma
● Thymoma
● Cystic lymphangioma

Proliferating multilocular thymic cyst

General
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● Resembles cutaneous proliferating epidermoid cyst and proliferating trichilemmal cyst

Micro description
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● Pseudoepitheliomatous hyperplasia of cyst lining cells (narrow tongues of squamoid epithelium extending deeply into fibrous cyst wall) with reactive changes but no dysplasia
● Typical mitotic figures present

Differential diagnosis
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● Squamous cell carcinoma: extremely rare in thymic cysts

End of Mediastinum > Cystic lesions > Thymic cyst

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