Localization

 anterior mediastinum

Synopsis

 homogeneous cut surface with numerous cystic structures that varied from 0.5 to 1.5 cm in greatest diameter
 numerous cystic spaces lined by flattened or cuboidal epithelial cells
 proliferation of small canalicular structures lined by round to polygonal epithelioid cells with vacuolated eosinophilic cytoplasm in the walls of the cysts

Immunohistochemical studies

 strong positivity of the epithelioid cells

• AE1/AE3 cytokeratin
• CK5/CK6
• calretinin

 negative stains

• CK7
• CK20
• alpha-fetoprotein
• CD31
• carcinoembryonic antigen
• MOC 31
• chromogranin

Electron microscopic examination

 numerous long microvilli on the cell surface
 abundant tonofilaments/desmosomal plaques in the tumor cells, characteristic of mesothelial cells

Differential diagnosis

 anterior mediastinal masses

References

 Plaza JA, Dominguez F, Suster S. Cystic adenomatoid tumor of the mediastinum. Am J Surg Pathol. 2004 Jan;28(1):132-8. PMID: #14707875#