Thymic Cyst

Definition

• Epithelial lined cyst involving the thymus

Diagnostic Criteria

• Frequently separated into congenital and aquired types
  • Common features
    • May occur at all ages
    • May be found in neck or mediastinum
    • Thymic tissue must be present at least focally within the wall of the cyst
  • Congenital type
    • Thin wall
    • May be unilocular or multilocular
    • Lined by squamous, glandular or ciliated epithelium
  • Acquired type
    • Walls thickened by inflammatory infiltrate
      • May have germinal centers
    • Virtually alwalys multilocular
    • Lined by squamous or columnar epithelium
      • May exhibit pseudoepitheliomatous hyperplasia
    • Frequent cholesterol clefts and granulomas
    • One report of four patients with associated fever and autoimmune disease (Izumi)
      • Cured by resection
    • May be associated with any neoplasm, but most frequently with:
      • Thymoma
        • Cysts usually lack epithelial lining
      • Basaloid thymic carcinoma
      • Lymphoma, Hodgkin or extranodal marginal zone
      • Germ cell tumor, especially seminoma
      • Close inspection and immunohistochemistry may be required
• Both types are benign (if not associated with a neoplasm)

Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Differential Diagnosis

• Thymic cysts, both primary and acquired are distinguished from other types of cysts by the identification of thymic tissue in their walls
  • Cervical
    • Branchial cleft
    • Cervial bronchial
    • Thyroglossal
  • Mediastinal
    • Bronchal
    • Esophageal duplication
    • Lymphangioma
    • Pericardial
• Associated neoplasm must be ruled out in all cases

Classification / Lists

Miscellaneous Thymic Lesions and Conditions

• Atrophy
• Cysts
  • Most are non-specific
  • Neoplasms most commonly associated with cysts
    • Thymoma
    • Seminoma
    • Hodgkin lymphoma
    • Extranodal marginal zone lymphoma
    • Basaloid carcinoma
• Castleman disease
• Dysplasia
• Hyperplasia
  • B cell/lymphoid/germinal center
  • Rebound
  • True thymic
• Microscopic thymoma / nodular hyperplasia of thymic epithelium
• (Ectopic hamartomatous thymoma)

Bibliography

• Shimosato Y, Mukai K, Matsuno Y. Tumors of the Mediastinum, Atlas of Tumor Pathology, AFIP Fourth Series, Fascicle 11, 2010
• den Bakker MA, Oosterhuis JW. Tumours and tumour-like conditions of the thymus other than thymoma; a practical approach. Histopathology. 2009 Jan;54(1):69-89
• Moran CA, Suster S. Cystic well-differentiated neuroendocrine carcinoma (carcinoid tumor): a clinicopathologic and immunohistochemical study of two cases. Am J Clin Pathol. 2006 Sep;126(3):377-80
• Izumi H, Nobukawa B, Takahashi K, Kumasaka T, Miyamoto H, Yamazaki A, Sonobe S, Uekusa T, Suda K. Multilocular thymic cyst associated with follicular hyperplasia: clinicopathologic study of 4 resected cases. Hum Pathol. 2005 Jul;36(7):841-4
• Kuroki S, Nasu K, Murakami K, Hayashi T, Sekiguchi R, Nishida H, Miyagawa K, Kuroki Y, Nawano S. Thymic MALT lymphoma: MR imaging findings and their correlation with histopathological findings on four cases. Clin Imaging. 2004 Jul-Aug;28(4):274-7
• Suster S, Rosai J. Cystic thymomas. A clinicopathologic study of ten cases. Cancer. 1992 Jan 1;69(1):92-7
• Suster S, Barbuto D, Carlson G, Rosai J. Multilocular thymic cysts with pseudoepitheliomatous hyperplasia. Hum Pathol. 1991 May;22(5):455-60
• Suster S, Rosai J. Multilocular thymic cyst: an acquired reactive process. Study of 18 cases. Am J Surg Pathol. 1991 Apr;15(4):388-98.
• Salyer DC, Salyer WR, Eggleston JC. Benign developmental cysts of the mediastinum. Arch Pathol Lab Med. 1977 Mar;101(3):136-9