Surgical Pathology Criteria–

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Definition

• Thymoma composed of alternating areas of epithelial cells and slender spindle cells

Alternate / Historical Names

• Biphasic thymoma, mixed polygonal and spindle cell type
• Low grade metaplastic carcinoma (at least some cases)
• Thymoma with pseudosarcomatous stroma

Diagnostic Criteria

• Alternating epithelial and finely spindled areas
  • Either component may predominate
  • Both are required for diagnosis
• Epithelial cells form islands and trabeculae/cords
  • Oval to polygonal cells
  • Nuclei usually vesicular
    • Usually small nucleoli
    • May be strikingly enlarged, hyperchromatic and pleomorphic
    • Intranuclear inclusions may be prominent
  • Mitotic figures rare
  • Keratin positive
  • May be squamoid but lack intercellular bridges or pearls
  • No perivascular spaces
• Spindled component forms intervening bands
  • Usually fine bipolar processes
    • Fascicular or storiform
  • Cytologically usually bland, no mitotic figures
    • Some cases may have large pleomorphic nuclei in the spindle component similar to those in epithelial areas
  • Keratin staining may be negative, focal and weak, to strong
  • May be focally actin positive
• Few lymphocytes
  • No immature T cells according to WHO
    • Two reports describe some immature cells
• Well circumscribed and variably encapsulated
  • Lacks thick fibrous septa
  • Invasion of adjacent tissues has been reported
• Two cases reported of sarcomatoid carcinoma arising in metaplastic thymoma
  • Defined by cytologic atypia and high mitotic rate
  • Followup too short to assess behavior

Supplemental Studies

	Keratin	E Cadherin	Vimentin	CD20	CD5
Epithelioid Cells	pos	pos	neg	neg	neg
Spindled Cells	weak	neg	pos	neg	neg*

*One report of weak CD5 staining

• Lymphocytes (few present)
  • Mature T phenotype
  • One report describes some immature T cells
  • Few B cells

Clinical

• Not associated with myasthenia gravis or other autoimmune disorders
• Only one out of about 22 reported cases in series died due to disease

Differential Diagnosis

Metaplastic Thymoma	Thymic Carcinoma with Sarcomatoid Differentiation
Cytologically bland	Usually cytologically atypical
Mitotic figures rare	Mitotic figures may be numerous

Metaplastic thymoma may have scattered bizarre nuclei but the general epithelial population is bland

Bibliography

• Shimosato Y, Mukai K, Matsuno Y. Tumors of the Mediastinum, Atlas of Tumor Pathology, AFIP Fourth Series, Fascicle 11, 2010
• Travis WD, Brambilla E, Burke AP, Marx A, Nicholson AG eds. World Health Organization Classification of Tumors. Pathology and genetics of tumors of the lung, pleura, thymus and heart. IARC Press: Lyon 2015.
• Suster S, Moran CA, Chan JK. Thymoma with pseudosarcomatous stroma: report of an unusual histologic variant of thymic epithelial neoplasm that may simulate carcinosarcoma. Am J Surg Pathol. 1997 Nov;21(11):1316-23.
• Yoneda S, Marx A, Heimann S, Shirakusa T, Kikuchi M, Müller-Hermelink HK. Low-grade metaplastic carcinoma of the thymus. Histopathology. 1999 Jul;35(1):19-30.
• Noh TW, Kim SH, Lim BJ, Yang WI, Chung KY. Thymoma with pseudosarcomatous stroma. Yonsei Med J. 2001 Oct;42(5):571-5.
• Lu HS, Gan MF, Zhou T, Wang SZ. Pathological Features of Sarcomatoid Carcinoma Arising in Metaplastic Thymoma: A Case Report. Int J Surg Pathol. 2011 Oct;19(5):677-80.
  
• Moritani S, Ichihara S, Mukai K, Seki Y, Inoue S, Yasuda A, Hakiri S, Yatabe Y, Eimoto T. Sarcomatoid carcinoma of the thymus arising in metaplastic thymoma. Histopathology. 2008 Feb;52(3):409-11.
• Liu B, Rao Q, Zhu Y, Yu B, Zhu HY, Zhou XJ. Metaplastic thymoma of the mediastinum. A clinicopathologic, immunohistochemical, and genetic analysis. Am J Clin Pathol. 2012 Feb;137(2):261-9.
• Kang G, Yoon N, Han J, Kim YE, Kim TS, Kim K. Metaplastic thymoma: report of 4 cases. Korean J Pathol. 2012 Feb;46(1):92-5. doi:
  10.4132/KoreanJPathol.2012.46.1.92.

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