Metaplastic Thymoma
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
- Occasionally hyperchromatic
- Occasionally with prominent nucleoli
- Mitotic figures rare
- Keratin positive
- May be squamoid but lack intercellular bridges or pearls
- Appearance of epithelial component may be similar to B3 thymoma except for lack of perivascular spaces
- Spindled component forms intervening bands
- Usually fine bipolar processes
- Fascicular or storiform
- Cytologically bland, no mitotic figures
- Keratin staining negative to focal and weak
- May be focally actin positive
- Usually fine bipolar processes
- Few lymphocytes
- No immature T cells according to WHO
- Two reports describe some immature cells
- No immature T cells according to WHO
- Well circumscribed and grossly encapsulated
- Lacks thick fibrous septa
- Invasion of adjacent tissues has been reported
Supplemental Studies
- Epithelial cells
- Keratin positive
- May be weak in spindled component
- CD20 negative
- CD5 negative
- One report describes focal CD5 staining
- Keratin positive
- 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
- Few cases reported, 1 of 11 died due to disease
Differential Diagnosis
| Metaplastic Thymoma | Carcinoma with Sarcomatoid Differentiation |
|---|---|
| Cytologically bland | Usually cytologically atypical |
| Mitotic figures rare | Mitotic figures may be numerous |
Bibliography
- Travis WD, Brambilla E, Muller-Hermelink HK, Harris CC eds. World Health Organization Classification of Tumors. Pathology and genetics of tumors of the lung, pleura, thymus and heart. IARC Press: Lyon 2004.
- 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.
