Type B2 Thymoma
Definition
- Thymoma with prominent large epithelial cells with an admixture of numerous lymphocytes
Alternate / Historical Names
- Cortical thymoma
- Mixed lymphocytic and epithelial thymoma
Diagnostic Criteria
- Prominent large epithelial cells with numerous admixed lymphocytes
- Lacks extensive areas of virtually pure epithelial cells
- Lacks extensive areas of virtually pure lymphocytes
- Prominent polygonal epithelial cell population
- Lack long thin processes
- Nuclei vesicular with prominent nucleoli
- May palisade
- Prominent lymphoid infiltrate
- Lymphocytes may outnumber epithelial cells but do not obscure them
- Perivascular spaces common
- Medullary foci and well formed Hassall corpuscles are absent to infrequent
Supplemental Studies
- Epithelial cells
- Keratin positive
- EMA negative
- CD20 negative
- CD5 negative
- Lymphocytes
- Immature T phenotype (CD1a, CD99, TdT)
- Few B cells
Clinical
- Associations
- Myasthenia gravis
- Pure red cell aplasia
- Hypogammaglobulinemia
- Usually presents at low stage (75% of cases)
- May be aggressive
- Widely variable long term survival depending on stage
Differential Diagnosis
| Type B1 Thymoma | Type B2 Thymoma |
|---|---|
| Predominantly lymphocytes with inconspicuous epithelial cells | Both lymphocytes and epithelial cell prominent |
| Numerous scattered pale medullary foci | Only occasional medullary foci |
| Type B2 Thymoma | Type B3 Thymoma |
|---|---|
| Both lymphocytes and epithelial cells are easy to see | Predominantly epithelial with very few lymphocytes |
| Lacks any epidermoid features | Frequently has epidermoid differentiation (but lacks overt keratinization and bridges) |
| Epithelial CD5 negative | Epithelial cells may be focally positive for CD5 |
| Type B2 Thymoma | Lymphoepithelial Carcinoma |
|---|---|
| Mild cytologic atypia at worst | Cytologically atypical |
| Epithelial cell mitotic figures rare | Epithelial cell mitotic figures may be frequent |
| Lymphocytes are immature T cells | Lymphocytes are mature T and B cells |
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.
- Nonaka D, Henley JD, Chiriboga L, Yee H. Diagnostic utility of thymic epithelial markers CD205 (DEC205) and Foxn1 in thymic epithelial neoplasms. Am J Surg Pathol. 2007 Jul;31(7):1038-44.
- Alexiev BA, Drachenberg CB, Burke AP. Thymomas: a cytological and immunohistochemical study, with emphasis on lymphoid and neuroendocrine markers. Diagn Pathol. 2007 May 11;2:13.
- Tateyama H, Eimoto T, Tada T, Hattori H, Murase T, Takino H. Immunoreactivity of a new CD5 antibody with normal epithelium and malignant tumors including thymic carcinoma. Am J Clin Pathol. 1999 Feb;111(2):235-40.
- Dorfman DM, Shahsafaei A, Chan JK. Thymic carcinomas, but not thymomas and carcinomas of other sites, show CD5 immunoreactivity. Am J Surg Pathol. 1997 Aug;21(8):936-40.
- Okumura M, Ohta M, Tateyama H, Nakagawa K, Matsumura A, Maeda H, Tada H, Eimoto T, Matsuda H, Masaoka A. The World Health Organization histologic classification system reflects the oncologic behavior of thymoma: a clinical study of 273 patients. Cancer. 2002 Feb 1;94(3):624-32.
