Type B3 Thymoma
Definition
- Thymoma composed of sheets of epithelial cells with mild to moderate atypia and scant lymphocytes
Alternate / Historical Names
- Atypical thymoma
- Epithelial thymoma
- Squamoid thymoma
- Well differentiated thymic carcinoma
Diagnostic Criteria
- Sheets of epithelial cells
- Mild to moderate atypia at most
- Round to oval irregular nuclei
- Frequently smaller than in B2
- Nucleoli variable, frequently smaller than in B2
- Round to oval irregular nuclei
- Mitotic figures usually <2/hpf
- Occasionally focally 4-10/10 hpf
- Usually pale to clear cytoplasm
- Prominent palisading around perivascular spaces and along septa
- May suggest epidermoid/squamoid appearance
- Lacks intercellular bridges in general
- At most focally keratinized
- Frequently has distinct cell borders with squared-off appearance
- Mild to moderate atypia at most
- Very few lymphocytes
- Most are immature T cells
- Lobular infiltration and growth pattern
- Prominent fibrous capsule and septa
- Perivascular spaces usually prominent
- Sharp borders
- Contain scattered lymphocytes
- Central vessel usually visible
- Scant reticulin between epithelial cells
- Infrequently may have spindle cell areas
- Tumor cell necrosis focal and rare
- Rare cases of true thymic carcinoma may arise in or adjacent to B3 thymoma
Supplemental Studies
- Epithelial cells
- Keratin positive
- EMA negative
- CD20 negative
- CD5 negative to focal (up to 50% of cases)
- CD117 <10%
- Lymphocytes (few present)
- Predominantly immature T phenotype (CD1a, CD99, TdT)
- T cells express most pan-T cell markers (CD3, CD5)
- Few B cells
Clinical
- Associations
- Myasthenia gravis
- Pure red cell aplasia
- Hypogammaglobulinemia
- Frequently invasive and aggressive
- Metastases may occur
- Usually restricted to pleura
- 40-70% long term survival
Differential Diagnosis
| Type A Thymoma | Type B3 Thymoma |
|---|---|
| Predominantly spindle cells | Predominantly polygonal cells |
| Lacks any epidermoid features | Frequently has epidermoid differentiation (but lacks overt keratinization and bridges) |
| Frequent glandular structures lined by cuboidal to flat cells | Frequent perivascular spaces surrounded by palisaded epithelial cells |
| Lacks cytologic atypia | Mild to moderate cytologic atypia |
| Epithelial cells may be CD20 positive | Epithelial cells CD20 negative |
| Epithelial cells EMA and CD5 negative | Epithelial cells may be focally positive for EMA or CD5 |
| Prominent reticulin within epithelial areas | Lacks prominent reticulin around epithelial cells |
| Type B2 Thymoma | Type B3 Thymoma |
|---|---|
| Both lymphocytes and epithelial cells are easy to see | Predominantly epithelial with 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 |
| Epithelial cell nuclei are large round vesicular with prominent nucleoli | Epithelial cell nuclei moderate in size, not typically large and vesicular |
| Type B3 Thymoma | Thymic Low Grade Squamous Carcinoma |
|---|---|
| Cytologic atypia usually mild, moderate at worst | Cytologically atypical |
| Epithelial cell mitotic figures usually <2/10hpf | Epithelial cell mitotic figures may be frequent |
| Lymphocytes few, but with immature T phenotype | Lymphocytes are mature T and B cells |
| Overt squamous differentiation usually scant and focal | Overtly squamous, with intercellular bridges usually identifiable |
| Lobular growth pattern | Infiltrative growth pattern |
| Frequent perivascular spaces surrounded by palisaded epithelial cells | Lacks perivascular spaces and palisading |
| Epithelial cells CD117 negative to rare | Epithelial cells CD117 65-80% positive |
| Epithelial cells CD5 negative to at most focally positive | Epithelial cells CD5 70% positive |
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
- 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.
- Kirchner T, Schalke B, Buchwald J, Ritter M, Marx A, Müller-Hermelink HK. Well-differentiated thymic carcinoma. An organotypical low-grade carcinoma with relationship to cortical thymoma. Am J Surg Pathol. 1992 Dec;16(12):1153-69.
- 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.
