Thymic Carcinoma
Definition
- Epithelial neoplasm of the thymus exhibiting clear malignant cytologic or architectural features
Alternate/Historical Names
- Type C thymoma
- Malignant thymoma (an imprecise term also used for usual thymomas exhibiting aggressive behavior)
Note
- Thymic neuroendocrine neoplasms are covered separately
- So called "well differentiated thymic carcinoma" is covered as thymoma type B3
Diagnostic Criteria
- Requires either cytologic or invasive features of carcinoma
- Unequivocal cytologic atypia is usually present
- Mitotic figures may be frequent and atypical
- Aggressive, destructive invasion with stromal response is usually present
- Unequivocal cytologic atypia is usually present
- Lacks dense capsule and fibrous septa typical of thymomas
- May be seen in basaloid type carcinomas
- May be lacking in micronodular thymoma with lymphoid stroma
- Lacks infiltrate of immature T cells typical of most thymomas
- May contain variable numbers of mature T cells and some B cells
- Any type of thymic carcinoma may arise in or form cysts
- Thymic carcinoma may rarely arise in the setting of a conventional thymoma
- Metastatic carcinoma must be ruled out
- A variety of patterns have been described
- Low grade carcinomas (grading is approximate due to rarity of most of these types)
- Well differentiated, keratininzing squamous cell carcinoma
- Most common type reported in Japan, but also relatively common in USA
- Basaloid carcinoma
- Rare
- Papillary/cystic and solid nest patterns
- Frequently arises in multilocular cysts
- May have comedonecrosis
- May form a capsule
- Papillary adenocarcinoma
- Rare
- Adenosquamous carcinoma
- Mixture of gland formations (may be scant) and squamous carcinoma
- Low grade mucoepidermoid carcinoma
- Rare
- Mixture of glandular, squamous and intermediate cells
- May be considered a subset of adenosquamous
- High stage low grade mucoepidermoid carcinoma behaves as high grade carcinoma (Minoka)
- Micronodular with lymphoid stroma
- Rare
- Undifferentiated large cell carcinoma associated with Castleman disease-like reaction
- Well differentiated, keratininzing squamous cell carcinoma
- High grade carcinomas
(grading is approximate due to rarity of most of these types)
- Lymphoepithelial-like
- Most common type reported in USA
- In spite of frequent association with EBV and histologic similarity to nasopharyngeal carcinoma, it does not share the good response to radiation therapy
- Large cell undifferentiated
- Rare
- Clear cell
- Rare
- Mucinous or signet ring adenocarcinoma
- Rare
- Sarcomatoid
- Rare
- May have heterologous elements
- NUT midline carcinoma with t(15;19)
- Rare, aggresive neoplasm of young patients
- 5-34 years old
- Variety of histologic patterns
- Poorly to well differentiated squamous carcinoma
- Usually exhibits foci of abrupt keratinization
- Rare, aggresive neoplasm of young patients
- Lymphoepithelial-like
- Low grade carcinomas (grading is approximate due to rarity of most of these types)
- Associated autoimmune disease is quite unusual in thymic carcinomas
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
