Thymomas and Thymic carcinomas are also classified by the type of cells that make it up. Thymomas are placed into 5 categories created by the World Health Organization.
- Type A thymoma. This is also called spindle cell thymoma or medullary thymoma. The chance of recovery for people with type A thymoma is good. Nearly 100% of people with this type live at least 15 years after diagnosis.
- Type AB thymoma. Also called mixed thymoma, type AB thymoma is similar to type A thymoma. However, type AB thymoma has lymphocytes in the tumor. The chance of recovery for people with type AB thymoma is also good. About 90% of people with this type live at least 15 years after diagnosis.
- Type B1 thymoma. This is also known as lymphocyte-rich thymoma, lymphocytic thymoma, predominantly cortical thymoma, and organoid thymoma. This type of thymoma has many lymphocytes in the tumor, but the cells of the thymus appear healthy. The chance of recovery for people with type B1 thymoma is also good. About 90% of people with this type live at least 20 years after diagnosis.
- Type B2 thymoma. Type B2 thymoma is also known as cortical thymoma and polygonal cell thymoma. This type of thymoma also has many lymphocytes, like type B1 thymoma. However, the thymus cells do not appear healthy. About 60% of people with this type live at least 20 years after diagnosis.
- Type B3 thymoma. Type B3 thymoma is also known as epithelial thymoma, atypical thymoma, squamoid thymoma, and well-differentiated thymic carcinoma. This type of thymoma has few lymphocytes, and the thymus cells look abnormal. Approximately 40% of people with this type live at least 20 years after diagnosis.
Thymic carcinomas are not included in the five WHO classifications for Thymoma. Instead, thymic carcinomas are classified by the histological type of cells with which they are composed. The carcinomas are more aggressive in terms of recurrence and metastasizing than thymomas, and are more resistant to chemotherapy.
Some of the histological types are:
1. Keratinizing squamous cell carcinoma
2. Nonkeratinizing squamous cell carcinoma
3. Lymphoepithelioma-like carcinoma
4. Adenosquamous carcinoma
5. Mucoepidermoid carcinoma
6. Clear-cell carcinoma
7. Papillary adenocarcinoma
8. Adenocarcinoma not otherwise specified
9. Basaloid carcinoma
10. Sarcomatoid carcinoma
For Those Who Want to Learn More
You’ll find a little more scientific detail on some of the more common histological type here.
Squamous cell (epidermoid) thymic carcinoma
This type of thymic carcinoma exhibits clear-cut cytologic atypia. In routinely stained sections, the keratinizing form exhibits equally clear-cut evidence of squamous differentiation in the form of intercellular bridges and/or squamous pearls, while the nonkeratinizing form lacks obvious signs of keratinization. Another subtype, basaloid carcinoma, is composed of compact lobules of tumor cells that exhibit peripheral palisading and an overall basophilic staining pattern due to the high nucleocytoplasmic ratio and the absence of keratinization.
Lymphoepithelioma-like thymic carcinoma
This type of thymic carcinoma has morphologic features indistinguishable from those of lymphoepithelial carcinoma (lymphoelithelioma) of the nasopharyngeal.. The differential diagnosis with germ cell tumors, particularly seminomas, can be difficult but important for treatment. It is considered a type of poorly differentiated squamous cell carcinoma.
Sarcomatoid thymic carcinoma (carcinosarcoma)
This is a type of thymic carcinoma in which part or all of the tumor resembles 1 of the types of soft tissue sarcoma.
Clear cell thymic carcinoma
This is a type of thymic carcinoma composed predominantly or exclusively of cells with optically clear cytoplasm.
Mucoepidermoid thymic carcinoma
This type of thymic carcinoma has an appearance similar to that of mucoepidermoid carcinoma of the major and minor salivary glands.
Papillary thymic adenocarcinoma
This type of thymic carcinoma grows in a papillary fashion. This histology may be accompanied by psammoma body formation, resulting in a marked similarity with papillary carcinoma of the thyroid gland.
Undifferentiated thymic carcinoma
This is a rare type of thymic carcinoma that grows in a solid undifferentiated fashion but without exhibiting sarcomatoid (spindle cell or pleomorphic) features.