Cellular Classification of Thymoma and Thymic Carcinomas
Thymic carcinoma (also known as type C thymoma) is a thymic epithelial tumor that exhibits a definite cytologic atypia and a set of histologic features no longer specific to the thymus but rather similar to those histologic features observed in carcinomas of other organs. In contrast to type A and B thymomas, thymic carcinomas lack immature lymphocytes. Any lymphocytes that are present are mature and usually admixed with plasma cells. Hypothetically, thymic carcinoma may arise from malignant transformation of a pre-existing thymoma. This hypothetical evolution could account for the existence of thymic epithelial lesions that exhibit combined features of thymoma and thymic carcinoma within the same tumor.
Thymic carcinomas are usually advanced when diagnosed and have a higher recurrence rate and worse survival compared with thymoma.[15,16] In a retrospective study of 40 patients with thymic carcinoma, the 5-year and 10-year actuarial overall survival rates were 38% and 28%, respectively. In contrast to the thymomas, the association of thymic carcinoma and autoimmune disease is rare.
Histologic subtypes of thymic carcinoma include the following:
- 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 caused by 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 of the respiratory tract. The differential diagnosis with germ cell tumors, particularly seminomas, can be difficult but important for treatment.
- Sarcomatoid thymic carcinoma (carcinosarcoma)
This is a type of thymic carcinoma in which part or all of the tumor resembles one 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, which may result 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.
Combinations of the above histologic types can occur within the same tumor. For these cases, the term combined thymoma can be used, followed by a listing of the components and the relative amount of each component.
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