Answer: Invasive thymoma

Histology: The tumor is composed of plump thymic epithelial cells. Rare lymphocytes are seen within the lesion. There are focal areas of pseudorosette formation. Occasional perivascular spaces characteristic of thymoma are also identified. The tumor invades through the thick capsule. In other areas tumor was seen within adipose tissue and lung parenchyma (not shown). The tumor is thus classified as an invasive thymoma.

Discussion: Germ cell tumors can occur in the mediastinum, however, the cells in this case are not as pleomorphic and cytologically atypical as those seen in embryonal carcinoma, and do not show the round nuclei with clear cytoplasm of the cells seen in a seminoma. The presence of pseudorosettes raises the possibility of a Ewing’s sarcoma/PNET of the mediastinum, however, such tumors are mitotically active and composed of primitive small round blue cells. Thymic carcinomas are fully malignant neoplasms and consist of cytologically malignant thymic epithelial cells. Thymic carcinomas may have various morphologic appearances, including squamous carcinoma, lymphoepithelioma-like carcinoma, carcinosarcoma, clear-cell carcinoma, basaloid carcinoma, mucoepidermoid carcinoma, and undifferentiated carcinoma, which can be indistinguishable from those of primary lung cancers. Immunohistochemical stains for CD5 may be helpful to differentiate thymic carcinomas from thymomas and pulmonary carcinomas, since thymic carcinomas are often immunoreactive for CD5.

The classification of thymic epithelial tumors can be rather confusing. The 1978 classification system separated thymomas into encapsulated, malignant thymoma type 1, and malignant thymoma type 2. The type 1 malignant thymoma refers to an invasive thymoma, whereas the type 2 malignant thymoma refers to a thymic carcinoma. In the 1999 WHO classification system there are 6 categories. Type A are tumors consisting of bland spindle cells with a few lymphocytes. Type AB thymomas consist of two components, one component resembling the Type A thymoma, and the other with plump cells and a predominant lymphocytic infiltrate. The Type B1 thymoma corresponds to any tumor composed of epithelial cells and an abundant lymphocytic infiltrate. The Type B2 thymoma is a predominantly lymphocytic thymoma with scattered plump thymic epithelial cells. The Type B3 thymoma, which the current case falls under, is composed of polygonal or round thymic epithelial cells with atypia. Some of these thymomas have, in the past, been referred to as well-differentiated thymic carcinoma. Finally, Type C thymoma refers to fully malignant thymic carcinomas.