Answer: Thymoma, minimally invasive

Histology: The tumor is composed of cytologically benign thymic epithelial cells interspersed with lymphocytes. The tumor has a variety of morphologic appearances. The majority of the tumor cells are spindled. Focal areas of pseudorosette formation are noted, as are perivascular spaces which are characteristic of thymoma. Tumor focally invades its thick fibrous capsule, but appears to be completed resected. Hence, this tumor can be classified as a minimally invasive thymoma. Such tumors have an excellent prognosis, similar to that of non-invasive thymoma.

Discussion: The presence of rosettes raises the possibility of a Ewing sarcoma/PNET of the chest wall/anterior mediastinum (Askin tumor). However, such tumors are typically mitotically active and composed of monotonous, primitive small round blue cells. These tumor cells label for the Ewing sarcoma antigen MIC2 (CD99). In contrast, in a thymoma, the immature non-neoplasticT lymphocytes that are recruited to the tumor by the neoplastic thymic epithelial cells stain for MIC2. Solitary fibrous tumors are composed of spindled cells with a cytologic appearance similar to that of the spindled cells of thymoma. However, solitary fibrous tumors stain for CD34 and not cytokeratin as thymomas do, and solitary fibrous tumors are not associated with thymic lymphocytes as thymomas are. Thymic carcinomas (previously referred to as malignant thymoma type 2) are fully malignant neoplasms composed of cytologically malignant thymic epithelial cells. Thymic carcinomas have a morphologic appearance similar to those of primary pulmonary carcinomas. Squamous carcinoma, small cell neuroendocrine carcinoma, adenocarcinoma, and large cell carcinoma are all recognized to arise in the thymus as well as the lung. Unlike thymomas or pulmonary carcinomas, thymic carcinomas are often immunoreactive for CD5