Clinical history: 41 year old male with a mediastinal mass.
What is the diagnosis?
Well-differentiated thymic squamous cell carcinoma arising from a type B2 thymoma
At least one third of all mediastinal masses are asymptomatic and discovered incidentally on radiographic examination. When these tumors become symptomatic, it is usually the result of invasion or compression of adjacent structures. Thymic carcinoma is defined as a poorly differentiated thymic epithelial neoplasm exhibiting obvious cytologic evidence of malignancy and total loss of the organotypical features of thymic differentiation. These tumors are usually not associated with myasthenia gravis or other autoimmune disorders.Thymic carcinoma may present as a de novo neoplasm, or it may develop from malignant transformation of a preexisting thymoma like in this case.
Histologic Types of Thymic Carcinoma
The distinction between primary and metastatic squamous cell carcinoma of the thymus cannot be made on histologic grounds alone and requires the demonstration of the absence of a primary tumor elsewhere by clinical and radiographic means.
Poorly differentiated (lymphoepithelioma-like) squamous cell carcinoma is nonkeratinizing squamous cell carcinoma and is the most common type of thymic carcinoma encountered in Western patients. The tumor often show prominent central comedo-like areas of necrosis.surrounded by a dense lymphoplasmacellular infiltrate. Mitotic activity is generally high (>10 mitoses/10 high-power fields). Another interesting association is with Epstein-Barr virus (EBV which is more stronger in Asian than in Western patients
Lymphoid-associated markers such as CD5 and CD70 are preferentially expressed in thymic carcinoma. A variety of mesothelial-associated antigens, including calretinin, mesothelin, HBME-1, and CK5/6, also react with thymic carcinoma.
The treatment of thymic carcinoma remains empirical. Tumors of low-grade histology are best treated by complete surgical excision followed by radiation therapy. High-grade tumors can be treated palliatively with combination chemotherapy and radiation therapy. Cisplatinum-based regimens have been used with apparent success in some cases of high-grade carcinoma