Thymic squamous cell carcinoma shows features of squamous cell carcinoma as seen in other organs, with or without clear-cut evidence of keratinization in routinely stained sections.¹ In contrast to thymomas of the B categories, thymic carcinomas lack immature T lymphocytes. Thymic squamous cell carcinomas may be derived from thymic epithelial stem cells.¹ Some cases of thymic squamous cell carcinoma are thought to arise from preexisting thymomas based on the observation of combined thymic epithelial tumors that harbor squamous cell carcinoma and conventional (usually B3) thymoma components.^2,3,4 Well-differentiated squamous cell carcinoma may rarely occur in a thymic cyst.^5,6

Squamous cell carcinoma is the most frequent subtype of thymic carcinoma, and the frequency is higher in Asia (90%) than in the West (30%).¹ Most cases occur at middle age, and the male-to-female ratio varies from 1 to 2.3.¹ The most frequent symptom is chest pain. Other symptoms are cough, fatigue, fever, anorexia, weight loss, and superior vena cava syndrome. There have been no reports of myasthenia gravis or pure red cell aplasia, but paraneoplastic polymyositis or hypercalcemia can occur.⁷

Macroscopically, squamous cell carcinomas usually lack encapsulation or internal fibrous septation that is common in thymomas. They are firm to hard with frequent foci of necrosis and hemorrhage.¹

Histologically, squamous cell carcinoma is composed of large polyhedral cells with eosinophilic cytoplasm, hyperchromatic or vesicular nuclei, prominent nucleoli arranged in nests, and cords separated by
broad zones of fibrous stroma. The neoplasm shows clear-cut cytologic atypia and evidence of keratinization and/or intercellular bridges (Figs. 107.1 and 107.2).^1,8 Foci of spontaneous necrosis are frequently seen, as is the invasion of intratumoral blood vessels. Lymphocytes, when present, are mature and usually admixed with plasma cells. The tumor is sometimes predominantly cystic.⁹