Thymomas and thymic carcinomas.

This review discusses epithelial proliferations of the thymus, excluding carcinoid tumor and small cell carcinoma. The clinical features of thymoma, possible etiologic mechanisms, and associated autoimmune and paraneoplastic conditions are summarized briefly. Histologic subtypes of thymoma, including lymphocyte predominant, mixed, and spindle cell tumors are described and illustrated. The concept of "medullary" and "cortical" differentiation in thymoma is reviewed. Immunohistochemical and electron microscopic features of this neoplasm are presented. Flow cytometric studies relating to its prognosis also are summarized. Distinctions between encapsulated thymoma, invasive thymoma, and metastatic or "malignant" thymoma are described in detail. Thymic carcinomas are reviewed, as distinguished from cytologically bland thymomas. Variants of thymic carcinoma include squamous cell, spindle cell, lymphoepithelioma-like, mucoepidermoid, adenosquamous, clear cell, basaloid, and adenoid cystic neoplasms.