Primary carcinomas of the thymus gland.

Carcinomas of the thymus are now well recognized as distinctive but rare entities, and several clinicopathologic variants of such neoplasms have been described. These include keratinizing squamous cell carcinoma, nonkeratinizing squamous cell carcinoma, lymphoepithelioma-like carcinoma, neuroendocrine carcinoma, adenosquamous carcinoma, mucoepidermoid carcinoma, clear-cell carcinoma, papillary adenocarcinoma, adenocarcinoma not otherwise specified, basaloid carcinoma, and sarcomatoid carcinoma. The application of electron microscopy, immunohistology, and other adjunctive pathological techniques is effective in refining the differential diagnosis between primary thymic carcinoma (PTC) and several other histological simulators. However, the distinction between PTC and carcinomas that involve the thymic region by metastasis from other sites is a difficult one, and ultimately must be predicated on detailed clinical and radiographic information. Well-differentiated squamous carcinoma, low-grade mucoepidermoid carcinoma, and basaloid carcinoma of the thymus usually are associated with a favorable prognosis, but other variants are aggressive and require multimodality treatment approaches.