Large cell neuroendocrine thymic carcinoma coexisting within large WHO type AB thymoma.

Large cell neuroendocrine carcinoma (LCNEC) is a rare type of thymic epithelial tumor. It is recognized as a different entity from other thymic tumors on account of it having a more aggressive biologic behavior and poor prognosis. We report an extremely rare case of a very small, "large cell neuroendocrine thymic carcinoma" coexisting within a large thymoma that could not be detected by usual biopsy. Surgery as the initial treatment has the significance of definitive diagnosis and curative treatment for LCNEC of the thymus. To make a successful differential diagnosis, application of detailed immunohistochemical stains may be of aid, since thymic epithelial tumor is not always morphologically homogenous.