Neuroendocrine differentiation is a common feature of thymic carcinoma.

Immunohistochemical evidence of neuroendocrine differentiation in the form of reactivity for synaptophysin, neuron-specific enolase, and/or chromogranin was found in 11 of 19 (58%) thymic carcinomas having the typical morphologic features of that tumor type. Four of these 19 cases were studied ultrastructurally, and neuroendocrine-type cytoplasmic dense-core granules were found in two. In contrast, 84 thymomas were negative for these markers, except for a focal immunoreactivity for neuron-specific enolase in areas of medullary differentiation in half of the lymphocyte-rich tumors. The results of this study show that in the thymus, similar to most other organs, neuroendocrine differentiation is not limited to tumors with an identifiable neuroendocrine appearance in hematoxylin-eosin-stained slides, such as carcinoid tumor and small cell carcinoma, but rather that it represents a common event shared by the major types of malignant epithelial tumors of that organ.