Carcinoid tumors of the thymus. An immunohistochemical study.
Five carcinoid tumors of the thymus were screened immunohistochemically for the occurrence of neuropeptides (ACTH, calcitonin, calcitonin gene-related peptide, cholecystokinin, gastrin, neurotensin, somatostatin, substance P), as well as of serotonin, chromogranin A, and neuron-specific enolase. Most of the patients exhibited local symptoms evoked by growing tumor masses in the upper mediastinum without any clinical evidence of endocrine activity. Light and electron microscopic examination showed characteristic uniform large epithelial cells in polar or palisade arrangement, containing variable amounts of electron-dense secretory granules. Only a few of the tested neuropeptide antisera reacted with the investigated tumors. Cholecystokinin-immunoreactive cell populations were seen in all tumors. Expression of neurotensin could be observed in three neoplasms, two of which also exhibited ACTH immunoreactivity. Chromogranin A-immunoreactive cells were found in two neoplasms. Neuron-specific enolase showed strong staining in three tumors, one of the tumors also being immunoreactive for calcitonin. The results were confirmed by control reactions. Apart from the demonstration that conventional marker proteins are not reliable in identifying all carcinoid tumors, the present study proves that the visualization of neuropeptide-immunoreactive cells in thymus carcinoids does not necessarily correspond to the manifestation of the clinical symptoms. Furthermore, each of the investigated neoplasms, as also known from other carcinoid tumors, appears to be able to produce more than one hormone.