Histologic Features

Another area shows cells with a low N/C ratio. Rosette formation is indicated at the arrows. At least one other rosette is also present. Rosettes are formed by cells aligned around a lumen. The cell nuclei are located basally, away from the lumen. Note the detachment of this tumor nest from the adjacent stroma. This artifact is a typical finding.

These tumors usually react with antibodies to one or more of the following: cytokeratin, chromogranin A, Leu-7, and synaptophysin. The tumor in this particular case reacted with antibodies to cytokeratin and Leu-7, but not with antibodies to chromogranin A and synaptophysin.

What is the most likely diagnosis of this man's tumor?

Lymphoma	Thymoma	Thymic carcinoma
Mediastinal thyroid gland or tumor	Carcinoid tumor	Germ cell tumor

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Diagnosis: Carcinoid tumor

The features of carcinoid tumor shown here are:

• Sheets of cells supplied by thin-walled vessels, an endocrine pattern of tumor growth
• Round nuclei with finely-divided chromatin
• Rosette formation
• Low mitotic rate.
• Reactivity with cytokeratin (excludes lymphoma)
• Reactivity with Leu-7, a neuroendocrine marker

Histologic features of other diagnoses: Thymomas are composed of a mixture of benign lymphocytes (leukocyte common antigen (LCA) positive) and benign-appearing, neoplastic epithelial cells (keratin positive). Thymic carcinomas are composed of malignant-appearing epithelial cells. Lymphomas are composed of lymphoid cells that are positive for leukocyte common antigen. Thyroid tissue shows follicles, and thyroid cancer shows follicular or papillary structures. Germ cell tumors show reactivity with antibody to placental alkaline phosphatase.

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