Differential diagnostic considerations and potential pitfalls in fine-needle aspiration biopsies of the mediastinum.

The mediastinum is the host for a number of relatively unusual primary neoplasms, as well as a frequent recipient of metastatic tumors. From the perspective of fine-needle aspiration cytomorphology, several distinct patterns are yielded. The polygonal (or epithelial-like) cell pattern may be seen with benign and malignant thymomas, germinomas, embryonal carcinoma, and many metastatic carcinomas. An intimate admixture of small lymphocytes with these epithelial cells may occur in this category. The small cell pattern may be produced by malignant non-Hodgkin's lymphomas, neuroblastoma, carcinoid tumors, and metastatic oat cell carcinoma. Uncommon morphologic forms of thymoma and carcinoid tumors, as well as benign mesenchymal lesions, may yield a picture of a spindle-cell proliferation. In addition to cytomorphology, the cytologist needs to integrate clinical, radiographic, immunocytochemical, and ultrastructural data to formulate a final diagnosis.