Cytologic diagnosis of medullary carcinoma of the thyroid gland.

The cytomorphologic features in fine-needle aspiration (FNA) biopsies from 91 histologiacally verified medullary carcinomas of the thyroid (MCT) were investigated. FNA was able to diagnose neoplasms with indications of surgical removal in 98.9% of cases and moreover, was accurate in specific tumor typing in 89% of cases. The most important cytologic criteria of MCT with FNA are: dispersed cell-pattern of polygonal or triangular cells, azurophilic cytoplasmic granules, and extremely eccentrically placed nuclei with coarse granular chromatin and amyloid. These and other cytologic features of MCT are discussed in detail. Fourteen cases of thyroid tumors originally diagnosed as MCT by cytology are illustrated to discuss the differential diagnosis of MCT and its potential pitfalls. If MCT is cytologically presumed but amyloid and azurophilic cytoplasmic granules are not demonstrated, the use of immunostaining is necessary for a correct tumor typing. The application of immunocytochemistry in MCT is discussed.