Cytologic diagnosis of medullary carcinoma of the thyroid gland

The cytomorphologic features in fine-needle aspiration (FNA) biopsies from 91 histologically verified medullary carcinomas of the thyroid (MCT) were i nvestigated. FNA was able to diagnose neoplasms with indications of surgica l removal in 98.9% of cases and moreover, was accurate in specific tumor ty ping 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 diagno sed as MCT by cytology are illustrated to discuss the differential diagnosi s of MCT and its potential pitfalls. If MCT is cytologically presumed but a myloid and azurophilic cytoplasmic granules are not demonstrated, the use o f immunostaining is necessary for a correct tumor typing. The application o f immunocytochemistry in MCT is discussed. Diagn. Cytopathol. 2000;22:351-3 58. (C) 2000 Wiley-Liss, Inc.