Cytopathologic features of medullary carcinoma of the thyroid.

Medullary carcinoma of the thyroid (MCT) presents a varied but characteristic cytologic pattern in cellular samples obtained by fine-needle aspiration (FNA) biopsy. In our experience with 17 cases of MCT, ten were identified by cytologic examination. One was acellular. The remaining six were typed as undifferentiated carcinoma (three cases), follicular variant of papillary carcinoma (one case), and cellular adenoma (two cases). Four Hürthle cell tumors and four papillary carcinomas were incorrectly typed as MCT. There was one false positive. Careful consideration of the listed cytologic features should make the FNA biopsy diagnosis of MCT of satisfactory sensitivity for the evaluation of "cold" thyroid nodules. Specificity may be considerably improved by using immunoperoxidase stain for calcitonin granules and/or by large-needle biopsy.