Fine-needle aspiration biopsy of mucus-producing medullary carcinoma of thyroid: report of a case with cytologic, histologic, and ultrastructural correlations.

Cytologic features of aspiration smears from a mucus-producing medullary carcinoma of the thyroid are described. Two patterns of mucin production by the tumor cells were noted. In some cells, mucin was present as numerous small cytoplasmic vacuoles with ultrastructural morphology characteristic of mucin granules. Other cells had larger, more well-defined cytoplasmic mucin vacuoles; on ultrastructural examination, they were found to be intracytoplasmic lumina. Although mucin production has been demonstrated in several histologic studies of medullary carcinoma, it has not been previously recognized in fine-needle aspiration biopsies from this neoplasm.