Fine needle aspiration of medullary carcinoma of the thyroid. Cytomorphology, immunocytochemistry and electron microscopy.

OBJECTIVE: To analyze of medullary carcinoma of the thyroid (MCT) diagnosed by fine needle aspiration (FNA) utilizing cytomorphologic features and ancillary studies. STUDY DESIGN: Nine cases of MCT were collected, and the cytomorphologic findings were reviewed. Additionally, immunocytochemistry, immunoelectron microscopy and ultrastructural examination results were reviewed for selected cases. RESULTS: In five cases, loose groups predominated over single cells, whereas single cells predominated in three cases. One case showed only highly cohesive groups of cells. Most cells were round to oval, and every case had some degree of plasmacytoid morphology. Spindle-shaped cells were predominant in one case and were occasionally noted as a subpopulation in the other cases. Binucleation was noted in seven cases, and scattered, abnormally large nuclei were identified in five cases. The cytoplasm was moderate to abundant and delicate in all cases. Routine immunocytochemical staining for calcitonin and chromogranin was positive in three of four cases, and staining positive for the markers was detected by immunoelectron microscopy in two cases. In four cases, electron microscopy revealed neurosecretory granules. CONCLUSION: The cytomorphologic appearance of medullary thyroid carcinoma is highly distinctive, and the diagnosis can be corroborated by appropriate ancillary studies.