Grading medullary thyroid carcinoma on fine-needle aspiration cytology specimens with the International Medullary Thyroid Carcinoma Grading System: A cytologic-histologic correlation.

Ki67FNA ≥5% shows low sensitivity but high specificity for predicting high-grade MTC. The presence of multiple mitotic figures in a single slide or definitive apoptotic bodies are both highly suggestive of high-grade MTC, and should warrant a close examination for necrosis and a careful Ki67 PR count.