Cytological Features of High‐Grade Medullary Thyroid Carcinoma: A Comparative Study With Low‐Grade Cases

OBJECTIVE: The cytological characteristics of high-grade medullary thyroid carcinoma (HG-MTC) remain insufficiently defined. This study aimed to elucidate these features and assess their potential in estimating HG-MTC. METHODS: Cytological and histological specimens from 12 patients with HG-MTC and 36 patients with low-grade (LG) MTC were analyzed. RESULTS: Amyloid was observed in 50.0% of LG-MTC nodules but in only 8.3% of HG-MTC nodules (p < 0.05). Necrotic materials were observed in only one HG-MTC nodule. The frequencies of intracytoplasmic mucin (25.0%), pseudoinclusions (66.7%), mitotic figures (58.3%), and cannibalism (25.0%) in HG-MTC nodules were significantly higher than those in LG-MTC nodules (p < 0.05). Nuclear grooves, fine chromatin pattern, and irregularly shaped nuclei were observed in 41.7%, 41.7%, and 25.0% of HG-MTC nodules, respectively. None of them were observed in LG-MTC nodules (p < 0.05). When at least one of the six cytological features including mitotic figures, nuclear grooves, fine chromatin pattern, irregularly shaped nuclei, intracytoplasmic mucin, and cannibalism was present, the sensitivity and specificity for HG-MTC diagnosis were 91.7% and 83.3%, respectively. CONCLUSIONS: Mitotic figures, nuclear grooves, fine chromatin pattern, irregularly shaped nuclei, intracellular mucin, and cannibalism are cytological findings suggestive of HG-MTC. Except for pseudoinclusions, PTC-like nuclear features can be considered important diagnostic clues for HG-MTC.