Multiple Auer Rods in Fine-Needle Aspiration Smears of Medullary Thyroid Carcinoma: An Unusual Finding

BACKGROUND: Medullary thyroid carcinoma (MTC) is a rare tumor. Fine-needle aspiration (FNA) cytology is a popular method for the preoperative diagnosis of MTC. CASE REPORT: A 45-year-old man was referred to our center for a palpable thyroid nodule. Ultrasound-guided FNA was carried out for the patient. Cytology slides showed many isolated and small clusters of round to oval cells, some with a moderate amount of cytoplasm and red cytoplasmic granules. Rare cells showed multiple red Auer rods. Histology confirmed the diagnosis of MTC. CONCLUSION: Cytologic findings of MTC in FNA are variable, and several rare cytologic findings have been reported. Cytoplasmic red granules are one of the usual findings, whereas detection of cells containing multiple Auer rods is an unusual finding, which has not been reported so far.