Background: Signet-ring follicular adenoma is a rare variant of follicular neoplasm, which has only been described using the conventional smear cytologic preparation. Here we report the unique cytologic findings of two cases of signet-ring follicular adenoma using liquid-based samples and corresponding histologic features and results of ancillary tests. Cases: Case 1: A 65-year-old man presented with a solitary nodule in the right lobe of the thyroid. Ultrasound-guided fine needle aspiration (FNA) yielded several groups of microfollicles containing colloid or mucin-like globules. The tumor cells had eccentrically located nuclei compressed by distended luminal globules. Case 2: A 51-year-old woman presented with a nodule in the left lobe of the thyroid. On liquid-based cytology, cord-like arrangements of microfollicules were noted. The tumor cells had large, clear vacuoles, but the nuclei maintained their round shape and central location. Conclusion: The liquid-based cytologic characteristics are quite different from those encountered in the conventional smear of FNA; therefore, pathologists must also be familiar with the cytomorphologic characteristics of liquid-based preparations. In addition, given the distinctive cytological and histological appearance and the benign clinical course, signet-ring follicular adenoma should be distinguished from other metastatic neoplasms displaying signet-ring cell features.

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