A 66-year-old female with multiple bilateral thyroid nodules underwent FNA biopsy. The cytology smears show a highly cellular specimen composed of numerous singly scattered and clusters of spindle cells exhibiting mild to moderate pleomorphism. Tumor cells have elongated nuclei with vesicular chromatin, conspicuous to prominent nucleoli, and scant to moderate cytoplasm. Mitotic figures and binucleated cells are present. No colloid or necrosis is present.

Given these morphological features, medullary thyroid carcinoma was considered as the first differential diagnosis, followed by anaplastic thyroid carcinoma. Upon performing an immunohistochemical work-up to differentiate these two entities, it was observed that the tumor cells were positive for S100, SOX10 and HMB45, and negative for CAM5.2, AE1/AE3, p40, Desmin, CD34, TTF-1, Calcitonin, Thyroglobulin, and SMA. This immunoprofile was consistent with metastatic melanoma to the thyroid gland. Concurrent core biopsies showed sheets of malignant pleomorphic spindle cells with numerous mitotic figures.

Thyroid is an unusual site for metastasis. Among all malignancies, renal cell carcinoma is the most common tumor that metastasizes to the thyroid (1,2). Melanomas metastasizing to the thyroid, although exceedingly rare, have been reported and account for around 10% of cases (2). While melanoma can display varied histological appearances, there are a few cytologic findings that are quite characteristic. Smears are usually very cellular and composed of isolated cells. The cells can have an epithelioid, spindled, or giant cell morphology. These morphologic features can be present individually or in combination with each other. The cytoplasm is usually granular and may contain melanin pigment. Binucleation, resulting in cells with double mirror image, giving the appearance of bug-eyed demons, is often observed. Prominent nucleoli and intranuclear pseudo-inclusions are also present. (3) Immunohistochemically, these cells show positivity for S100, HMB45, and Melan-A.