Merkel cell carcinoma metastatic to the thyroid gland: Aspiration findings and differential diagnosis

Clinically diagnosed metastasis to the thyroid gland is exceptionally rare and may present diagnostic issues on fine needle aspiration. The most common primary sites of metastases to the thyroid are cancers of the lung, breast, skin (especially melanoma), colon, and kidney. Herein, we report a case of metastatic Merkel cell carcinoma to the thyroid presenting as a 2.1-cm solid nodule in a 50-year-old male with a previous history of Merkel cell carcinoma of the upper extremity. The aspirates were moderately to highly cellular featuring small to intermediate sized cells with scant to no cytoplasm, round-to-oval nuclei with finely dispersed chromatin, and predominantly arranged as scattered single cells. There was focal nuclear molding, numerous mitoses, and karyorrhectic nuclei. The differential diagnosis centered on the "small round blue cell" tumor group such as medullary thyroid carcinoma and non-Hodgkin lymphoma. However, in light of our patient's previous history, the FNA findings were most consistent with a metastasis of Merkel cell carcinoma. In patients with a known history of a primary neoplasm, the differential diagnosis of a thyroid nodule should always include potential metastasis.