Fine needle aspiration cytology of metastatic tumours to the thyroid

Background: Metastasis to the thyroid gland from non-thyroid sites is relatively rare and often poses a diagnostic difficulty on fine-needle aspiration cytology, as it often mimics primary thyroid neoplasms.Methods: All cases of fine needle aspiration cytology (FNAC) of metastasis to the thyroid gland (2014-2022) were selected from the pathology database. The detailed cytopathological features and histopathology of the cases were studied.Results: There was a total of 18 cases of secondary tumours of the thyroid. All cases had confirmed histopathological data. The most common primary tumours in our study were squamous cell carcinoma of the oesophagus (nine cases) followed by infiltrating ductal carcinoma of the breast (four cases), and one case each of renal cell carcinoma, neuroendocrine carcinoma of the lung, adenocarcinoma stomach and malignant melanoma and squamous cell carcinoma from vallecula.Conclusion: Metastasis to thyroid carcinoma is relatively uncommon. A history of malignancy, the presence of malignant cells amid benign thyroid follicular cells, unusual malignancy in a FNAC smear and immunocytochemistry are helpful in diagnosing such cases.