Thyroid gland metastasis from small cell lung cancer: an unusual site of metastatic spread.

Metastasis to the thyroid gland is uncommon compared with the frequency of primary thyroid tumors. The primary sites of metastatic thyroid tumors usually include the breast, lung, kidney and stomach. Among lung cancer types metastasizing to the thyroid, adenocarcinomas are the commonest followed by squamous and large cell carcinomas. Small cell lung carcinoma has not been frequently reported to cause thyroid metastatic deposits. Herein, we describe a patient with small cell lung cancer who developed metastatic lesions to the thyroid and brain simultaneously. Thyroid ultrasonography-guided fine-needle aspiration cytology (US-FNAC) and particularlyimmunocytochemistry documented metastasis from primary lung cancer. Clinical, cytopathological and therapeutic aspects of this unusual site of extrathoracic metastatic disease are discussed laying special emphasis on the paramount importance of the immunocytochemistry in distinguishing primary thyroid tumors from thyroid metastasis due to lung cancer.