Tumor-to-Tumor Metastasis: A Case of Lung Carcinoma Metastasizing to Thyroid Neoplasm

ABSTRACT Objective: Metastases to the thyroid gland are rare. The most common tumors that metastasize to the thyroid are kidney, lung, gastrointestinal tract, and breast tumors. Metastases from other primary cancer sites into a primary thyroid cancer (defined as tumor-to-tumor metastases) are even rarer. Methods: In this case report we present the 10th published case of lung cancer metastasizing into a thyroid neoplasm. Results: A 53-year-old male patient had been diagnosed with metastatic non-small cell lung carcinoma 18 months previously (adenocarcinoma of the lung that had spread to both lungs) and was admitted to the endocrine surgery department for a mass in the thyroid. The biopsy of the mass revealed follicular neoplasia of the hurthle cell type. He underwent total thyroidectomy and the final histopathologic evaluation was reported as papillary thyroid cancer (hurthle cell type) with invasion of the adenocarcinoma with papillary structures showing different morphology inside the tumor. Conclusion: The most common tumors that metastasize are kidney, lung, gastrointestinal tract, and breast cancers. Routine pathological examination may not be adequate to diagnose other, less common metastases and careful review of medical history and systemic examination should be performed to reach the correct diagnosis. Immunohistochemical staining could aid in diagnosis given that trefoil factor 1 is positive in thyroid neoplasms, adenocarcinomas, and small cell tumors of the lung. Identification of the primary or metastatic status of a tumor is a diagnostic challenge, however it is essential as thyroid malignancies are usually treated by excision while widespread metastatic tumors of other organs could require chemotherapy and radiotherapy.