Metastatic tumors in fine needle aspiration biopsy of the thyroid

A series of almost 25,000 thyroids examined by fine needle aspiration (FNA) biopsy was reviewed to ascertain the incidence and presentation of metastatic cancers in thyroid FNA samples. Metastatic cancers in FNA samples from the thyroid were identified in 25 cases (0.1%); the primary tumors were carcinomas of the kidney (8 cases), lung (7 cases), breast (5 cases), cervix uteri (1 case) and colon (1 case) and 1 case each of malignant melanoma, malignant pleural mesothelioma and rhabdomyosarcoma. FNA cytology was positive in all 25 cases. In 11 cases, the primary tumor was clinically known at the time of FNA biopsy; of the other 14 cases, cytology suggested that the malignancy was metastatic in only 5. Metastases of renal and mammary adenocarcinomas were almost indistinguishable from follicular and papillary thyroid carcinomas on cytologic grounds. The results demonstrate the rarity of this finding and the difficulty of diagnosing a metastatic tumor in the thyroid by FNA biopsy, in the absence of a clinical history of a prior primary neoplasm.