Primary mucoepidermoid carcinoma of the thyroid: diagnosis by fine-needle aspiration biopsy.

Fine-needle aspiration biopsy (FNAB) has been recognized as a safe and reliable procedure in the evaluation of thyroid nodules. We herein report a case of a low-grade mucoepidermoid carcinoma of the thyroid that was diagnosed with this technique. Examination of an intra-operative FNAB showed cohesive clusters of polygonal squamoid cells with distinct cellular borders, uniform round nuclei, distinct nucleoli, and homogeneous amphophilic to cyanophilic cytoplasm. Focal keratin "pearl" formation was apparent, along with extracellular, lightly basophilic mucin deposits mantled by squamoid cells. These cytologic features are characteristic of low-grade mucoepidermoid carcinoma, as seen in other anatomic sites. This impression was confirmed by examination of paraffin sections. Previous reports have indicated that mucoepidermoid thyroid carcinoma is an indolent, locally recurring lesion. However, in spite of low-grade histology in our case, the neoplasm presented with distant metastases to bones, pleura, and lung.