Mucoepidermoid carcinoma of the thyroid with concomitant papillary carcinoma: comparison of findings on fine-needle aspiration biopsy and histology.

We report two cases of mucoepidermoid carcinoma (MEC) of the thyroid gland coexisting with, and possibly arising in, papillary thyroid carcinoma (PTC). In the first case, CT-guided fine-needle aspiration (FNA) was performed on a paratracheal mass representing extrathyroidal invasion of a right thyroid lobe tumor. The aspirate showed papillary fronds and cells in honeycombed arrangements with fine chromatin, enlarged nuclei, nuclear grooves, and intranuclear inclusions in a background of mucus and blood; a diagnosis of PTC was rendered initially. However, examination of histologic sections of the mass showed nests of malignant squamous cells with interspersed mucous cells and extracellular mucin, concordant with MEC, as well as PTC. A retrospective review of the FNA specimen identified MEC. In the second case, ultrasound-guided FNA was performed on a right thyroid lobe nodule. The aspirate contained two populations of epithelial cells: larger cells showing foci of both squamous and glandular differentiation that were interpreted as MEC and smaller follicular cells with nuclear changes characteristic of PTC; both were addressed in the diagnostic report. Primary MEC of the thyroid is a rare neoplasm typically exhibiting indolent clinical behavior, although our first case demonstrated extensive local invasion. It is thought to arise from squamous metaplasia associated with PTC, Hashimoto thyroiditis, or other inflammatory or neoplastic processes. In thyroid FNAs, the presence of neoplastic mucous cells and extracellular mucin plus malignant squamous cells is diagnostic of MEC. As MEC is thought to arise in PTC, the finding of the latter in these aspiration specimens is not unexpected.