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.

Eosinophilic replacement infiltrates in cystic Hashimoto's thyroiditis: a potential diagnostic pitfall

A 50-year-old woman underwent a fine-needle aspiration biopsy for progressive enlargement of the left thyroid lobe which was cystic and solid on ultrasound exam. The smears contained innumerable eosinophilic leukocytes along with lymphocytes, Hurthle cells, cells from a papillary thyroid carcinoma (PTC), and atypical glandular and squamous cells. The cytologic interpretation was Hashimoto's thyroiditis (HT), suspicious for epithelial neoplasm. The associated diagnostic comment stated concern for a sclerosing mucoepidermoid carcinoma with eosinophilia (SMECE) arising in a PTC. Thyroidectomy demonstrated a PTC, HT with multiple lymphoepithelial cysts, and extensive multifocal infiltrates of eosinophils, generally confined to the cyst walls. As the cytologic findings mimicked a SMECE, we report these specimens as a most unusual diagnostic pitfall.