Histology: The background thyroid gland exhibits Hashimoto thyroiditis. There is an ill-defined, unencapsulated tumor consisting of nests and trabeculae of squamoid cells forming duct-like structures. There is a tumor-associated inflammatory cell infiltrate that has a conspicuous number of eosinophils. The tumor cells are relatively bland.

Discussion: Sclerosing mucoepidermoid carcinoma with eosinophilia (SMECE) is a rare form of thyroid carcinoma that typically arises in middle aged women, almost always in the setting of Hashimoto thyroiditis. It consists of a mass-forming proliferation of nests, sheets, cysts, or cords of squamoid cells similar to intermediate cells of salivary mucoepidermoid carcinoma. Overt squamous differentiation, ducts, and mucinous cells may be present, but are not required. The tumor cells exhibit only mild to moderate nuclear pleomorphism, and mitotic rates are low. In addition, there is a sclerotic stromal reaction with an infiltrate of eosinophils. It can be difficult to distinguish from foci of squamous metaplasia commonly seen in Hashimoto thyroiditis – in fact, SMECE may arise from these foci. In contrast to metaplasia, however, SMECE forms a distinct tumor mass. The prognosis of SMECE is generally good; it was originally reported to be extremely indolent, but more recently some aggressive cases have been reported.