Answer: Sclerosing mucoepidermoid carcinoma with eosinophilia

Discussion:
In the setting of Hashimoto thyroiditis, there is a mass-forming proliferation of nests, sheets, cysts, or cords of bland squamoid cells similar to intermediate cells of salivary mucoepidermoid carcinoma. Scattered mucinous cells are also present. There is a sclerotic stromal reaction with an infiltrate of eosinophils. The findings are diagnostic for sclerosing mucoepidermoid carcinoma with eosinophilia. This tumor type is encountered most commonly in women, with a peak in 50s and 60s. It almost always arises in the setting of Hashimoto thyroiditis, and may arise from foci of squamous metaplasia that are commonly seen in Hashimoto thyroiditis. Despite its name, it does not resemble very closely salivary mucoepidermoid carcinoma, and rare reports suggest that it does not harbor the MAML2 translocations seen in most salivary gland mucoepidermoid carcinomas. It may be difficult to distinguish from Hashimoto thyroiditis with florid squamous metaplasia, but the presence of a tumor mass, mucinous cells, and stromal eosinophilia favor carcinoma. The prognosis is good, though while this tumor was originally reported to be extremely indolent, more recently some aggressive cases have been reported.