Thyroid sclerosing mucoepidermoid carcinoma with eosinophilia distinct from the salivary type.

We report three cases of thyroid sclerosing mucoepidermoid carcinoma with eosinophilia (SMECE), which is an extremely rare variant of mucoepidermoid carcinoma (MEC). The aims of this report were to describe the clinicopathological findings, including results from immunohistochemical and fluorescence in situ hybridization analysis of thyroid SMECE, as well as to discuss the distinction between thyroid SMECE and its salivary counterpart. The cases included a 63-year-old female, a 44-year-old male, and a 66-year-old female, with all patients presenting with Hashimoto's thyroiditis. Nodal metastasis was not found in any of the three cases. Neither regional recurrences nor distant metastases were found in any patient during the follow-up, which was 20 years, 3 years, and 18 months, respectively. Histologically, tumors were composed of epidermoid carcinoma cells, intermediate type carcinoma cells, and goblet cell-type mucus-secreting carcinoma cells, with all tumors displaying a sclerotic stroma with eosinophilic and lymphocytic infiltration. The formation of eosinophilic abscess in the tumor nests that might be a novel characteristic finding of SMECE was observed. Immunohistochemically, the carcinoma cells were positive for cytokeratin 34βE12, TTF-1, and PAX8, but negative for thyroglobulin. In two cases, increased IgG4-positive plasma cells were observed. Mastermind-like transcriptional coactivator 2 (MAML2), according to fluorescence in situ hybridization, was intact in all cases. In conclusion, thyroid SMECE has favorable outcomes and seems to be genetically different from salivary MEC. This is the first report to describe the presence of increased IgG4-positive plasma cells in the stroma of SMECE.