Clear cells and thyroid transcription factor I reactivity in sclerosing mucoepidermoid carcinoma of the thyroid gland.

Clear cells have been described in mucoepidermoid carcinoma of the salivary glands, but to our knowledge these cells have not been recognized as a distinctive component of sclerosing mucoepidermoid carcinoma with eosinophilia of the thyroid gland. We report four examples in which sclerosing mucoepidermoid carcinoma of the thyroid gland contained a minor component (10% to 30%) of clear cells. The four tumors occurred in women with Hashimoto's thyroiditis whose ages ranged from 38 to 52 years. All four patients presented with thyroid nodules, but two also had enlarged cervical lymph nodes. Although two tumors metastasized to multiple cervical lymph nodes and one recurred in the soft tissues of the neck, all four patients are disease-free 6 months to 6 years after surgical treatment. The locally aggressive behavior of the first three tumors was probably related to their size, rather than to the presence of clear cells. Microscopically, these tumors showed the classical features of sclerosing mucoepidermoid carcinoma with eosinophilia, as well as nests of clear cells that appeared to be glycogen-rich squamous cells. Although the four tumors were thyroglobulin negative, two were immunoreactive to thyroid transcription factor 1 (TTF-1), supporting the idea that they originated from squamous metaplasia of follicular epithelium rather than from solid cell nests of the ultimobranchial body.