Papillary and mucoepidermoid carcinoma of the thyroid with anaplastic transformation: a case report with histologic and immunohistochemical findings that support a provocative histogenetic hypothesis.

A unique thyroid tumor in a 62-year-old woman is reported. Foci of papillary carcinoma (PC), mucoepidermoid carcinoma (MEC) and undifferentiated carcinoma (UC) were found in the surgical specimen. Acid mucosubstances were observed in the two histologically differentiated areas of the neoplasia. The PC showed immunoreactivity for thyroglobulin, and both PC and MEC foci were positive for high-molecular-weight keratins. Papillary carcinoma, MEC and UC were stained with antibodies against keratin CAM 5.2, vimentin, S-100 protein and neuron-specific enolase. No immunoreactivity was found for calcitonin, calcitonin-gene-related peptide, chromogranin, keratin 1, carcinoembryonic antigen and p53 suppressor gene. The diagnosis of this peculiar carcinoma of the thyroid exhibiting papillary and mucoepidermoid features together with undifferentiated (anaplastic) areas, reinforces the existence of a close relationship between papillary carcinoma and mucoepidermoid carcinoma, and supports the importance that ultimobranchial multipotential stem cells may have in the histogenesis of thyroid carcinomas.