Mucoepidermoid carcinoma of the thyroid.

Clinical and morphological features of three cases of primary mucoepidermoid carcinoma of the thyroid are described. The tumours were composed of two cell types. One of these resembled squamous epithelium and ultrastructurally showed tonofilaments and numerous desmosomes. The other cell type contained Alcian blue and mucicarmine positive mucin and, on electron microscopy, showed mucigen granules. Marked stromal fibrosis and psammoma bodies were seen in all tumours. Immunohistochemical studies showed that the tumour cells were negative for thyroglobulin. A few calcitonin-containing cells were seen in one metastatic tumour. One tumour showed, in addition to the histological features of mucoepidermoid carcinoma, anaplastic areas with obvious transition between the two histological patterns. The same thyroid also had a small thyroglobulin-positive papillary carcinoma in the opposite lobe. All tumours presented lymph node metastases. In two cases the primary tumour was confined within the thyroid capsule but that with anaplastic areas invaded surrounding structures. This patient died 13 months after diagnosis; the other patients are alive and symptomless one and 10 years since diagnosis. Mucoepidermoid carcinoma of the thyroid appears to be a clinicopathological entity that resembles papillary carcinoma in its natural history. The origin of the tumour is unclear. There is, however, some histological and immunohistological data suggesting that the tumour might be related to the ultimobranchial system although some histological features also appear to favour a common origin with papillary carcinoma.