Mucoepidermoid carcinoma of the thyroid gland.

An extremely rare case of mucoepidermoid carcinoma of the thyroid in a 56-year-old woman is presented. The patient clinically having Hashimoto's thyroiditis was noted a nodule in her neck. The tumor was sited in the midportion of the left lobe of the thyroid, and histologically it showed both squamous features and mucin production. The squamous cells were arranged in solid sheets with horny pearls and the mucous cells tended to line dilated duct-like elements. Ultrastructurally, the epidermoid cells had aggregates of tonofilaments and well-developed desmosomal attachments, and the mucous cells contained numerous mucin granules in their cytoplasm. Immunohistochemical studies revealed that cytokeratin antibodies showed positivity for both the lining cells and squamous cells, whereas carcinoembryonic antigen positivity was found in the lining cells and intraluminal material. The tumor cells were negative for thyroglobulin, calcitonin, vimentin, chromogranin, and neuron-specific enolase. These unusual histologic and immunohistochemical features are suggestive of a tumor related to the so-called "solid cell nest" of the thyroid.